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THE INTERNATIONAL SCIENTIFIC SERIES, 


ANIMA, Lae 


AS AFFECTED BY 


THE NATURAL CONDITIONS OF EXISTENCE. 


BY 


KARL SEMPER, 


PROFESSOR OF THE UNIVERSITY OF WURZBURG. 


WITH TWO MAPS AND ONE HUNDRED AND SIX WOODCUTS. 


NEW YORK: 


D. APPLETON AND COMPANY, 
1, 8, anv 5 BOND STREET. 
1881. 


PREFACE. 


—_—+r— 


Ir 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 pro- 
cess 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 expres- 
sions 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. 

J 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 at- 
tained 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. 


Wirzbure: Sept. 1879. 


CONTENTS. 


INTRODUCTION. 
PAGE 
Impetus to study given by Darwin. Morphological Pedigrees, 
Inheritance and Adaptation. General Sensibility of the Living 
Cell. Morphological and Physiological Characters 1 


SECTION I. 
GENERAL PRELIMINARY CONSIDERATIONS. 


CHAPTER I. 
THE PHYSIOLOGY OF ORGANISMS. 


Morphology and Physiology. Topography of wee i of 
the Book. Division of the Subject . ‘ i - 25 


SECTION II. 
THE INFLUENCE OF INANIMATE SURROUNDINGS. 


CHAPTER II. 
FOOD AND ITS INFLUENCE. 

The Equivalence of Food to Tissue. The Optimum of Food. Sti- 
mulants. Monophagousand Polyphagous Animals. Modifica- 
tions caused by Food and ced of Food. Changes in 
Structure and in Colour . . ° F . . F 40 


vill CONTENTS, 


CHAPTER MII. 


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. 
Hurythermal 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 Cli- 
mates. Acclimatisation of Parrots. Periodicity. Equable 
Temperature of the Sea. : % Z : 3 101 


CHAPTER V. 
THE INFLUENCE OF STAGNANT WATER. 


Water indispensable to every Form of Existence. Chemical Com- 
position 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 Experi- 
ments on Articulata. Beudant’s on Mollusca. Transition 
from Branchipus to Artemia. Influence of Volume. Experi- 
ments on Lymnea stagnalis. An Unknown Stimulant. In- 
fluence of Oxygen or Air in the Water. Respiration a 
Function of Protoplasm. Organs specially adapted for Re- 
spiration. The Skin; Gills; the Intestine; the Air-bladder. 
Power of enduring Desiccation. Eggs of Apus and Cypris. 140 


CONTENTS. ix 


CHAPTER VI. 


THE INFLUENCE OF A STILL ATMOSPHERE, 


PAGE 
Action of the Lungs. Composition of Air. Trachez in Insects. 


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 (Godiid@). Amphibious Mollusca (Am- 
pullaria). Land Crabs (Birgus latro). Transformation of 
the Branchial Cavity intoa Lung. : F ‘ 7 . 178 


CHAPTER VII. 
THE INFLUENCE OF WATER IN MOTION. 


Presence of Salts in Fresh Water. The Mechanical Factor. Re- 
sistance 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 . 7 : . : ‘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. 3 : ‘ A F : 7 5 - . 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. Simi- 
larity of Fresh-water Mollusca less great than Darwin assumes. 
American Astacide. 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. ° F 5 . 276 


CHAPTER X. 


A FEW REMALKS 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. Accommo- 
dation to Pressure. Possible Effects of Electricity 317 


SECTION ITI. 


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. lima in and on a 
Holothurian, Hybridisation. In Confinement. Ina Free State 
of Nature. Its Effects on Colouring and on Specific Characters 330 


CONTENTS. xi 


CHAPTER XII. 


THE SELECTIVE INFLUENCE OF LIVING ORGANISMS 


ON ANIMALS, 


PAGE 
Indirect Selection. Competition for Similar Conditions. Rela- 


tions of the Pursued and the Pursuer. Transition Forms, in 
Lamellicorn Beetles. Modifications leading to Physiological 
Function. The Dorsal Eyes of Onchidium. Its Defensive 
Glands. Protection by Imitation or Mimicry; in Insects. 
Difficulties with regard to Polyps and Fishes. New Instances 
among Univalve Mollusca in the Philippines. False Mimicry 
in a Worm in the Mediterranean. Caution as necessary as 
Research . . . 5 : f : . . . + 360 


Nores 


INDEX . ' ‘ . ‘ n 


LIST OF ILLUSTRATIONS. 


8 
MAPS. 
PAGE 
J. Sketch Map of the Pelew Islands . 3 3 ee 5 » 235 
II. The Atoll of Kriangle é 2 . ° 3 $ - . 240 
WVOODCUTS., 
Fic. 
1. The Internal Structure of a Chameleon, in its natural 
position. . 4 
2. Diagram of the Lungs and Ciroulation of Bir ua late, the 
Palm Crab . ‘ - . x , . ae 5 
3. Section of a Bleak . . . > 7 


+. a, The Bone of a Cat with the NMarcowe intbess 3 3, the — 
of a Bird, with Air-cavities instead of Marrow-tubes 3 
ec, the Skull (sawn through) of Buecrvs, with Air- 


cavities in all the Bones . : ‘ , . ae 8 

5. a aFlying-Fish; d,a Bat . “ & : 9 
6. The Branchie, @ to c, of Annelida; d, of a ftuazal : va dS 
7. The Skull of the Female Dugong . . . . . » WV 
8. The Casting of the Skin in Reptiles . . : 5 » . 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 : > AT 
13. Section of the Gésophagus and Stomach ofa Picen . » 65 
14. Larus argentatus - < 3 . : F . > - 60 
15. Myopotamus Coypu . : ‘ - ‘ . . - 61 


16. Nestor mirabilis . ‘ F ij 5 ™ -« » €2 


xiv 


LIST OF ILLUSTRATIONS. 


. Animals in which Chlorophyll Grains have been detected 
. Collozoum inerme (Haeckel) f - . ‘ 

. Section through Sphenapus Steenstrupii . 

. a, Proteus from the Adelsberg Grotto, reduced ; 3, "Seotion of 


a Rudimentary Eye . 3 a so 
, Pinnotheres Holothuria, nat. size ; b, depenemie Water- 
lung of the Holothuria . é 


. Zoea-larva of Pinnotheres Holothurie 

. Ablind Cymothoe; in fresh water . 

+. Siredon pisciforme, the Mexican Axolotl 

5. Section of a Frog’s Skin . ‘ . 

: Chromatophores of a Frog's Skin 

. Rossbach’s Curves of Temperature for the ‘Action of the Con- 


tractile Vesicles of the Infusoria . ‘ 


. The Infusoria experimented on by Rossbach 

. Desoria glacialis, the Glacier Flea . 

. Aphis Beccabunge : : 4 2 : 

. a, Diploznon paradoxum ; b, Polystomun eS wm, from 


the Bladder of a Frog . : 


. Phasma sp., a ae Orthopterous Tracer: 

. alpus if 

. Buplectelia epee ium 

. Halvbates, new sp. 

. Pachydrilus, new st ., living in 1 ihe alt Waters na Kiestnaen 
. Platurus vulcanicus . : : 

. Bopyrus ascendens 

. An Oyster from the river Gumdiaran 3 in Basifen 

. Cordylophora lacustris 

. a, Branchipus stagnalis ; b, Aptenite atin ; ; 

. Transformation of Artemia saline into A. Milhausenti 

. Four Shells of Lymnea es all of the same Age and 


Brood . é A ‘ 


. Curve of Volume for Lymneca ita anette 

5. Curve of Time of Growth of Lymnea stagnalis 

. A similar Curve altered by the Effects of Temperature 

. a, Anahas scandens; b, a Tadpole ; ¢, ayoung Polypterus from 


the Nile; d, Embryo of a Shark . . . 


. Gills of Mollusca 

. Portion of the Stomach of a Helothurtan 

. Cobitis fossilis 

. Section through the Tie of the Embryo of a Pig , 
. Systems of Trachex sketched in the Outline of the Insects. 
. Geonemertes padacnsis, a Nemertean ; nat. size . 

. Talitrus saltator . 


106 
107 
117 
123 


124 
127 
128 
137 
144 
145 
146 
147 
148 
152 
156 
157 


161 
162 
164 
164 


168 
170 
171 
172 
179 
180 
187 
188 


. A piece of solid inestone; shetty ered, ivi Teta 
. Part of a Stock of Campanularia, « Hydroid Polyp, with 


. Antipathes filix, Pourtales , a A 


LIS£ OF ILLUSTRATIONS. 


. The Branchial Lungs of Ampullaria 
. Gecareinus Hones a Land Crab 

. a, Lapea; b, Ocypoda . 

. Univalves cling to Rocks — means sat he Foot 
. Various Animals that swim by means of Fins 

. Creeping Mollusca . . 

. Diagram of Section through Moritiea and N avinells, showings 


the position of the Operculum 


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

. The Inhabitants of Coral Galls . 

. Seriatopora hystrix, with Galls inhabited o aaieds CANUS 


marsupialis . 


. A Gall on Seriatopora hystrix, dened 
. Sideropora palmata, with a Gall or Cyst ‘ 
. Goniastrea Bournoni, M. Edw., with a Funnel in iideh lives 


a Cryptochirus coralliodytes, Hiller . 


. Diagram of the Growth of a Colony of Porites 
. @, Section through Kriangle; b, through Dabo at 


Aibukit; ec, through Pelelew . 


. Pyrosoma gigas s s 

. Onchidium tonganum, nat. size . 

. The Shells of Philippine Snails 

. Amphidromus maculiferus, Sow. 

. Trocomorpha sp. 

. Lemnocephala chilensis, Dinca’ 

. Two operculated Fresh-water Snails 
. Various Fresh-water Univalves : 
. a, an Amoeba in a Locomotive State ; 3, the same, eneveied:, 

. Cypris sp. from the Philippines 

. Trochosphera equatorialis, from the Pei pinks 

. Coregonus hiemalis 

. A piece of Wood bored by Honnarta opie & fron Helis. 


land 


closed pear-shaped Galls, age a Sea- ete Pye- 
nogonida . : : % 


. Heteropsammia Michelini 

. Heterocyathus philippensis 

. Rhizochilus antipathum, Steenstrup . ; 

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


Individuals of the Parasitic Sacculina carcini 


xV 


PAGE 
191 


192 
196 
203 
206 
208 


211 
213 
214 
217 


218 
219 
220 


222 
225 


258 
279 
281 
284 
285 
288 
293 
299 
300 
302 
304 
306 
321 


326 
326 


332 
335 
336 
337 


340 
341 


104. 
105. 


106. 


LIST OF ILLUSTRATIONS. 


. Section through a calcareous Sponge, showing the simple 


central Cavity; b, the Sponge in an uninjured State 
(after Haeckel) 


. Spongia cartilaginea, Esver, half nat. size. 

. Spongia cartilaginea, Esper . ; ‘ : ‘ 

. Entoconcha mirabilis, Miller Z 

. Two undescribed Species of Eulima . ; : 

. Cladognathus dorsalis, Erichson . 

. Chalcosoma Atlas, from the Philippines . 

. Section of Eyes . 

. Section of the Dorsal yer of. Aucbidions vern vente m 

. Periophthalmus Koelreuteri, a Fish which pursues the Onchidium 
. Development of the Eyes of Onchidium 

. Grasshoppers protected by their likeness to Leaves 

. a, Doliops sp., mimics b, Pachyrhynchus orbtfer, c, Dette 


curculionides, mimics d, Puchyrhynchussp.; ce, Scepastus 
pachyrhynchoides (a Grasshopper), mimics /, Apocyrtus; 
g, Doliops sp., mimics h, Pachyrhynchus sp.; i, Phor- 
aspis sp. (a Grasshopper), mimics k, a Coccinella 
Spiders which mimic Ants ‘ ‘ 
a, Rhysota Antonti, mimicked by 8, Festa pebndanaonets ¢, 
Helicarion tigrinus, mimicked by d, Vesta Cumingi 
Myzicola infundibulum (after Claparéde) . : . . 


PAGE 


342 
343 
344 
B48 
351 
366 
367 
370 
371 
373 
378 
383 


390 
391 


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 develop- 
ment 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 advis- 
able 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, 


Z INTRODUCTION. 


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 it is 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 otherwize than of a 
somewhat doubtful character. In all zoological investiga- 
tions, 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 

.on 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 inter- 
pret 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 applica- 
tion of the facts of animal morphology. 

We may, however, rest satisfied with these illustrations, and 
proceed to the wider question, how it was possible that nature 
should have produced such an immense variety of forms as 
is exhibited 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 


4 INTRODUCTION. 


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- 


Fig, 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 genera- 
tions; 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 


Fic. 2.—Diagram of the lungs and circulation of Birgus latro, the Palm Crab. I. The 
Inng-vessels indicated within the outline of the animal—a,, @,, a,, the three upper affer- 
ent vessels (veins) ; e 7, the efferent vessel (artery); e br, section of the opening of the 
arterial gill-vessel; h, heart ; hb, p lium. II. Diagram of section of the same, 
lettered as above —bdr, gills or branct ;¢,, the lower afferent vessel (vein) ; 7, 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 consider- 
able advances in the art of discriminating between those typical 


e 


6 INTRODUCTION. 


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. 

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 
origmated 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 regard- 
ing them as, in some degree, characters of adaptation. 

Another instance. It 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 ina band, 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 iden- 
tical, and if we were to attempt to regard the pectoral fins of 


CHARACTERS OF ADAPTATION, vi 


fish as identical with those that occur in manuals, 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 indi- 
vidual 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 


Fie. 3.—Longitudinal section of a Bleak. s, anterior ; s’, posterior portion of the 
air-bladder ; «, cesophagus ; J, air-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 members of the body, 
in themselves not comparable, have, by adaptation to new con- 
ditions of existence, become such organs with similar physio- 
logical functions. 

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


8 INTRODUCTION. 


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 


Fig. 4.—a, the bone of a Cat, showing the marrow-tube; }, that of a Bird with 
cavities containing air instead of marrow; c, the skull of a Buceros sawn tarough. 
Air-cavities traverse every part of the bone. 


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 de- 
ductions from them coincide with conclusions drawn from 
other facts. In agreement with this we see that the peculiar 


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 develop- 
ment 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 con- 
ceived of as having originated through modification of a simple 
organ—perhaps a bladder-shaped lung—which may have been 


2 


10 INTRODUCTION. 


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-saes, 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 crusta- 
ceans. 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, ¢.¢. of parts which may be made use 
of for investigating the evolution and modification of these 
classes—or, as muy be, orders—one from another, and for 
establishing such a natural system of the Vertebrata as may in- 
dicate 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 Verte- 
brata, these seem to have the value merely of characters of 
adaptation. The whale has fins as efficient as those of the stur- 
geon 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 adapta- 
tion, and so valueless for any determination of their affinity. 
But if we now turn our attention to the same organs within 


ORGANS OF LOCOMOTION. ll 


the limits of a single order or even of a single family of the 
Vertebrata, the case is wholly different. The wings of birds 
have the same typical structure throughout the whole class, and 
the same is true of the wings of the bats, and of the parachutes 
of the flying reptiles. Within the limits of these narrow groups 
each organ acquires a quite different value. Their constant 
occurrence and the similarity of their structure and development 
make us suppose that they must have originated through the 
modification of one or more simple organs in the parent form of 
each family. It is the same with regard to the fins of fishes, 
whales, and other Vertebrata. Here, as in the first example, 
we see that the organ or member which, within the limits of a 
small group, is a transmitted character, and helps in determin- 
ing the affinity of the individual forms, appears as a character 
of adaptation when we compare the great systematic groups 
with one another. And choose what organ we will for this 
comparison, the result will be the same ; parts which have little 
or no value as characters of adaptation assume a conspicuous 
diagnostic importance when we have to trace out the relations 
of affinity under a wide systematic aggregate, because, within 
the limits of the smaller groups, they may always be regarded 
as hereditary. Thus, too, we arrive at the conclusion that the 
distinction drawn, in the most recent zoology, between characters 
of transmission and those of adaptation, has only a partial value ; 
for every organ which originated by adaptation, and in the first 
instance was worthless for the determination of the relations of 
affinity, may quite easily—nay, must—become transmissible by 
inheritance if it is rendered permanent simultaneously with 
improvement in other directions, in several varieties or species 
all derived from the same parent form. But it may be trans- 
formed into an hereditary character in a yet wider sense, particu- 
larly when an organ which has originated by adaptation in one 
single species, or even in one individual, is transmitted to along 
series of generations which branch off into different families, 
though all descended from the one parent form. 

Tt is not difficult to show by an imaginary instance how 
such a change in the organ might be effected side by side with 
permanence of the fundamental form. Suppose, for instance, 


12 INTRODUCTION. 


that from the skin of one of the lower animals, say a worm, a 
ramified and villous prolongation arose by local excrescence 
which, as contrasted with the general respiration hitherto carried 
on by the skin, was a specially qualified organ of respiration—a 
true gill. This gill must be in such connection with the vessels 
of the body, or with the cavities which contain the circulating 
blood, that the absorption of oxygen by the blood may be more 
easily effected here than in other parts of the skin; it cannot 
otherwise be designated as a true gill (or branchia). But, in 
order to exercise the same respiratory activity as the skin, 
these gills must possess a certain rigidity, so that their whole 
surface may be in contact with the water that surrounds them, 
for this would be impossible to soft and pendulous gills ; more- 
over, certain auxiliary organs must be connected with them, to 
sceure the requisite change of water by producing a constant 
current. This renewal of the supply of water for respiration is 
frequently effected by the active movement of the branchie 
themselves, or by the constant motion of the animal; but in 
every case where such organs have ceased to be superficial on 
the skin by its induplication, or have become internal, special 
auxiliary organs are found, as in Crustacea for instance, Fishes, 
Mollusca, &e., whose sole duty is to keep up a constant stream 
bathing the gills. Thus the physiological efficiency of the 
principal organ depends not alone on the capability of the 
epidermal cells to absorb oxygen from the surrounding water 
(by osmosis), but also on those auxiliary organs which con- 
stantly supply the branchise with fresh water for respiration, 
and, by keeping up their rigidity, prevent any diminution of 
the respiring surface by collapse. 

If furthermore we suppose that the branchie, which 
originated, perhaps, by adaptation to an increased demand on 
the respiratory organs, were permanent during the transmu- 
tation of the first species into several new ones, while at the 
same time they preserved their character of independent appen- 
dages of the outer skin, they might very likely come to act as 
organs not merely of respiration, but also of locomotion. For 
by their position, rigidity, and power of independent movement 
—all indispensable to their efficiency as branchie—they are, 


ADAPTATION TO NEW FUNCTIONS. 13 


from the first, able to offer a certain resistance to the common 
motion of the whole body, and to serve as a fulerum for the 
movements of limited sections of the body in a way that might 
certainly be advantageous to the whole animal. Thus a gill 
might be partially or whol'y transformed into an organ of 


Fic. 6.—Gills, a, b, c, of Annelida; d, of a bivalve Mollusk. a, Nauphanta celox (Greeff) 
enlarged to three diameters, with broad gill-fins. 6, foot of Vanadis ornata (Greeff), 
with two broad gill-fins. c¢, section of a segment of £unice ; br, the ramified gill- 
appendages of the rudimentary foot, d, Mytilus edulis, with 6r, the gill-folds, and 1, the 
lips separated from them. 


locomotion, and accordingly we find in many Annelida gill- 
bearing organs (fig. 6, a, 5) which at the same time serve for 
creeping or swimming, and which present that more specialised 
form in which the functions of respiration and locomotion, 
originally exercised simultaneously by the same organ, have been 


14 INTRODUCTION. 


transferred to two separate sections of the same organ, though 
this is still morphologically one. The branchie might likewise 
become internal, as in Fishes, Crustaceans, Molluscs, Ascidians, 
and so forth. The current requisite for respiration might, in 
such a case, be induced simply by the development of cilia on 
the cells of the epithelium of the branchixl membrane, as occurs 
in all molluscs and in ascidians. The current might then serve 
another purpose, namely, that of bringing food to the mouth ; 
and this is the case in the above-mentioned animals, which re- 
ceive their nutrition, consisting of microscopic organisms, ex- 
clusively by the aid of the current drawn into the branchial 
cavity. Now, if the function of respiration were transferred 
by any means to some other part of the animal, or restricted 
to a limited section of the branchiz themselves, the remaining 
portion might be transformed into an organ serving exclusively 
to obtain nutrition. The lips lying near to the mouth of 
molluscs would, in fact, appear to be such modified portions of 
the folds of the branchiee. 

But this is by no means the limit of such change of function. 
Each animal cell in the living organism is sensitive to various 
molecular movements which impinge on it from without. 
General sensibility is an attribute of the living sabstance of the 
cell. Now it would obviously be a considerable advantage to 
the animal that the organs of respiration or locomotion should 
be connected with certain organs of sense—in our example, for 
instance, if the lip-like appendages of the branchie of the 
molluscs could be transformed into organs of taste or touch. 
As every living cell, including of course the cells of the mucous 
membrane of the branchie or the labial fold, possesses this 
general sensibility, and this in a certain sense includes the 
capability for developing a special sense of touch or taste, we 
perceive that an epidermal member which originated as a 
simple gill may, by virtue of its inherent properties, easily be- 
come an organ of locomotion, sensibility, or taste, and it might 
equally easily be converted into an organ for the acquisition of 
nourishment (as in the Ascidians) or for any other purpose. 
At the same time be it observed, such transformations have 
not taken place suddenly in an abrupt and, so to speak, revo- 


GENERAL SENSIBILITY OF THE CELL. 15 


lutionary manner ; for their existence need not in the first in- 
stance be conditional on the introduction of new influences, 
since they depend on the fact that the branchie must from the 
first—or they could not have lived, grown, and exercised their 
functions—have contained all the elements and have exercised 
all the elementary functions which fitted them for differentiation 
in the direction indicated by those functions, and for transfor- 
mation into organs apparently intended for one function only. 
We arrive at the same conclusion by a simple general con- 
sideration. We know that the simplest and lowest animal, a 
mere gelatinous mass, sayan Ameeba, exercises, and must exercise, 
all the vital functions as well as the ovum-cell which is so rich 
in protoplasm, or even as the young and growing cells which 
constitute animal tissues—functions which in the higher animals 
are apparently fulfilled exclusively by special organs. The pro- 
toplasmic cell or the Amceba takes up nutrition, often indced 
of a solid nature; it moves more or less quickly and voluntarily ; 
it is sensible to impressions transmitted to it by the agitation or 
chemical properties of the surrounding medium ; it assimilates 
organic matters, and breathes, inasmuch as it expires the carbonic 
acid formed in the process ; it is capable of more or less definite 
sensations, for it selects the food that suits it, and it grows 
and multiplies often by highly complex processes. All these 
characters are to be found in each living protoplasmic cell of 
every growing organ; but it is true that it is not every cell of 
an organism that is in this sense living. Thus in the hair and 
nails, for instance, there are horny cclls which no longer contain 
any fresh and unchanged protoplasm, and consequently can no 
longer grow or multiply, their increase being effected by fresh 
and living cells lying in the deeper skin-layers ; from these, new 
horny cells are constantly produced and pushed forward to 
replace the old cells as they wear off at the angles of the nails 
or tips of the hairs. We may apply Briicke’s expression, 
‘elementary organisms,’ as a name for those deep-seated cells, 
which, being richly supplied with protoplasm and capable of 
multiplication, are, in the strictest sense, living cells. For as 
the life of every organ is the sum of the individual life of the 
living cells which compose it, it is clear that every living and 


16 INTRODUCTION. 


growing organ, without exception, must, in a certain sense, be 
capable of becoming modified in any such various directions as 
are indicated by the common properties of the living substance 
of the protoplasm. The abstract and paradoxical formula for 
this position might be put thus :—Every living organ may, by 
virtue of the properties inherent in its living cells, become any 
other organ. 

Let us now return to our starting-point. We have seen 
that every character of adaptation must be in a certain sense 
hereditary ; for if those individuals of a species which first 
acquired any given character by adaptation were incapable 
of transmitting it to posterity as a part of their inherited 
nature—particularly if the exciting causes were to be re- 
moved—every newly acquired character would presently be lost. 
The inheritability of this newly acquired character might also 
be greatly increased if, for instance, it were transmitted through 
a long series of varieties or species, while at the same time it 
was undergoing modification. This might occur if, from the 
first, its fundamental character were such as must induce 
specialisation of its function. Now we have seen that even so 
highly specialised an organ as a gill is, or seems to be, must 
yet be capable of numerous modifications ; for its primary use 
depended partly on other functions which were capable of 
further modification, @.e, specialisation, in a mode analogous 
to that by which the gill became an organ of respiration. And 
the more manifold the independent and latent properties are of 
a newly constituted organ of adaptation, the greater will the 
probability be that it will be transmitted by inheritance to all 
the divergent descendants of the parent form, and be at the same 
time modified to meet thcir altered functional requirements. 
But the more specialised an organ is—that is to say, the more 
one single purpose is developed to the prejudice of the latent 
functions—the harder will it be for it to adapt itself to new pur- 
poses, and so it will probably be transmitted to the descendants 
of the parent form but little altered. Hence it is impossible to 
establish an @ priori distinction between Characters of Adapta- 
tion and Characters of Inheritance, and we perceive that most, 
perhaps all, of the characters now in a great measure hereditary 


RUDIMENTARY ORGANS. 17 


originated through modifications of those originally adaptive 
organs which bore within them the elements of continuous 
and extensive gradual transformation.? 

This inference includes another: That all the structural 
pecularitics of animals are true organs which must subserve 
some function and can never be mere useless ornaments. 
Otherwise, from the Darwinian point of view—which, as I have 
said, I accept as a standard—it would be quite unintelligible 
how wholly useless portions of the body could have been in- 
herited and modified through a long series of divergent descen- 
dants from the parent form. If it could be strikingly shown that 
organs actually exist which are of no physiological use to the 


Tic, 7.—Skull of female Dugong; the colossal tusks in the upper jaw never pierce the 
thick fleshy lip, although they continue to grow with the jaw. a, the root of the tusk; 
b, the point. 

possessors, but which determine the types of form in whole 

Classes, Orders, or Families, then the conclusion would seem 

inevitable that these functionless organs have been formed in 

accordance with some transcendental law (or plan) of develop- 

ment. 

Now it is sometimes asserted, oftener, itis true, by botanists 
than by zoologists, that such functionless organs do in fact 
exist. Ido not here allude to rudimentary organs ; ? for although 
these appear in sundry groups of animals to be in fact devoid 
of any recognisable function, they are derived, no doubt by 
degeneration, from true organs whose functions are conspicuous 
in other animals. The best example known to me of such 
rudimentary organs is offered by the female Dugong (Halicore 


18 INTRODUCTION. 


Dugony). This creature has enormous tusks which continue to 
grow as the animal grows, and are even larger in proportion in 
the adult than in the young. Thus the tusks of the female 
grow more quickly than the skull. They are nevertheless wholly 
functionless in the female ; that is to say, they are used neither 
as tusks nor as teeth, for they are completely encased im the 
upper jaw-bone, and the blunt point is covered bya huge fleshy 
upper lip forming a snout. The male Dugong uses his tusks, 
which project at the sides of the mouth, as weapons or for other 
purposes, as is proved by the fact that the outer surface of 
the points of the tusks is, without exception, much worn in 
every male skull found in our collections. The tusks of the 
female Dugong are rudimentary and functionless as teeth ; 
however, like all similar rudimentary organs, they are not 
included in the above-mentioned class of functionless organs, 
which, in spite of their immense variety and often conspicuous 
size, cannot be regarded either as true organs now exercising 
their functions, or even as truc organs in a degraded condition, 
such as are known in scientific parlance as ‘ morphological cha- 
racters,’ in contradistinction to ‘ physiological characters,’ that is 
to say, those whose use is obvious or well-known. The exist- 
ence of such morphological characters has been affirmed, as I 
have said, even of animals; but it still seems doubtful whether 
those organs and parts of the animal body which we include in 
this category do in fact belong to it, and are not perhaps thus 
classed merely because as y2t we know nothing of their func- 
tional importance. Even when we assign to such parts the 
smallest possible importance in the life of the animal, we ought 
not to forget that they consist of living cells, or are directly 
dependent on them. Tence we are justified in propounding the 
thesis that every part which we are accustomed, from its lack 
of conspicuous physiological character, to regard as a morpho- 
logical character, must nevertheless have a certain functional 
value in the general economy of the animal, since it must pro- 
duce a fraction, however small, of the material which is formed 
in the living body, and must possess a proportionate share of all 
its properties. 

At the same time it cannot be disputed that even in 


MORPHOLOGICAL CHARACTERS. 19 


animals, though certainly less often than in plants, peculiarities 
of structure do occur which appear to be absolutely useless to 
the life of the individual, although they are not rudimentary 
organs. Such, for instance, are the colours of the skin of many, 
and especially of marine, animals; many expansions of the skin 
and the sculptured character of the skin of Reptiles, Crus- 
taceans, and Insects would seem to be of this nature; the rela- 
tive positions of the various organs, which may sometimes be 
said to be highly inappropriate, the number of the extremities 
in various animals, and many other circumstances a'so come 
under this head. It would be a highly important task for a 
zoologist, and, I believe, fertile in results, to discuss all these 
cases ‘in detail, in order to see whether, or how far, our pre- 
sent knowledge suffices to explain them; %.¢. to show that 
such morphological characters do not in fact exist in animals. 
In this place it must suffice to discuss one single example. 

It is known that the skin of Reptiles encloses the hody with 
scales. These scales are distinguished by very various sculptur- 
ings, highly characteristic of the different species. Irrespective 
of their systematic significance they appear to be of no value in 
the life of the animal; indeed they are viewed as ornamental, 
without regard to the fact that they are microscopic and much 
too delicate to be visible to other animals of their own species. 
It might therefore seem hopeless to show the necessity for their 
existence on Darwinian principles, and to prove that they are 
physiologically active organs. Nevertheless recent investiga- 
tions on this point have furnished evidence that this is possible. 

It is known that many Reptiles, and above all the snakes, 
cast off the whole skin at once, whereas human beings do so by 
degrees. If by any accident they are prevented doing so, they 
infallibly die, because the old skin has grown so tough and 
hard that it hinders the increase in volume which is insepar- 
able from the growth of the animal. The casting of the skin is 
induced by the formation, on the surface of the inner epidermis, 
of a layer of very fine and equally distributed hairs, which 
evidently serve the purpose of mechanically raising the old 
skin by their rigidity and position. These hairs, then, may be 
designated as casting hairs. That they are destined and 


20 INTRODUCTION. 


calculated for this end is evident to me from the fact established 
by Dr. Braun that the casting of the shells of river Cray-fish 
is induced in exactly the same manner by the formation of a 
coating of hairs which mechanically loosens the old skin or 


Fie. 8.—Casting process in the skin of reptiles. a@,in the clinging bristles of tle foot 
of the Gecko; within the epidermis are seen the casting hairs, Hh, destined to form 
the new clinging lvistles. 5, in the Adder ; Ak, the casting hairs ; the portion of skin 
above them is pushed away, and the hairs themselves form the ridges on the new skin. 
c, a scale of Phyl/odactylus, with the little sensitive hairs si at the right edge. @, Theca- 
dactylus ; sh, the sensitive hairs; and hh, the casting hairs which do not change 
during casting. From Cartier. 


shell from the new. Now the investigations of Braun and 
Cartier have shown that these casting hairs—which sezve the 
same purpose in two groups of animals so far apart in the 


systematic scale—after the casting are partly transformed into 
the concentric stripes, sharp spikes, ridges, or warts which 


THEIR PHYSIOLOGICAL VALUE. 21 


ornament the outer edges of the skin-scales of reptiles or the 
carapace of crabs. Hence we are justified in regarding the 
sculpture thus produced on th2 epidermis of these animals as 
a rudimentary organ ; for the microscopic casting hairs, after 
they have done their duty in inducing the casting, remain 
where they were formed, somewhat altered in form, it is true, 
and without any further visible use. 

Occasionally, however, these hairs, after they have fulfilled 
their office, are transformed into organs which are capable of 
serving other useful purposes to the reptile. Thus, for instance, 
Von Leydig discovered certain organs in the skin of reptiles, 
which he designated as organs of a sixth sense, regularly 
communicating with long elastic hairs which project far above 


Fig. 9.—Stages of castirg in the carapace of the freshwater Cray-fish, from Braun. 
l. First stage; a, the two old cuticular layers; 6, the layer of casting hairs; c, the epi- 
dermis cells. II, Second stage; a, b, e, as in I.; between) and c the new cuticle d has 
intervened. 

the surface of the skin, and seem admirably adapted to transmit 

every impact or molecular movement to the sensitive and 

guiding organs that are connected with them. These sensitive 
hairs belong to those casting hairs just mentioned, of which some 
fow, that are placed in suitable positions, have been transformed 
into such sensitive hair-organs (fig. 8, sh). Many of the teeth 
and ridges which are formed on the surface of the scales by 
the coalescence of casting hairs are so placed as to be of use in 
the difficult process of stripping off the whole skin on whose 
surface they are situated, for they serve as holdfasts to the 
rough surface of stones. Another still more striking example 
is exhibited by the family of the Geckos, which are all dis- 
tinguished by having an immense number of long stiff movable 


22 INTRODUCTION. 


bristles on the soles of their feet, which give these creatures their 
well-known power of running with great rapidity along vertical 
walls or the ceiling of a room back downwards, without falling.‘ 
These hairs, like those described above, are nothing more than 
specially developed casting hairs, for they originate in the same 


woe erm a 


ji fyi 

vin 

Wl We i 
a i 7 


FG. 10,—Structure of the Gecko's foot ; a, from above; 6, a toe with its clinging ridges, 
from below, slightly magnified ; ¢, diagram of section through a toe, exhibiting the 
ridves in section; @, a few of these, magnified, with their bristles ; ¢, four rows of bristle- 
cells, Much magnified ; f, two separate bristle-cells, more magnifica. From Cartier. 

way as those, and assist in the process of casting (fig. 8, @). 

The clinging hairs are absent in the embryo of the Gecko; they 

do not appear till the first casting, and assist in causing the 

process ; this sufficiently proves that they were not primarily 
destined to be used as organs for clinging, but have acquired 


FORM DETERMINED BY FUNCTION. 23 


that use after first serving their other function. The sculptured 
patterns on the scales of reptiles, on the other hand, may be 
regarded as the transformed and now useless remains of those 
casting hairs whose utility ended with the preparing the old 
dead skin for its casting by slightly loosening it; while the 
remains of others of these casting hairs have grown to be 
other functional organs—as sensitive hairs and clinging bristles 
—hbecause they possessed such characters as qualified them 
for such special functions. Hence we may say that the sculp- 
tured markings on the scales of those reptiles which cast their 
skins are no longer to be designated as morphological characters, 
since it has been shown that they originate by the transforma- 
tion of parts which have a determinate, highly specialised and 
indispensable, or at any rate most useful function to perform in 
the life of thé animal. 

This one example may, and in this place must, suffice to 
show that we need not abandon the hope of explaining mor- 
phological characters on Darwinian principles, though their 
nature is no doubt often difficult to understand. If it be 
granted that it is possible—or if we are at any rate allowed to 
attempt—to show, that in fact all those hitherto inexplicable and 
so-called morphological characters have stilla determinate func- 
tion, or have at any time had one, and may be regarded as true 
or as rudimentary organs which were enabled by their living 
elements to undergo further transformations and changes of 
function—if so much as this is hypothetically granted, the 
direction of our researches is clearly pointed out, and we are 
justified in prosecuting them. For since we consider ai the 
parts of the animal body as true organs, and see that the sum 
total of their functional activity determines the vital fitness of 
the species, we perceive that it is the task of the zoologist to 
enquire how the conditions of life must act upon individual 
animals and their organs, in order to be able to deduce our 
inferences as to the physiological causes of the origin of 
different animal forms. We shall not, however, follow the 
morphologist, who seeks to trace the affinities which must exist 
between all living animals by investigating and comparing the 
-forms and organs of living and extinct animals as well as the 


24 INTRODUCTION. 


processes of their development from the egg. For although the 
morphological section of animal biology teaches with much 
probability that this species or that organ has undergone this or 
that course of modification in the animal series, and that 
in the process of modification it has passed through a whole 
series of various forms, still it is only physiological research 
that can elucidate the necessity for their existence by revealing 
their causative conditions. 


SECTION I. 
GENERAL PRELIMINARY CONSIDERATIONS, 


CHAPTER I. 


THE PHYSIOLOGY OF ORGANISMS. 


Tue general direction of zoology is, as we have seen in the 
introduction, determined by two branches of science—Morpho- 
logy and Physiology. Although both make it their task to 
learn to understand the phenomena presented to us by the 
animal kingdom, they are so widely different, both as to their 
details and as to the paths they have struck out for solving the 
problems, that we are fully justified in keeping them separate 
as two independent branches of science. 

The problem for Morphology is to discover those affinities 
of relationship in animals which actually exist, and to found on 
them a natural system of the animal kingdom. It attains this 
end, or endeavours to attain it, by investigating morphological 
differences, as well as those similarities which indicate a true 
affinity, by means of the comparative method—comparative 
anatomy and embryology. Physiology, on the other hand, does 
not seek to establish those affinities, but, on the contrary, to 
investigate those universal conditions of existence and those 
functions of living organisms which may elucidate from the 
point of view of the laws of causation, among other things, the 
natural system arrived at from morphology. Morphology, indeed, 
only establishes the relations of affinity between individual 


26 GENERAL PRELIMINARY CONSIDERATIONS. 


species ; if it ever should succeed in finding a truly natural 
system corresponding to these relations, this in itself would be 
the best morphological evidence of the accuracy of one of the 
principal propositions of the Darwinian theory, 1c. of the 
genealogical relationship of all animals. But Physiology, 
taking this result for granted as a fact, endeavours to explain 
it by revealing its physiological necessity, 7.e. its dependence 
on external and internal causes whose united action has, 
slowly or rapidly, caused the transmutation of one animal form 
into another. 

It will be advisable to illustrate this proposition by shortly 
discussing an example. Morphology teaches us that two pairs 
of organs of locomotion—limbs—are a marked characteristic of 
the Vertebrata, and that two pairs only do not occur in any other 
animal group. Moreover, we have learned that these two pairs 
of extremities must have possessed the highest degree of plas- 
ticity, since they are found, throughout the vertebrate series, of 
the utmost variety of form and structure; while at the same 
time their variations are so characteristic that they furnish us 
with an easy means of tracing even very close relations of 
affinity between different animals. But Physiology has hitherto 
been wholly unable to detect the causes which led to the 
development of only two pairs of limbs in the Vertebrata; since 
no self-evident usefulness can be directly ascribed to the exact 
number of four organs of locomotion, and it is undeniable that 
many vertebrate animals could move just as well with six or 
more legs as with four; there are fishes, too, as the Eel, which 
are wholly devoid of them, and yct move forward with great 
rapidity by a wriggling motion of the body ; and Snakes, which 
have not four legs either, run, as is well known, with extreme 
rapidity on the points of their numerous ribs. To find a reason 
for the prevalence of four limbs in the Vertebrata, and at the 
same time the cause of their origin, is precisely a problem for 
Physiology. Even if an invertebrate animal were to be found, 
which, on general grounds, might be regarded as the nearest 
invertebrate ally of the Vertebrata, and which, moreover, in its 
larva or embryo stage exhibited organs which in position and 
structure might be regardcil as comparable with the simplest of 


AIMS OF PHYSIOLOGY. 27 


the four typical organs of locomotion of the Vertebrata, such a 
‘discovery would indeed be hailed with delight by the morpho- 
logist.6 But the physiological problem would remain unsolved ; 
it would merely be transferred from the Vertebrata as a class to 
that group to which this hypothetical animal might belong. From 
the most general point of view the purely physiological problem 
is, to say the least, of just as much importance as the morpho- 
logical. 

After this illustration we may set morphology wholly aside, 
and pass on to a general preliminary consideration of the sub- 
ject of the present volume, #.e. the ‘General Physiology of the 
Animal Kingdom,’ or, as I first named it, the Physiology of 
Animal Organisms—a title by which I intended to convey a 
certain opposition or contradistinction of the subject of which 
it treats to the general conception of Animal or Human Phy- 
siology. 

Everyone is aware that the science which is usually known 
simply as Physiology endeavours almost exclusively to explain 
the functions of different organs, and is not unfrequently con- 
fined within even narrower limits, in accordance with the 
assertion of a well-known German physiologist, that this science 
is useful only or principally in practical medicine, and must be 
regarded as subservient to it. This familiar form of physiology 
is merely the physiology of the organs ; its aim is to verify the 
laws by which the organs of sense—such as the brain—the 
muscles, stomach, heart, spinal cord, lungs, kidneys—in short, 
each of the various organs—exercise their functions. I am, I 
need not say, far from disputing the immense value of this 
branch of study, or even from thinking that its results can ever 
be disregarded by the zoologist. J nevertheless must maintain 
that another field, as yet almost unworked, lies open to physio- 
logical enquiry—nay, more, that organic physiology has not 
afforded such assistance to zoology as it might have done if it 
had been less exclusively forced into the service of practical 
medicine. An immense number of questions bearing the 
highest general scientific importance lie open to physiological 
enquiry in the vast number of different species of animals ; but 
they are never, or but rarely, answered or even worked out, for 


28 GENERAL PRELIMINARY CONSIDERATIONS. 


they cannot be solved by the few animal forms on which 
physiology is wont to make her experiments. It is certainly 
no exaggeration to say that not more than six or eight—at most 
twelve—kinds of animals from among the many thousand 
existing forms, have hitherto been investigated by physiology 
as it is understood in our universities. 

But even supposing that the laws of organic physiology had 
been deduced from the investigation of the greater number of 
living animals instead of merely a few, it still could not avail to 
answer the questions which arise from the reciprocal relations 
of animals, and which bear upon the external conditions of 
existence. But these, above all others, are those that claim our 
interest, if our point is to establish the most universal laws of 
the development of organisms and of the transmutation of one 
form into others. 

In order to set this in a clear light, I think it will be 
advisable to compare, in the most general manner, two groups 
of facts which apparently have no common point of coincidence 
—the geographical distribution of animals, and the normal 
arrangement and functions of organs in the individual animal. 

All the organs of an animal are in co-ordination, physio- 
logically as well as morphologically. Although the liver and 
blood-vessels, brain and muscles, and all the organs appear to 
act independently of each other, they are so absolutely depen- 
dent on each other that they are wholly incapable of doing 
their duty as soon as their relations, sometimes very remote, 
to the other organs are interrupted. Thus the muscles of the 
arm, though the arm itself were uninjured, would cease to act 
with any purpose if they were made independent of a healthy 
will; and this again depends on the normal activity of our 
vascular system, for if the blood-vessels of the brain are exces- 
sively or insufficiently supplied the functional activity of the 
will must suffer. And every separate organ is in the same 
way influenced, and its activity determined, by others, or by all 
the rest. If one organ is in any degree changed, every other 
will be affected and changed more or less. 

The same law applies in a certain measure to the present 
distribution of animals on the surface of our globe. It is, no 


INTERDEPENDENCE OF ANIMALS. 29 


doubt, evident that the animals now inhabiting Australia are 
so widely separated from those of England that, irrespective 
of other circumstances, it would be quite impossible for them 
to have any influence on each other. But if we turn our 
attention to a defined region, such as North or South America, 
where the most widely different animals live in the most 
intimate and ever varying contiguity to each other and to the 
plants which occur there, the case is altogether different. If 
the American prairies were to cease to produce grass, the first 
result would be the rapid and utter extinction of the now 
numerous herds of buffaloe ', and on their existence depends that 
of the surviving remnant of the ancient Indian population of 
America. If the various insectivorous birds of North America 
were exterminated, within a very few years beyond a doubt all the 
produce of the rich agricultural districts of that continent would 
be destroyed. If we change the mode of life of any single 
animal, the change will instantly have an influence on all the 
other animals whose healthy existence was in any way depen- 
dent on its normal function before it was altered. Although 
it is certainly true that the various animals inhabiting a country 
are not so intimately interdependent as the organs of the 
individual, the relations in the two cases may be very directly 
compared. The normal numerical proportion, mode of life, and 
distribution of animals would be altered or destroyed by the 
extermination of one single animal, just as the whole body 
suffers, with all its organs, if only one of them is destroyed or 
injured, And in both cases nature has analogous remedies at 
her command. In the one case the function of the incapacitated 
organ can be assumed, at any rate to a certain extent, by some 
other uninjured organ, exactly as, in the other case, the function 
of the exterminated animal may be fulfilled with regard to the 
whole fauna of the country by some other animal. But a 
perfect compensation for the loss sustained is impossible in 
either case. 

This parallel between an individual organism and the con- 
ditions of distribution at present existing may be carried out 
with reference to the purely morphological relations. 

Every animal body is constructed to a certain extent in 


30 GENERAL PRELIMINARY CONSIDERATIONS, 


accordance with a determined type, and every individual of a 
species repeats the organisation of its parents without any 
considerable or abnormal deviation from it. And it is well 
known that this is the case not only with reference to the mere 
existence or reproduction of the same organs, but with regard, 


Section of the young embryo, 


from Kiulliker. y 
ch the organs originate by gradually in- 


showing the first (germinal) layer from whi 


Fig. 11.—Germinal layers of the Chick, 
creasing differentiation. 


above all, to their position. This topography of the organs of 
an animal is, to say the least, of quite equal significance with 
the existence of the individual organs; so much so that the 
development of our modern views as to the embryology of 
animals—the history of their individual growth—rests essen- 


STRATIFICATION IN STRUCTURE. 31 


tially on the axiom that the topographical relations of the in- 
dividual organs must always be of the same general type, not- 
withstanding the ulmost variety in the forms of the organs 
themselves or of the animals to which they belong, so long as 
these are inciuded in the same systematic group. Thus, for 
instance, a vertebrate animal with a brain in the foot, as some- 
times occurs in Mollusca, or with an ear in its tail, asin cer- 
tain Crustaceans, is simply an impossibility ; among the Ver- 
tebrata these organs are always necessarily located in the skull, 
or brain-capsule, which is invariably in the head ; every organ, 
even the most insignificant, as it might appear, has its deter- 
mined position from which it is but rarely displaced. This 
topography or distribution of the organs is indicated and recog- 
nisable at a very early period in the life of the individual. Be- 
fore any kind of organ is constructed and adapted to any 
determined use, two or three embryonic layers or strata of 
cells are formed (fig. 11), known to embryologists as the 
germinal layers. Each of these is gradually formed or differ- 
entiated into certain organs. Thus, for instance, it is now 
ascertained that the central nerve-system originates in almost 
all animals directly from the outer layer, the so-called ‘ Ectoderm,’ 
of the embryo; and we know also that the eyes and ears are 
formed in the same way from this outer germinal layer, and at 
the same time from the central nervous system, which has been 
already separated from it. The same obtains of all the other 
organs, of which some always originate from the inner germinal 
layer, others again as invariably from the median layer. This 
stratification of the body, which is, on the whole, tolerably 
uniform in all animals, and the early appearance of the three 
principal germinal layers, can no longer be doubted. 

If we now look at a map of the world on which the dis- 
tribution of the fauna into districts is indicated by different 
colours, and compare this with the lists of Birds, Mammal'a, 
or Reptiles which usually accompany such a map, we perceive 
that a great number of species, genera, and even families occur 
in only one district and not at all in the others. This matter 
has lately been admirably treated by the distinguished English 
naturalist Wallace, whose work on the geographical distribution 


32 GENERAL PREL!MINARY CONSIDERATIONS. 


of animals will long be the chief book of reference for all who 
take an interest in the subject. I would refer the reader to 
Wallace’s book, and think I may therefore fairly refrain from 
adducing numerous instances to prove that in the present 
distribution of animals on the globe an equally sharp demar- 
cation of the stratification of the groups of animals can be dis- 
cerned as of the topography of the organs in an individual 
organism. No one will expect to find living Marsupials in 
England or Germany, or buffaloes, stags, and other Ruminants 
in Australia. The reader who is acquainted with Wallace’s 
work on the contrast between the faunas of the Australian 
and Indo-Malayan provinces will not have been surprised—on 
the contrary will have felt a certain satisfaction—at hearing 
that an animal has lately been discovered in New Guinea be- 
longing to the group of the Monotremata and nearly allied to 
the Ornithorhynchus. 

If we could describe the embryology of these groups of fauna 
—1i.e. if we were in a position to represent the development of 
each distinct fauna from the earliest geological periods down 
to our own time with anything approaching to the same com- 
pleteness as we can now attain with regard to the embryology 
of many animals—we should undoubtedly be able to trace the 
present distribution of animals up to very remote geological 
periods. The brilliant results, far transcending all that had 
been previously achieved, of the paleontological investigations 
in North America by the American naturalists, who annually 
risk their lives to obtain scientific results, will undoubtedly 
ere long put them in a position to give us an almost complete 
history of the secular evolution of the North American fauna ; 
and I am convinced that tho elucidation of this branch of 
natural history will not only reveal many more important inter- 
mediate forms between different vertebrate types than are now 
known, but also that the main features of the present distri- 
bution of the North American Vertebrata will be recognisable 
in that of the fossil fauna, precisely as the general arrangement 
of the organs, in which the most dissimilar animals agree, is 
indicated from the first in the primary germinal layers. 

Enough has been said, I hope, to prove that we are justified 


STRATIFICATION IN TIME. 33 


in comparing the organs of the individual, their action and 
their distribution, with the different species of animals, and 
their present distribution and functions on the globe. The 
fauna of a district thus takes the aspect of a vast organism 
whose separate members—the different species of animals—are 
living parts of the body, and which has had too its embryology, 
i.e. its development in time. These species, as regards the 
laws of their local distribution, may be regarded morphologically 
as the limbs of a gigantic organism which throws one or another 
of them up into the air on to the top of some mountain peak, 
while others are flung into ocean depths, subterranean caves, 
lakes, or rivers. But they may also be studied physiologically, 
and compared to organs which by their functions and import- 
ance influence the life of the whole mass, and are interdepen- 
dent by the most various physiological relations, like the organs 
of a healthy living organism. 

It is in agreement with these arguments that we apply 
the expression ‘ Universal Physiology,’ or ‘the Physiology of 
Organisms,’ in contradistinction to the Physiology of Organs, 
to that branch of animal biology which regards the species 
of animals as actualities and investigates the reciprocal rela- 
tions which adjust the balance between the existence of any 
species and the natural, external conditions of its existence, 
in the widest sense of the term. Each separate organ, even 
when influenced by other parts of the body, must exercise its 
own specific function. The sum, or, to speak more accurately, 
the resultant, of all the forces simultaneously at work in an 
individual, constitutes its individual life ; and ite general well- 
being and its capability for maintaining the place it has once 
acquired in the struggle for existence are the result of a 
combination of the numerous different and often antagonistic 
functions of the individual organs. This is equally true of 
species, if we regard them as members of a single vast organ- 
ism ; and this organism can only maintain its place in existence 
—the distribution, that is, of its members on the surface of the 
earth—by the efficient action of its functions, t.e. by the reci- 
procal activity of the species which constitute it, sometimes 


3 


3-4 GENERAL PRELIMINARY CONSIDERATIONS. 


co-operative and sometimes antagonistic, sometimes under the 
influence of the external conditions of life and sometimes opposed 
to them. 

Under the term ‘ Physiology of Organisms’ we may there- 
fore comprehend all those laws which ave known to us from the 
investigation of the relations of various species to each other 
and to those conditions of life which maintain, destroy, or 
modify their existence as species. Special physiology, on the 
other hand—which in its present stage is often termed simply 
human physiology—or, more accurately, the ‘Physiology of 
organs, includes all those facts and laws which refer chiefly 
or exclusively to the specific action or re-action of the organs 
of individuals. 

The subject-matter of the following chapters has now been 
exactly enough defined. We shall leave the laws of the re- 
lations of affinity as revealed by morphology entirely out of 
the question, accepting them as they stand, without criticism. 
In the province of physiology we shall in the same way dis- 
regard, as far as possible, the physiology of organs; for, at any 
rate in the first instance, it is of no importance to our more 
genera] problem that the use of each organ should be deter- 
mined. The interest of these specific enquiries extends only 
co'far as they may be of value in determining fora species its 
capability of existence as such. At the same time it must 
never be forgotten that the results of the more general enquiry 
can never contradict the really well-founded facts and laws of 
special physiology, and we shall consequently be obliged again 
and again to refer to them, especially when a species depends 
principally or exclusively on the healthy and vigorous action of 
its organs for the possibility of ma‘ntaining its place in the 
struggle for existence. 

Before going on to this particular enquiry it seems desirable 
that the expression ‘External Conditions of Existence’ should 
be as accurately defined as may be. I have already said that I 
wish to see as wide an application given to it as possible, so as 
to include every influence, however insignificant and difficult to 
detect, that can affect the ‘fitness for survival’ of a species, and 
to investigate its mode of action. This explanation might 


GENERAL CONDITIONS OF LIFE. 35 


suffice, but I prefer to illustrate my meaning by a few further 
considerations. 

Everything which tends to hinder or to favour the con- 
tinuance of the life of the individual and the propagation of the 
species, as such, must be regarded as a condition of existence 
for that species. In this sense every organism éxisting on the 
face of the globe, as well as every inorganic constituent of the 
earth’s surface and of the atmosphere, is a condition of existence 
for all animals. Their relations to those organic and inorganic 
elements differ only in degree, in being more or less remote. Heat 
or cold, light as well as nourishment, the density of the atmo- 
sphere, the water or the soil in or on which animals pass their 
lives, electricity and the chemical constituents of the media 
surrounding them, whether air or water, the plants or other 
animals with which they live, either in the closest connection 
or in mere association—everything, in short—may and must 
exercise a certain influence on animals, and may be harmful or 
prejudicial to them ; and there is nothing on the face of the earth 
that may not be regarded as an essential condition of existence 
to some species of animal. It is self-evident that the influences 
of these manifold conditions must be in the highest degree 
various. One animal requires a high temperature in order to 
live, another a low one; one form prefers a very damp atmosphere, 
another adry one; many are destined to live always under water 
or in the soil, while quite as many disport themselves in the 
freer medium of the air. If we could suddenly reverse all the 
conditions of existence which are indicated by these modes of 
life, we should annihilate all the animal life on the earth; for 
no fish can swim in the air, no bird can live permanently under 
water, a mole cannot climb, a salamander cannot exist in a 
desert, nor a desert-snail in the virgin forests of the tropics. 
If, on the contrary, we reverse the conditions slowly, but still at 
a perceptible rate, it is probable that most animals would perish 
while a few wouldsurvive. But if we suppose that such changes 
—in the atmosphere, for instance, in the constituents of water or 
of the soil, &c.—were effected so slowly as to be perfectly in- 
appreciable by man, it is highly probable that the number of 
surviving forms would ke very considerable. The influence of the 


36 GENERAL PRELIMINARY CONSIDERATIONS. 


conditions of existence thus changed is sometimes very different 
on nearly allied forms; for instance, one species of Neritina can 
live equally well in fresh, brackish, and sea water, while others 
occur only in one or the other, and cannot survive any diminu- 
tion or increase of the saltness of the water they live in. The 
simple reason of this phenomenon is the fact that the life of an 
animal depends not merely on the influence of the external con- 
ditions, but on the reaction of its own organisation. If we 
transfer a stickleback (Gasterosteus aculeatus) directly from 
fresh to sa]t water, and leave it there for days or weeks, it will 
not perish if it is supplied with sufficient food. But if at the 
same time we place one of the common fresh-water mussels 
(Unio or Anodonta) in sea-water it will soon die, sometimes in 
afew hours. The remarkable difference in the behaviour of 
these two creatures is easily explained by the following hypo- 
thesis : In both animals the salt water is transmitted through 
the skin to the tissues of the body; but this takes place to a 
much greater extent in the mussel than in the fish, and thus 
injures it, while the fish can bear the small quantity of salt it 
has absorbed. If our migratory fishes, as the salmon, had as 
great an affinity for the salt of the sea-water as the mussels 
have, they would soon cease to exist, or would have to become 
adapted to live wholly in fresh water. Thus every change in 
the conditions of existence influences different animals in dif- 
ferent ways. The problem, then, is to investigate more accurately 
these different effects of changed conditions. 

If we suppose that some such secular change in the condi- 
tions has been effected, or that certain animals have in some 
way or other been transferred from their original stations into 
other circumstances, the effects of such a change in either case 
may be the same or quite distinct ; and this in two ways. It 
might occur, in the first place, that, the whole species not hav- 
ing perished under the new conditions of existence, a certain 
kind of selection was made among the survivors, as in the 
above-mentioned instance of the fresh-water mussel and the 
stickleback. This selective power, however, may be exerted 
not merely on a species as a whole, but on the more or less 
dissimilar individuals that compose it, and even on the organs 


SELECTIVE INFLUENCE. 37 


of each individual. An organ exclusively adapted to a certain 
medium, or fitted only for one restricted use, must degenerate 
and at last disappear if it becomes useless by the change of 
conditions, even though the animal itself does not suffer from 
this on the whole. Or, secondly, the animal, though not ex- 
terminated, may be more or less crippled or altered. An organ 
no longer needed for its original purpose may adapt itself to 
the altered circumstances, and alter correspondingly if it con- 
tains within itself, as I have explained above, the elements of 
such a change. Then the influence exerted by the changed 
conditions will be transforming, not selective. 

This last view may seem somewhat bold to those readers 
who know that Darwin, in his theory of selection, has almost 
entirely set aside the direct transforming influence of external 
circumstances. Yet he seems latterly to be disposed to admit 
that he had undervalued the transforming as well as the selective 
influence of external conditions; and it seems to me that his 
objection to the idea of such an influence rested essentially on 
the method of his argument, which seemed indispensable for 
setting his theory of selection and his hypothesis as to the 
transformation of species in a clear light and on a firm footing. 
By a rearrangement of the materials of his argument, however, 
we obtain, as I conceive, convincing proof that external condi- 
tions can exert not only a very powerful selective influence, but 
a transforming one as well, although it must be the more 
limited of the two. We shall presently see that in many indi- 
vidual cases direct effects of this kind have been actually 
observed and perfectly established by a systematic series of ex- 
periments. The discussion of these must naturally be reserved 
for the chapters to which they belong. 

Finally, I have yet a few words to say on another objection 
which has already been frequently made to the view which is 
here brought forward. It is pretty generally supposed—and 
indeed the facts often seem to bear it out §—that those changes 
of organs or of organisms which are brought about by the 
direct influence of any external cause are neither constant nor 
hereditary, so that the varieties that have originated in such a 
manner seem incapable of any share in the process of trans- 


38 GENERAL PRELIMINARY CONSIDERATIONS. 


forming one species into another; for every subsequent change 
in the conditions of existence would give rise to fresh changes, 
of advance or retrogression, so that it would become impossible for 
them to develope any further in one particular direction. Under 
such a theory as this it would evidently be quite superfiuous to 
investigate the influence of outer circumstances on animals and 
on their organs and mode of life. 

But this objection rests, as it seems to me, on the false 
assumption that the external conditions are constantly and 
rapidly altering, so that each variation caused by them is coun- 
teracted at once by some antagonistic external influence. This 
assumption is, as we know, in direct contradiction to the fact 
that the external conditions in reality remain constant through 
extraordinarily long secular periods. Thus the assumption would 
seem well-founded, that animals might be acted upon merely 
by the constant, uniform repetition of certain influences,’ and 
strongly enough affected to become capable of maintaining 
the characters thus acquired, even when the external causes 
which gave rise to them were removed by some fresh change. 
Hence it is impossible in our enquiry to ignore the transform- 
ing influence of the conditions of existence, merely in order to fall 
in with a somewhat commonly accepted dogma; for it is only 
by assuming that such effects are possible, and directing our 
enquiries and experiments accordingly, that we shall be able to 
arrive at any decision on the question whether such trans- 
forming influences have played any part in the development of 
animal types or not. 

After these somewhat long, but indispensable, general con- 
siderations, I must briefly indicate the classification and arrange- 
ment of the material which was best fitted to elucidate the 
study of the action of external conditions on animal life. At 
first sight it might seem that it would be well to distinguish 
the Transforming from the Selective influences. Such a classifi- 
cation would, however, involve us in many inconveniences. In 
the first place the two divisions would be widely different in 
extent ; for while we have no particularly rich store of experi- 
mentally grounded facts, even with regard to the selective 
influences of external conditions, with regard to direct trans- 


DIVISION OF THE SUBJECT. 39 


forming influences we have next to none. I therefore prefer to 
adopt an apparently arbitrary and illogical division, classing the 
external influences as (a) those that belong to inorganic or inani- 
mate nature, and (b) those which are due to living organisms, 
and above all to living animals of other species. To the first class 
naturally belong all the relations which originate in the need 
of animals for inorganic nourishment; and this, though it is 
not unfrequently consumed in the form of living animals, is 
not able to exert its specific influence until they are dead. 

This division is, as I have observed, somewhat illogical. 
But, irrespective of the impossibility, at present, of adopting 
any other, it has this advantage—that it indicates at once the 
fundamental differences between the two groups. The influ- 
ences of the first group may be both selective and transforming, 
while those of the second are exclusively selective. However, 
this division is not altogether sharp and accurate, as will be 
seen. 


40 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


SECTION II 


THE INFLUENCE OF INANIMATE SURROUNDINGS. 


CHAPTER II. 


FOOD AND ITS INFLUENCE. 


The necessity of nourishment.—It is universally known that 
most animals begin their existence as very minute, often indeed 
microscopic, elementary bodies, as eggs which are simply cells 
and usually immeasurably smaller than the parent animal. 
This disparity of size is most marked among the mammalia, the 
most highly developed group of the animal kingdom; the ovum 
cell being always microscopically small, while the animals are 
often of gigantic size. This difference of size shows, without 
any further proof, that in most cases the nutriment present in 
the ovum must receive further additions of organic matter ® to 
enable the animal to acquire its proper size; and as animals 
cannot, like plants, form these matters themselves by the decom- 
position of carbonic acid, they must take it up from external 
sources in the form of ready elaborated organic tissues—which 
is equivalent to saying that animals must derive the organic 
portion of their nutriment from other organisms. The need of 
the growing animal for such organic nourishment is, as we well 
know, very great. 

But this imperative need of constantly adding new supplies 
of organic matter during the period of growth to the nutriment 
which the young animal has derived from the egg is not the 
only cause which obliges it to be always seeking nourishment; 


BALANCE OF FOOD AND LOSS. 41 


there is a second, which in later life is at least equally press- 
ing. If there were no other cause, the animal might cease to 
eat as soon as it had attained its full growth. But everyone 
knows that regular and, in some cases, numerous meals are 
required, and consequently every animal is forced, to the very 
last day of its existence, to seek food, although growth has long 
since ceased. The reason for this is very simple. That sum of 
functional activity which we call life can only be maintained by 
using up the organic matter contained in the tissues of the 
living body. The activity of the muscles and of the brain, the 
sensitiveness of the sense-organs to external impressions, the 
secretion of urine or perspiration, respiration, propagation, and 
the assimilation of food—in short, all the vital processes that are 
carried on in the living individual—are only possible through the 
consumption, or, more correctly, the decomposition, of a corre- 
sponding amount of the organic matters contained in the organs 
that are exercised. The minimum of matter thus destroyed 
may be greater or less in different animals, sometimes even 
inappreciably small; but the loss of even this minimum of 
organic matter must sooner or later endanger the life of the 
animal if it is not soon made good. In order to make it good 
and to be at the same time in a position to carry on uninter- 
ruptedly the normal process of loss of its own tissues by secre- 
tion, the animal must consume nourishment in various pro- 
portions according to its needs. There are apparent exceptions 
to this rule: for instance, the well-known cases of animals— 
Amphibia, Mollusca, and others—which are able to live for years 
without food. I myself kept various species of land-snails for 
years wrapped in paper and quite dry in wooden boxes, and thus 
wholly without food, and many of them are at this day alive 
and active.? The explanation of this striking instance is easily 
found. The amount of nourishment required daily by any 
animal must naturally be equivalent to the organic matter 
which is daily used up in the various organs to keep up the 
vital processes : the more active an animal is, the more food will 
it require. But the vital processes of animals that are as low in 
the scale as the Amphibia or Univalves are extvemely feeble ; 
their respiration, even under the agitating influence of pro- 


42 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


pagation, is not sufficiently energetic to raise the temperature 
of the body perceptibly higher than that of the surrounding 
medium, whether air or water. In such animals the need for 
food may be in fact suspended for a long period, as their 
vital processes can easily be reduced to a minimum without 
endangering life. But, notwithstanding the privation of 
nutrition, a certain consumption of organic constituents, how- 
ever small, must be constantly going on ; for such a consumption 
is inseparable from respiration, and this, even when reduced 
to the lowest point, can never be wholly suspended without 
endangering the life of the animal. Thus, in such cases, the 
cessation of consumption of nutriment in no way proves that 
the animal could have carried on an active life without food, 
but only that its vital activity can be to a certain extent latent 
for a long series of years; still, not for all eternity; on the 
contrary it is perfectly certain that, even in an apparently latent 
life, acertain consumption of organic tissues goes on, since with- 
out it respiration, which is indispensable even when reduced 
to the lowest point, is impossible, and so death must ensue even 
with those animals that have the utmost powers of resistance. 
Thus distinctions can only properly be made between the 
difference in the amount consumed and the greater or less 
resisting power as affected by that difference. Thus, for 
instance, warm-blooded animals generally can scarcely live a 
week without food, while cold-blooded animals can often support 
life for many months without nourishment ; and it is extremely 
interesting to observe that animals so high in the scale as the 
Mammals that hybernate can, in the same way, carry on a 
latent life for months without any nourishment, like Land-snails 
or Amphibia; not only do they not suffer, but they actually 
require this period of negative existence during their winter 
sleep for the maintenance of their normal vitality. For certain 
reasons to be discussed presently, these animals during their 
hybernation have been compared, and apparently with justice, 
to the cold-blooded animals. 

The amount and kind of nourishment —The amount of 
nourishment required within a given time stands, as has 
been observed, in the closest relation to the greater or less func- 


MAXIMUM, MINIMUM, AND OPTIMUM. 43 


tional activity of the individual organs, to the size of the animal, 
and also, as I must now add, to its special adaptation to a certain 
mode of life. It is clear that a large animal must consume 
actually more food than a small one, but with relation to the 
mass of the animal the proportion may be precisely inverse. 
Thus we know that a caterpillar, at the period of its most 
rapid growth, eats a great deal more in proportion than a dog 
or an elephant. The determination of the absolute and relative 
amount of nourishment needed by different kinds of animals 
is extremely difficult and of no importance to the present 
enquiry. It offers, indeed, only two points of more general 
interest, of which one shall at once come under discussion, while 
the other—the relative amount of nourishment required by 
carnivorous and herbivorous animals—will be treated later. 
The amount of daily nourishment needed differs very widely 
in individuals of the same species ; one will eat, another will 
drink, more than others; but they will all be apparently 
equally thriving, excepting in cases of actual over-eating or 
privation, Between these two extremes—which both result in 
death, because the maximum of utilised nutrition is exceeded or 
the minimum is not attained—there is a graduated scale of 
quantities, which are less and less favourable as they approach 
these dangerous extremes. Hence a point must exist between 
the two, which is the most favourable as regards the mass of 
food introduced into the stomach. This may be briefly desig- 
nated as the optimum of food. But this optimum does not 
lie, as it might be supposed that it should, exactly halfway 
between the two extremes, but may lie, according to the 
creature’s needs, nearer to the one or the other. It is, of course, 
of the highest interest to ascertain what the optimum of daily 
nourishment is for different animals, since this must be one of 
the most potent influences which govern the constantly vary- 
ing numbers of species and individuals. Unfortunately no data 
of general value exist on this point.!° We know, with tolerable 
accuracy, the optimum of nutrition for man, for the domestic 
animals, and for those that have been subjected to physiological 
experiment ; also for some others, such as many birds and 
insects, which are of interest to the husbandman. But this 


44 THE INFLUENCE OF INANIMATE SURROUNDINGS, 


knowledge, in itself but small, has been acquired either by 
casual observations or by experiments which relate almost 
exclusively to such animals as are useful or injurious to man, 
and the general iological bearing of these proportions has as 
yet been in no way verified by investigation. Hence I shall 
avoid giving any specific data, and it will suffice to repeat once 
more that every deviation from the optimum of nutrition (as 
to quantity) must be more or less injurious to the creature. 
The quality of the nourishment has, if possible, an even 
greater influence on the life of the individual and consequently 
on the species, and it constitutes one of the most powerful 
influences for adjusting the relations between animals and their 
surrounding circumstances. There is scarcely a constituent of 
the earth’s crust, whether on land or in water—not an animal 
nor a plant, whether living, dead, or even in decomposition— 
which does not afford nourishment to some living animal. Some 
insects live in dried wood, others on living leaves or roots. 
Almost all the species of Holothuria (sea-cucumbers), many 
sea hedgehogs, and one genus of Mollusca (Onchidiwm) swallow 
sand or mud, while neglecting the animals and plants which 
lie close at hand. Parasites suck the b!ood of their host or 
absorb the juices of a particular organ; certain larve of Ascaris 
(Ascaris nigrovenosa, in the frog) consume the organs of their 
own parent ; and human flesh is a tit-bit to some of the human 
race. But in these, as in all other cases, animals require two 
quite different kinds of food; it must be of organic and of in- 
organic origin. If one kind of nutrition is omitted, the other 
kind, exclusively supplied, will no longer have the same favour- 
able effect on growth and the other vital processes that it had 
when duly mixed with the other kind. This fact is universally 
recognised with regard to man and the domestic animals; but 
it obtains throughout the animal kingdom, though it is not in 
all cases so plainly apparent. Thus, for instance, Parasites— 
such as tapeworms, threadworms, é&c.—seem to require one kind 
only of organic food, since they live in certain organs only, 
one species in the liver, another in the intestines, others again 
in the brain (as the worm which gives sheep the staggers) or in 
the eye, the skin, and even in the bones. All these species of 


STIMULANTS IN FOOD. 45 


animals take in only the one form of nourishment which they 
find in those organs in which they take up their residence; and, 
in a certain sense, we are no doubt justified in saying that these 
animals live solely on organic food. But when we remember 
that the fluids which permeate those organs invariably contain 
a larger or smaller quantity of salts in solution, this contradic- 
tion does not seem, accurately speaking, to exist; for it must 
evidently be quite immaterial whether an animal takes up the 
earthy salts and water which are indispensable to its existence 
directly in their original form, as we do, or indirectly in the 
juices of the animals or plants on which they feed ; only in the 
latter case, if the amount of inorganic matter contained in the 
organ is sufficient for its needs, it will require no further addition 
of salts or of water. 

Recent physiology establishes the fact that in man and in 
the few animals physiologically experimented on, the propor- 
tion of inorganic and organic food must always be approxi- 
mately the same, if health is to be maintained unimpaired. 
We know moreover that nourishment, at least for man, is taken 
in combinations in which a conspicuous part is played by 
stimulants, which by their presence excite the glands in the 
mouth, stomach, &c., so that they fulfil their office more effec- 
tually. The most universally used stimulant is salt. We may 
very fairly suppose that a similar proportion between organic 
and inorganic food is necessary to all other animals, and also 
that they need a mixture of innutritious stimulant with their 
food ; for instance, we know that Ruminants are very fond of 
salt. But we have no general and verified data on this sub- 
ject, and tue only theory we can assert with any degree of pro- 
bability is that the stimulants, if any, needed by the lower 
animals must be quite different from those required by man and 
the higher animals. 

Irrespective of salt, these stimulants—or excitants—consist, 
for man, principally of wine, beer, and other alcoholic drinks, of 
coffee, tea, &c., and the various spices. Although we are now 
speaking of them as in contrast to nutritious food, properly 
speaking, because they are not transformed or assimilated into 
living organic tissue, they would seem to be almost indispen- 


46 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


sable aids to the assimilation of the true nutriment. They 
may he compared to the oil needed for the working of every 
machine; this does not add to the effective power of the 
machine, whatever that power may be, and yet it cannot work 
smoothly for any length of time without it. It is in the same 
way that stimulants enable the body to exert its digestive 
powers to the utmost. Can other animals dispense with such 
an ‘oiling of the machine’? This gives rise to the question 
as to what sort of stimulants they need; and to this other 
one: Whether substances which certainly are by no means 
stimulants to man may not prove to be such to the lower 
animals. And finally a third question occurs: Whether other 
influences, irrespective of the actual reception of food into the 
intestinal canal, may not act as powerful stimulants for the 
absorption of true nourishment. This last question may for 
the present be regarded as superfluous, but it seems advisable to 
point out that in Chapter VI., ‘On the Influence of Stagnant 
Water on the Creatures inhabiting it,’ evidence will be adduced 
that in certain of the Mollusca (Zymmea) the assimilation of 
nourishment depends not merely on the food itself, on the 
healthiness of the organs, on the temperature, &ec., but also on 
the influence on the skin of a certain constituert of the water 
at present unknown to us. 

Organs for taking in, preparing, and assimilating the 
food.—Everyone knows that digestive organs of some kind are 
possessed by every sort of animal, and it may be taken for 
granted that the general structure of these organs.and their 
mode of action are generally well known, so that it will be 
superfluous to describe here the endless variety of such parts; 
every text-book of zoology gives ample information on sucli 
points. A general outline of the relations of the parts must, 
however, be briefly given. In the first place, their position, 
invariably within the body, is woithy of remark, since it is 
this which necessitates the presence of other organs which 
have certain auxiliary duties to perform in the service of their 
masters—the stomach and the intestines. The organs for 
taking up food, the mouth, teeth, and, more remotely, the fore 
extremitics or other external parts, are specially adapted to 


MEANS OF TAKING IN FOOD. 47 


secure food, to divide it, and to transmit it to the stomach after 
being well comminuted ; while man also requires the assistance 
of the cook in the preparation of his food. Now although, 
from a physiological point of view, these auxiliary organs are of 
less importance than the digesting intestinal canal, they are of 
the highest interest for us, inasmuch as they involve an endlessly 
varied series of links between animals and the conditions under 


Fic, 12.—Sacculina careini, with the tuft of clinging roots which itinserts into the body 
of its host; 8, its larva (Nauplius) ; c, Zhompsonia globosa (Kossmpnn) 3d, its larva, 
Cypris stage. : 


which they live, in addilion to those which arise from the mere 
quality and quantity of the nourishment required. 

The peculiar mode of taking up nourishment exhibited by 
various Parasites must also be shortly described. As a rule, 
almost without exception, the larvee of parasites swim or move 
freely about in water (leading a very unfitly termed active life). 
During this stage of free locomotion the larve are usually high 
in the scale of structure. The larva of the parasitical Copepoda 
or Cirrhipedia (for instance, of « Sacculina, fig. 12) is known 


48 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


to zoologists as a Nauplius. This animal has a nervous system, 
external organs of locomotion of a complicated character, a mus- 
cular system of the crustacean type, a well-developed intestinal 
canal such as is found in the Nauplius larve of the lower crabs 
that are not parasites, and usually even special organs of sense— 
eyes. Gradually this Nauplius, after attaching itself to the 
gill or skin of a fish, or under the tail of a crab (Sacculina), 
loses its organs of locomotion, the greater part of its muscular 
and nervous system, its organs of sense, nay, often its mouth, 
stomach, and intestinal canal. Thus the lively crab-like larva 
is transformed into a shapeless sac, exhibiting no trace by which 
its crab-like nature can be recognised. Still the creature needs 
a limb by which to cling to the animal that is to be its host 
and provide it with nourishment; peculiar clinging organs are 
developed instead of the lost motory organs (fig. 12, @), and 
these not unfrequently also assume the office of absorbing 
nutrition from the host. Such, for instance, is the case with 
the parasitical crabs, which, like Sacculina (fig. 12), live on the 
abdomen of the hermit crab (Pagurus) or of other crabs. 
They have, without exception, long filamentary processes at the 
fore-end of the body, with which they cling and bore through 
the skin of the crab into its abdominal cavity, and then they 
clasp portions of the crah’s internal organs, particularly the 
liver, in the long entangled filaments. These slender threads 
are thin-coated tubes which open into the body cavity of the 
parasite, so that it is highly probable that these clinging threads 
also serve the purpose of suckers, since they are capable of 
absorbing nourishment ina fluid form through the’r thin tissue; 
at any rate, they do not convey it into an intestinal canal, for 
the parasite has none. This, however, is no argument against 
the assumption that the fluid thus absorbed by endosmosis 
through these roots or suckers serves as food ; for we know that 
in all animals which have a body cavity and dispense with a 
vascular system the food must first pass into the body cavity, 
in order to be conveyed from thence to the organs situated on 
it. So far as regards the part taken by the clinging filaments 
as organs of nutrition to the parasite, it is perfectly indifferent 
whether the nutritious fluids to be assimilated first pass through 


OSMOSIS. 49 


the intestine into the body cavity, or, as in Sacculina, are con- 
veyed to it directly by the suckers. 

We hereby see that the ways and means by which animals 
obtain the food they need are very various ; even mere external 
appendages of the body may, like the roots of Sacculina, be 
transformed into organs of nutrition. But there are other ways 
in which we see an essential difference in the mode of obtaining 
nourishment that characterises different animals. All animals 
that have well-developed internal organs of nutrition are com- 
pelled, under the influence and guidance of their will and their 
subjective sensations of hunger and thirst, to make more or less 
vigorous voluntary efforts to obtain the amount and kind of 
nourishment they require. Even animals of such simple struc- 
ture as the Infusoria obey this law, though their intestine, 
stomach, and mouth do not constitute separate organs, but only 
are portions of their protoplasmic body, and though, like all 
other one-celled animals, they absorb their solid food direct 
into the digesting protoplasm; still they manifest liking and 
aversion for different kinds of food, just as much as the higher 
animals; they never swallow that which they dislike, even 
when by some accident it comes into contact with their mouth 
at the same time as the food they prefer. 

The case is quite different with all those true Parasites which - 
dispense altogether with a true intestinal canal; they can take 
up nourishment only in a fluid state, as their skin alone is 
capable of absorbing it by osmosis, and this is quite independent 
of the will of the creature. Thus the Cestodea, the curious 
parasit.cal snail Entoconcha, several parasitical Crustacea in their 
fully developed state, the parasitical Trematoda (as the liver 
Distoma), and even some insects—all of which absorb their 
nourishment through the skin without having any intestinal 
canal—depend for their life and wellbeing on the osmotic rela- 
tions between their skin and the fluids surrounding them. 
Now, as osmosis through the skin—+.e. the absorption of the 
flu'd nutriment in the surrounding medium—can never, so 
far as we know, be interrupted, all these animals are compelled 
to be incessantly taking up and assimilating food. Thus the 
parasite is quite incapacitated from making any choice in the 


50 TIE INFLUENCE OF INANIMATE SURROUNDINGS. 


nutriment supplied to it, since it is fixed in the skin or in some 
particular organ ; and moreover it must always perish if by any 
circumstance the fluids which bathe its skin are so far altered 
as to cease to be fit to nourish it. 

From these circumstances it will be at once understood that 
there is a fundamental difference between true nutritive matters 
and certain other substances. If indeed, as is sometimes done, 
we choose to call everything food which may be in any way 
concerned in digestion and the absorption of the gastric juice or 
a partial conversion into organic substances, heat, and motion— 
irrespective of how or through what means the matters were 
conveyed into the organism, and so rendered efficient—we shall 
be forced to apply the term not merely to oxygen and ozone, 
which are taken into the body by respiration, or water and 
salts, which are introduced in the most various modes—often 
through the skin—but to all the other influences which are in- 
dispensable to the life and growth of every individual. Nay, 
even the sunbeams with their waves of heat and chemical light 
must be included, for without their aid the stomach and in- 
testines could not fulfil their functions, any more than the gills 
or lungs, the brain or the organs of sense could carry on theirs 
without healthy nutrition through the intestine. Hence we 
are justified, while investigating the effects of nutrition on the 
animal organism, in directing our attention solely to those in- 
ternal organs of digestion which demand the collaboration of 
external auxiliaries, and in leaving absorption by the skin quite 
out of the question; for although this process, as regards its 
effect on the life of the individual, acts precisely like the true 
nutritive function, it induces no other connection with the ex- 
ternal conditions of existence than those which subsist in all 
animals through the skin and its relations to the medium 
surrounding it, whether air or water, &c. These relations 
never demand any special auxiliaries that depend on will, in- 
clination, or disinciination, since the efficient action of the skin, 
in all such cases, depends merely on the molecular relations 
between it and the fluid matter with which it is in contact."! 

The results we have so far arrived at may be thus shortly 
recapitulated. We have seen that food, in the strict sense, 


RATIO OF CARNIVORA TO HERBIVORA. 51 


gives rise to various relations between the animals and their 
surroundings in the following manner. All animals need a 
certain optimum of food; being compelled to take organic as 
well as inorganic food, they are dependent on plants, which 
alone are able to form organic compounds by the decomposition 
of carbonic acid; both the quality and quantity of the food 
lead to a vast number of very various relations between the 
animals and inorganic nature on one hand, and living beings on 
the other; finally, the organs which are auxiliary to the acqui- 
sition of food are in direct connection with the animal’s mode 
of life, 

Every modification of these relations once established must 
necessarily exercise an influence on the animals in contact with 
it, and in this case, as in all others, this influence may be two- 
fold: selective or transforming. The great variety, as has been 
briefly indicated, of these conditions and relations requires us to 
discuss a few cases of more conspicuous interest, in order to un- 
derstand how far food does in fact exert a direct or sli 
influence on different animal forms. 

Monophagous and polyphagous animals.—Any division of 
animals into two such groups as are here indicated has obviously 
none buta purely physiological value, nor is ita thoroughly com- 
prehensive one, as we shall immediately see; although a very 
conspicuous contrast exists, and has a certain value, between 
Monophagous animals—consuming, that is to say, only one-kind 
of food—and Polyphagous creatures, which eat a variety of 
food or even anything that comes in their way. 

If we confine our attention to the distinction between the 
two kinds of food, vegetable and animal, we may regard all 
purely carnivorous or purely herbivorous animals as monopha- 
gous; but within each of these groups there are animals that 
are monophagous in the strictest sense of the word, several 
species being fitted in fact to feed on one kind only of organic 
food. A closer enquiry into the conditions resulting from this 
will be of interest. 

In the first place, it is clear that a certain interdependence 
between flesh- and plant-eating animals must exist, and find 
its expression in the proportional numbers of individuals of 


52 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


the two groups generally distributed over the face of the earth. 
We know that its surface—dry land as well as land covered 
with water—is capable of producing only a certain limited 
number of plants, depending on the conditions of the locality. 
Assuming then that a given number of plants—the maximum 
number being present at the time—offered, let us say, a thou- 
sand units of food to these two classes of animals, the car- 
nivorous and herbivorous species would not be able to have an 
equal share of the space and of the food it would afford. The 
flesh-eaters would only obtain food from the soil indirectly 
through the plant-eaters. Now the transmutation of the nutri- 
ment derived from the plants into the flesh of the plant-eaters 
is inseparable from a certain loss in the whole mass, since the 
oxidation of a certain amount of the organic constituents is 
necessary for the production of animal heat and for the move- 
ment and due use of all the functions of the body. Now we will 
assume—quite arbitrarily—that the proportion of the whole 
mass of plants produced by the soil is to the animals which can 
subsist on them—converting them into animal tissue—as ten to 
one; then, in the area we have assumed, only 100 units of 
feeders—individual Herbivorous animals—can live on 1,000 
units of plant food. The maximum of nourishment, then, which 
exists for monophagous carnivorous animals, can amount only to 
100 units. In the transmutation of these 100 units of food in 
the organs of the Carnivora 2 considerable loss will be incurred; 
organic matter will be consumed, the indigestible portions, as 
hairs, hoofs, and horns, will be ejected, and if the proportions 
were such that ten units of animal food could suffice only for 
one unit of the animal body, the maximum of food as supplied 
by 100 herbivorous animals would enable 10 carnivora at most 
to exist. Thus the same area can never produce and maintain 
so large a number of carnivorous as of herbivorous animals, an 
inference which is perfectly confirmed by the facts. It is well 
known that the number of Herbivora is much greater than 
that of Carnivora; and in connection with this fact is this 
other, that among the Vertebrata, those at any rate that com- 
monly live in large herds, are vegetable feeders, while the indi- 
vidual Carnivora, which are on the whole much less numerous, 


LIFE AT GREAT DEPTHS. 53 


display a much greater disposition to separate themselves into 
small families. Thus the number of individuals of the mono- 
phagous animals depends in a great degree on the nature of 
their food; and even the most primitive habit of life, ze. the 
instinct of living apart from their fellows, or of living associated 
in large herds, is very decidedly influenced by it, if not actually 
produced by it. 

The dependence of the Carnivora on the Herbivora thus 
clearly indicated, leads to another question—that, namely, as to 
the possibility of animal life existing where no plants can grow, 
and where consequently no vegetable feeders can live. We 
know now that, contrary to the opinion which for a time 
prevailed that the bottom of the sea was uninhabitable, a 
considerable number of the most various creatures live in spots 
where the sun’s rays never penetrate, and where, therefore, no 
plant can grow. According to Forel, plants containing chloro- 
phyll cease to be found in the Lake of Geneva at about one 
hundred fathoms, and the limit in the sea seems to be about 
the same. Nevertheless, in the Lake of Geneva, which is much 
more than one hundred fathoms deep, and everywhere on the 
floor of the deepest Atlantic, we find a multitude of living 
animals. These, at such great depths, cannot feed on living 
plants; they must all be flesh-eaters, as has been confirmed by 
observation. But as they cannot form organic matter from 
carbonic acid, water, and ammonia, they must soon infallibly 
perish if no substitute were provided for the animals destroyed 
for food. Hence we may be allowed to assume that the organic 
food found in the plants at the surface of the sea is in some way 
conveyed to some of them. Professor Mobius of Kiel has 
lately undertaken the investigation of this problem. He came 
to the conclusion that the organic matter produced at the 
surface of the sea by the decomposition of plants and animals is 
carried down to the bottom by the Sinking Current, as it is called, 
which results from the difference of temperature at the bottom 
and at the surface ; this theory, however, cannot be regarded as 
proved by the experiment which Mébius made for that purpose. 
In small aquaria which were perfectly protected from any 
shock, variation in temperature produced sinking currents 


54 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


which sufficed to level gradually, but completely, the uneven 
mud at the bottom of the aquarium. Now it is true that we 
know that many currents occur in the ocean which reach to 
the bottom, and which, as they take their rise at the surface in 
remote localities, seem specially adapted to convey food from 
above to the deep-sea creatures. But Mobius himself points 
out that one assumption is still unproved by this—namely, that 
the organic portions of the plants and animals that fall from 
the surface must remain undecomposed in their journey if they 
are actually to serve as food for the animals there. Whether 
this is the case we do not know, and this really ingenious theory 
remains for the present unproved, and may perhaps be ere long 
replaced by another not less plausible. 

We will now enquire more closely into a few striking 
phenomena connected with the monophagous habits of certain 
animals, and endeavour to demonstrate, by the discussion of a 
few examples, the extremely diverse conditions which appear to 
be produced by the adaptation of various animals to one single 
kind of food. It is clear that an animal to whose existence one 
particular sort of food is indispensable must be the slave of 
that plant or animal which alone can supply it; such a mono. 
phagous creature must consequently, in many cases, be adapted 
to the same mode of life as the organism on which it lives. 
Many birds live, as is well known, exclusively on hard seeds. 
Now, as the beak of a bird is but rarely adapted to crush such 
seeds or grains, the grain-eating birds must possess another 
organ with which to reduce them. This organ is what is 
known as the gizzard, This has on its inner side a very thick, 
hard, brown skin, which is admirably suited to triturate the 
hard grains by the aid of the grains of sand and small pebbles 
which are swallowed at the same time, and to protect the softer 
portions of the stomach against the ill effects of the sand and 
stones. Thus we here find a peculiarity which enables its pos- 
sessor to avail itself of a particular supply of food, which the birds 
of prey with their soft stomachs are unable to take advantage of. 
A still more striking illustration of the fact that such organs, 
calculated for a single kind of food, sometimes appear under 
very unexpected aspects, such as by all school theories would be 


MODIFYING EFFECTS OF FOOD. 55 


considered impossible, is afforded by the genus Dasypeltis among . 
the snakes; the species occur in Africa and live on birds’ eggs. 
They swallow their food—the eggs—as other snakes swallow 
frogs and fishes, snails or mammals. But the nourishment 
contained in the egg is enclosed in a calcareous envelope— 
the egg-shell ; if the snake, in order to get at the contents of 
the ege, were able to crush it by its teeth and jaws, it would 
certainly lose the greater part of the fluid contents. The only 
way to lose nothing of it, therefore, is to swallow the egg 
whole; and in point of fact it does reach the stomach un- 


Daeg 


Fic, 13.—Section of the cesophagus and stomach of the Pigeon. dr, the glands; 4, the 
gizzard, enclosed in a thick brown skin. 
broken. But here organs have developed in a most marvellous 
way, which, in all other cases without exception, are confined to 
the bones of the mouth, namely, teeth. These occur firmly set 
on the lower side of the vertebre and in the forepart of the 
stomach, and their points pierce through the coat of the 
stomach so far that they seem to be purposely fitted for break- 
ing the eggs passing through it ; in fact they must work in this 
way, for they are the only part of the stomach strong enough to 
be able to answer this purpose. This is, as has been said, the 
only instance of true teeth, acting as such, occurring in any 
other situation than on the bones which surround the cavity of 


56 TIE INFLUENCE OF INANIMATE SURROUNDINGS. 


the mouth; but it is an exception which, as we see, is due to 
the propensity of the species of Dasypeltis to make the eggs of 
birds their exclusive food. 

These few examples, to which others could easily be added, 
for many are universally known, will here suffice to prove 
that monophagy in animals is often connected with the 
occurrence of special organs or relations of structure, and that 
the preservation of such species is solely due to their efficiency. 
Their inefficient development would infallibly lead to the 
destruction of the species—taking it for granted, of course, 
that it was unable to accustom itself to any other food. 

Sometimes adaptation to a single kind of nourishment does 
not depend, as in the cases here considered, on the existence of 
a special organ, but on a peculiar cycle of development in each 
individual animal. This, for instance, is the case with all the 
Intestinal Worms. These must become extinct if their larve 
were not able, or even forced, to migrate and to seek food in 
other spots away from their parents. If we suppose that the 
Tapeworm, or even the Trichina, were capable of going through 
the whole cycle of its development within the same host, its 
permanence as a species would be possible only if all men were 
habitually cannibals. Corresponding to this we find that all 
Intestinal Worms have to go through a longer or shorter 
period of migratory existence as young and sexless creatures 
or as larve. They at the same time change their host several 
times—for they often become parasites from the first, after a 
short period of free life in the water—till at length they are 
sexually mature, and have found their way into an animal or 
an organ similar to that which they left in the embryo or larva 
state. All internal parasites are subject to this inevitable law 
of migration—such, that is to say, as live in the interior of an 
animal structure or of its organs. It is applicable even to the 
well-known Trichina spiralis, which is capable of going through 
all the stages of its development in the same animal, but which 
nevertheless travels, in its youth, from the intestine outwards to 
the muscles. From them, however, it is incapable of returning 
to the same intestine, although it would be perfectly capable of 
achieving sexual development there and of producing eggs ; it 


MIGRATION. 57 


must absolutely pass out from the muscles to the intestine of 
some other creature—a rat, a mouse, &c.—in order to pass, in 
the second generation, back again into the human intestine. If 
the Trichina from the muscle does not pass again into the 
intestine of some other creatwre—which is, of course, commonly 
the case as regards human beings—it infallibly dies, although its 
tenacity of life is enormous; such a Trichina can live for ten 
years enclosed in a muscle. Here the permanence of the species, 
as such, depends on the capability of the larve for migration, 
and for finding their nutriment in other animals which may 
secure their transfer into those in which alone they can find 
the special food that is necessary for their full development, 
and for the exercise of their sexual functions. Ifa young para- 
site were to lose its way, or to be swallowed by an unsuitable 
host—a Trichina, for instance, by a Fish—it would infallibly 
perish unless it were able to accustom itself quickly to the 
food which is unsuited to it. No such cases, however, are 
known of adaptation of parasitical worms, when sexually 
mature, to an unwonted form of nutriment. Thus it would 
appear asif in these cases, without exception, the change of food 
involved in the migration of the young animal were of the 
same service to the species as are special organs contrived for 
special nutrition in others; the preservation of the species in 
the former class depends on change of food and migration 
just as much as, in the latter, it depends on the adaptation and 
functional activity of individual organs. 

A similar dependence of the species on its food does not of 
course exist among the truly Polyphagous Animals. Their 
polyphagous habits allow of their changing their food at 
pleasure without suffering in any way, or at any rate seriously, 
when, from any external cause, they are obliged to alter their 
mode of life. Itmust not however be forgotten that even these 
animals depend toa certain, if not to a very great degree, on the 
nature of their food. It is now universally admitted that in 
many animals a definite relation must subsist between the 
amount and kind of food if the animal is to derive the greatest 
possible advantage from the food consumed. Man in this 
respect offers the best known instance. Starchy food or sugar, 


4 


58 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


fat or meat, salt, water, and stimulants must be obtainable in 
certain proportions—which may be designated as the optimum 
of nutrition—if they are actually to produce all the effects 
proper to themselves and beneficial to the human organism. 

We are certainly justified in supposing that similar relations 
exist between the various constituents of the food of the other 
polyphagous animals. But we know nothing or very little on 
this subject, although it would be very interesting to learn 
whether similar relations in the admixture of these constituents 
subsist for the lower animals as for man; or, on the other hand, 
quite different ones—for low forms of polyphagous animals, for 
instance, as Insects, Crustaceans, and Molluscs. At present, 
therefore, an enumeration of polyphagous animals has no 
interest, since we cannot learn from it anything as to the 
dependence of the polyphagous animals on any definite mixture 
of food, or as to their absolute independence of it. 

Many cases of polyphagy are of the highest interest as con- 
sidered from another point of view. In connecting and compar- 
ing the physiological activity of an animal with its position in 
the general system, we might perhaps expect to find that all the 
species of a genus, and still more all the individuals of a species, 
would be equally dependent on the same mixture of food; and 
we should be particularly inclined to this assumption in all those 
cases in which, as we know, the consumption of food directly 
depends on the presence of one particular organ of definite 
structure and action. Such a conclusion would nevertheless be 
wholly unjustified. We will for the present postpone the ques- 
tion as to how far different individuals of the same species may 
be capable of varying their nutrition, and will here only investi- 
gate those cases which show that many polyphagous species are 
found in genera which otherwise contain none but monophagous 
Carnivora or Herbivora. 

The greater number of Parrots are, as is well known, vege- 
table feeders—live, that is to say, on grains and fruits. Many, 
however, eat insects eagerly, and even meat; and it seems to be 
a tolerably general custom in zoological gardens to add a 
certain proportion of fat to the vegetable food of the larger 
parrots. The Lizards of the Hastern hemisphere are, almost with- 


CHANGE OF FOOD. 59 


out exception, carnivorous; those of the Western, on the contrary, 
chiefly herbivorous. But among the former there are certain 
species—Lacerta agilis, L. muralis, and others—which some- 
times, like dogs, eat grass and even fruits. On the Balearic 
Island of Ayre, close to Majorca and Minorca, lives an entirely 
blue-black variety of Z. muralis, which I myself found there. 
The island is very barren ; only low shrubs grow on the stony 
soil, and during the dry months, from June till October, not 
even burrowing insects are to be found. During this period the 
lizards feed on plants, and above all on the fruit which is brought 
in by the inhabitants. I have been able to keep numerous speci- 
mens which I brought away with me, for months together, even 
during our northern winter, on sweet fruits, juicy or softened 
by soaking. Now in all text-books of zoology it is stated that 
the lizards of the Old World are distinguished by having teeth 
connate with the jaw, while the vegetable-eating lizards of the 
West have teeth which grow in sockets in the jaw. The facts 
above given suffice to show that this parallel between the 
nutrition and the animal’s place in a system—such as seems to 
be indicated by the teeth—is in fact defective in individual cases, 
and we may even hazard a suspicion that it may in great part 
depend only on insufficient observations of the habits of lizards 
on the part of zoologists. Most small apes feed on fruits ; 
amongst them, however, Jachus vulgaris, known as the Mar- 
moset, is distinguished by an inordinate liking for the ill-smell- 
ing cockroach, a species of Blatta. Our common perch, as wellas 
a few Cyprinoide, frequently eat duck-weed (Lemna), although 
they belong to a group of carnivorous fishes ; squirrels are the 
greatest enemies of our singing birds, whose eggs and young they 
devour in great quantities ; individuals of the Russian brown 
bear will feed on oats, others on honey, others again on ants or 
meat. In conclusion I will only mention one fact frequently 
observed in aquaria by myself and by others. The well-known 
European pond-snail, Lymnea stagnalis, belongs to a group of 
Mollusca which all live on vegetable matter; and their lingual 
teeth are regarded by malacologists as typical of true plant- 
eaters. Nevertheless, the Lymnea is fond of eating the little 
water salamander, Triton. I have often observed them sud- 


60 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


denly attack quite healthy living specimens of Triton teniatus, 
overcome them and devour them, although the aquarium was 
full of luxuriantly growing plants, on which these water-snails 
usually feed. 

These instances!? will, I think, suffice to warn us to be 
cautious when, from the systematic position of an animal and 
the structure of its organs, we are called upon to determine 
what may be its mode of life and nutrition ; they further teach 
us that a polyphagous animal can occasionally be easily trans- 


Fic. 14.—Larus angentatus, one of the species of gull experimented on by John 
Hunter. 


formed into a monophagous one without suffering any serious 
injury. 

Thus, in general, polyphagous animals are less dependent on 
their food than monophagous species, and hence food can exert 
only a weaker selective influence on the former than on the latter. 
Assuming, for instance, that there were an animal which, up to 
the present time, had been fitted to use a certain species of 
animal or plantas food, and that it were suddenly transferred to 
a foreign country where such food was lacking, or that the 
animal or plant serving it for food were extirpated, while the 
creature itself was not; in either of these cases the continuance 


CHANGE OF FOOD IN GULLS. 61 


of the species as such might be made possible if the surviving 
specimens could quickly accustom themselves to the effects of a 
change of food. Such an accommodation to a new diet, not 
properly suitable to the animal, might be expected to be almost 
impossible to monophagous creatures, but to the polyphagous 
far less so. 

However, many animals of both groups are already known 
which are able, intentionally or under compulsion, to change 
their food, and in a corresponding degree their mode of life. 
The well-known anatomist and physiologist, John Hunter, long 


Fig. 15.—Myopotamus Coypu. / 


since communicated his observation that a kind of gull, Larus 
tridactylus—can live on grain, although its stomach is adapted 
to flesh diet; it commonly feeds on fish. Another species, 
Larus argentatus, is said by Dr. Edmonstone to live in the 
Shetland Islands on grain in the summer, and on fish in winter. 
In the same way the Coypu—J/yopotamus Coipu—living in 
the Chonos Islands, off the western coast of South America, 
has accommodated itself to an animal diet ; it there chiefly eats 
the marine mollusca of the coast, where alone the creature is 
found ; on the mainland, high up the country, it feeds exclu- 
sively on roots, which it digs out on the shores of streams and 


62 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


brooks. A very interesting example is offered by the Kia— 
Nestor nivabilis—of New Zealand; it is allied to the parrots, 
and formerly fed on the juices of plants and flowers, but lately 
it has become accustomed to sipping the blood of newly 
slaughtered sheep ; and it is asserted that this bird, originally so 
harmless. has actually become a serious foe to the flocks of New 
Zealand by its constantly increasing love for the blood of sheep, 
for it even pecks and sips the most minute wounds on a living 
sheep, and so sets up an irritation which not unfrequently leads 
to the death of the animal. Dr. Philippi, the best known 


Fic. 16.—Nestor mirabilis, a New Zealand parrot, 


zoologist of the University of Santiago in Chili, has recently com- 
municated a still more remarkable case. Two horses on the 
estate of a certain Mr. Nicholas Paulsen, according to him, had 
for weeks indulged in the bad habit of eating every day some 
of the young pigeons and chickens in the poultry-yard. 

In the Zoological Institute of Wiirzburg, I have kept for six 
years a pair of fully grown and perfectly tame prairie dogs. The 
male, to which I gave the old-fashioned German name of Hans, 
differs entirely in his tastes from the female, Gretel. She, in every 
respect an ornament to her sex, always gentle, unassuming, and 
aflectionate, but very timid too, prefers a vegetable diet—fresh 


ANTHROPOPHAGOUS CROCODILES, 63 


plants, bread, nuts, corn, &c.—although she sometimes does not 
disdain meat and liver. Hans, on the contrary, bold, eager, and 
suspicious, a true tyrant withal over his wife, is passionately fond 
of everything he can get in the way of animal food. Formerly, 
when aquaria stood in the room in which Hans and Gretel lived, 
he often tried to catch fish or crustaceans, which he devoured 
eagerly ; fat, liver or meat, eggs or frogs, ant’s eggs or insects—in 
short, every kind of anima] food—is acceptable to him,and he laps 
the blood of freshly slain beasts with the utmost satisfaction. It 
is evident that Hans first became accustomed in my laboratory 
to most of these articles of diet. In itself the matter certainly 
is not so very surprising, since most—or very many—rodents 
are polyphagous, or even omnivorous animals ; but it is rendered 
interesting by the fact that the female has by no means accus- 
tomed herself to an animal diet in the same way as the male. 

This brings me to an observation which, in the course of my 
travels, I once had occasion to make very much against my will. 
The Egyptian crocodile — Crocodilus biporcatus —is, as we 
know, very widely distributed, and it lives in great numbers in 
the rivers and on the sea-shore of the Philippine Islands. In 
Egypt this creature is considered extremely dangerous, and is 
said to have a particular predilection for human flesh. When 
I was travelling in the Philippine Islands I was often told 
by the natives that they distinguished two sorts of crocodiles, of 
which one ate men in preference to other food, while the other 
did not ; several of the former were said to be well known to the 
natives, and in Cagayan in Luzon, where I saw the skeleton— 
quite 22 feet long—of a crocodile caught not long before, I was 
assured that a gigantic anthropophagous crocodile lived in the 
river and could not be caught, and had for years been known 
to the natives by a particular nickname. I was much inclined 
to doubt this story till I went through a little adventure which 
made it seem to me certainly by no means improbable. On 
one of my excursions in the north-east of Luzon we (my servant 
Antonio and I) had crossed a wide but shallow river early in 
the morning in a canoe; when we returned in the evening the 
canoe had disappeared, and not a living soul was to be seen any- 
where. After long waiting in vain we decided on walking 


64 THE INFLUENCE OF INANIMATE SURROUNDINGS, 


through the stream. I,in order to preserve my watch and other 
instruments from a wetting, seated myself astride on Antonio's 
shoulders. When we were about halfway across, where the 
water reached nearly to my bearer’s neck, a man appeared on the 
shore. Seeing him I shouted out, half in jest, ‘ Are there any 
crocodiles in the river here?’ My feelings may be imagined 
when I received the answer, ‘ Oh, yes ; there are plenty of croco- 
diles in the water, but they will not eat men.’ Everyone will 
be reminded by this story of the many similar ones of sharks, 
alligators, and other animals, which all concur in proving that 
these creatures exhibit the most remarkable preferences in the 
choice of their food, and that even individuals of the same species 
differ widely. : 

It will be unnecessary to adduce any further examples or 
even to investigate the credibility of these current stories 
regarding crocodiles; for, even without these, the instances 
given above suffice to show that polyphagous or monophagous 
habits are not immutable characters, but that, on the contrary, 
almost every species is able more or less to vary the nature 
of its food. Hence the dependence of an animal on its 
nutrition ig not absolute, and consequently the selective 
influence of the nutrition is, as we see, in some degree 
limited by the animal’s capability for accommodating itself, 
with very various results, to a diet hitherto unknown to it. 
The selective influence must, at any rate, remain tolerably 
great, particularly on monophagous creatures; and it is more 
than probable that a sudden and rapid change of nutrition, such 
as may sometimes be forced upon animals by external circum. 
stances, will inevitably lead to the equally rapid death of most 
species. }8 

Now, if we suppose that such a sudden change of nutrition 
actually were imperative on several species at once—such, for 
instance, as always occurs in the migration of many marine 
creatures, and of all parasites—some species must perish, because 
they would not be capable of living on the unaccustomed food ; 
others might survive because they were omnivorous or because, 
even though monophagous, they were able to adapt their 
functions promptly to the new conditions of life. In the latter 


TUK INDISPENSABLE OPTIMUM. 65 


case the structure of the animal and of its organs might remain 
unaltered in spite of the alteration in the nutrition, as, for 
instance, seems to bave been the case with the Coypu, those of 
the Chonos islands not differing in any way, so far as we know, 
from their congeners on the mainland. Bat, finally, this 
change in the food might have altered the structure of some 
organs, particularly of those most directly interested, so far as 
to make these changes conspicuous ; and a direct modifying 
influence exercised by the conditions of existence afforded by 
the food would be thereby proved. 

The direct modifying influence of food.—It is universally 
known, and has never been denied, that the amount of food 
exerts a very decided influence in determining the growth of 
the individual and of its organs, as well as on its whole size ; 
but this has often been tendered as a means of explanation in 
certain cases which have not been submitted to careful investiga- 
tion. It can never, of course, be disputed that an animal must 
take up the optimum of daily nutrition, which constantly varies 
with its advancing age, in order to attain its normal size; we 
might even declare our opinion that very many living or 
extinct animals might have grown to a size far beyond that 
which they have in fact attained, if they had had more abundant 
supplies of food at their disposal; but it would be in the 
highest degree illogical to assume, on the contrary, without 
any experimental proof—as is unfortunately almost universally 
done—that the small size of any particular animal in any 
particular locality is invariahly induced by a deficiency in the 
food attainable there, where the optimum is seldom or never 
attained. In these cases, as in all others, in consequence of the 
extreme complication of the animal body and of its functions, 
the same effect may be produced in many different ways. 

But the amount of food attainable affects not merely the 
size of the animal, but also determines, and even modifies, 
certain vital functions. It is self-evident that an optimum 
of nutrition can alone insure the normal functions of all the 
organs ; if it does not attain the optimum, the functional activity 
of all the organs is impaired; modifications at the same time 
occur in their structure, ¢.e. the animals grow leaner, become 


66 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


incapable of exercising their sexual functions, &c. In this 
respect the most interesting examples are those of the influence 
of deficient nutrition on the larva forms or on the conditions of 
development. Unfortunately next to nothing reliable is known 
on this subject, and it is much to be wished that the various 
observations that have been accidentally made and interpreted 
‘to taste’ should be made the starting-point for actual experi- 
mental investigations on this question. I will here mention 
a few of what seem to be the most trustworthy of these. Mr. 
T. Gentry, of Philadelphia, has shown that the larve of a moth 
—Acronycta sp.—entirely lose the habit of spinning a cocoon 
before assuming the pupa state when their food is insufficient, 
and tbat both the pupe and moths are then smaller. The 
observations independently made on the Hydroid Polyps by 
Hincks, Allman, and Schneider are highly interesting. Ac- 
cording to these, in the first place a Medusa of the group of 
the Hydroida can be induced by lack of nourishment to assume 
the polyp-form, 7.c. the larva form of the species. Secondly, 
the hydroids of the higher Discoid Meduse—as Jfedusa chrysaora 
and others—produce much fewer Medusz in confinement than 
in the open sea; and this has been accounted for, somewhat 
hastily, by the assumption that deficiency of food is the cause. 
Experimental proof of the accuracy of this hypothesis has not, 
however, been adduced. 

The quality of the food, next to the quantity, exerts a direct 
modifying influence which in many cases exhibits itself in the 
organs most nearly interested—those, namely, of digestion— 
though others may become subject toit. More rarely the whole 
size attained by the animal may be conspicuously affected by 
it. But we possess only a few trustworthy observations on this 
point, interesting as it is, and still fewer available physiological 
experiments, though such are indispensable. The lack of 
materials on this subject renders it necessary to discuss it briefly 
here. 

In the first instance the statements of Wallace and others 
as to the influence of food on coloration must be mentioned, since 
Seidlitz in his various works attributes great importance to 
them, although, as it seems to me, he assumes something to be 


EFFECTS OF FOOD ON COLOUR. 67 


proved which, fundamentally considered, is not so. Wallace, for 
instance, relates thata Brazilian parrot—Chrysotis festiva—can 
be made to change the green in its feathers to yellow or red 
if it is fed on the fat of certain fishes allied to the shad—a method 
largely adopted by the Indians. The same traveller further 
asserts that the splendid Indian bird, Lori Rajah, is said to 
preserve its gorgeous colouring by a peculiar mode of feeding. 
The bullfinch is said to turn black when fed on hemp-seeds ; 
recently a splendid orange-coloured variety of the canary has 
been introduced into commerce, and it is said it is produced by 
feeding ordinary specimens of the bird on Spanish pepper. The 
statement is well known that butterflies, and more particularly 
species of the genus Huprepia, assume an abnormal colouring 
when the caterpillars are fed on leaves which they are not 
accustomed to; thus Huprepia caja becomes quite brown when 
the larve are fed entirely on walnut-leaves. This assertion, 
however, has been frequently contradicted, and no systematic 
and experimental investigations, as directed expressly to this end, 
have ever been made to my knowledge, for the independent 
experiments in feeding made accidentally or by a happy chance by 
different entomologists cannot in fact be regarded as physiologi- 
cal experiments. Still less can the statements made by travellers, 
as by Wallace, count as such, since they rest entirely on hearsay 
from wild Indians, and not on the results of their own investi- 
gations. Of course I am far from asserting that no such direct 
modifying influence of food on the colour of animals exists, or 
that it is improbable; I only would point out that up to the 
present time we know nothing exact on this point, and that 
nothing is actually proved beyond the possibility or probability 
of such an influence affecting the skin-pigment of various 
animals. As to the nature of this chemico-pbysiological process, 
which is what is truly worth knowing in the matter, so far as 
I know, not even a hypothetical view has as yet been ex- 
pressed. 

A. few experiments are better established which prove that 
certain structural relations may be entirely changed by the 
direct influence of food. The English anatomist Hunter pur- 
posely fed a sea-gull—ZLarus tridactylus—tfor a whole year on 


68 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


grain, and he thus succeeded in so completely hardening the 
inner coat of the bird’s stomach, which is naturally soft and 
adapted to a fish diet, that in appearance and structure it pre- 
cisely resembled the hard horny skin of the gizzard of a pigeon. 
Dr. Edmonstone assures us that this experiment is annually 
repeated by nature ; that the herring-gull—Larus tridactylus— 
of the Shetland Islands twice every year changes the structure 
of its stomach, according to its food, which consists during 
the summer of grain, and during the winter of fish. This 
gull then has, in fact, during the summer the stomach of a 
grain-eater, and during the winter that of a carnivorous bird of 
prey. The same naturalist observed a similar transformation 
in the structure of the stomach in the raven, and Ménétriés 
makes a similar statement with regard to an owl—Strix 
grallaria. 

These experiments suffice to prove that the stomach of a 
carnivorous bird (an owl, a gull, and a raven) can be trans- 
formed to that of a grain-eater if supplied for a sufficiently long 
period with the food requisite for this result. The question then 
obviously suggests itself whether the converse is equally true, 
a.e., whether the gizzard of a true grain-eating bird can be 
transformed into the soft-skinned stomach of a carnivorous 
bird. The experiments of Dr. Holmgrén in fact prove that in 
pigeons which are fed on meat for a sufficiently long period, the 
gizzard is gradually transformed into a carnivorous stomach.'4 

I have not been able to collect a larger number of really 
credible or experimentally proved data, and I believe that [ 
cannot have overlooked many really important and available 
communications. I except, of course, the cases briefly given in 
Note 15 of the influence of nutrition on sexual maturity and 
on the secondary sexual characters of domestic animals, since 
we are not justified in directly applying the results derived 
from the artificially bred races of domestic animals to all others 
living wild. Meagre as is the list here given, it amply suffices 
to prove that changes in nutrition are able to exert even a direct 
influence on many structural relations of organs, although it 
must be admitted that we know nothing—absolutely nothing— 
of the limits of the variations called forth by this direct influence 


MODIFYING EFFECTS OF FOOD. 69 


of a certain diet. The variations in the structure of the stomach 
of birds experimentally proved by Hunter, Edmonstone, and 
Holmerén, have only a superficial importance, since we do not 
know whether modifications of other parts were connected with 
them or might subsequently have originated from them. If 
now we reflect that, in spite of the great general interest of the 
experiments of Hunter and Holmgrén, not thesmallest additional 
fact has been established experimentally since their time either 
by modern zoology or, on the other hand, by organic physio- 
logy—no one having investigated the subject—it may be 
regarded as a not improbable opinion that experiments purposely 
carried out on a large number of animals, as widely different as 
possible, would offer a much greater mass of results than are at 
present at our disposal. Moreover, it is not impossible that 
change of food may lead to more fundamental modifications 
in other animals than those in the stomachs of the pigeon and 
herring-gull, since we know that different species react in 
very various ways under identical influences. The above- 
mentioned cases of the variations in external colouring pro- 
duced by food in birds and butterflies sufficiently prove this ; 
for there is a large number of animals in which a change of 
food has no influence whatever on the skin-pigment. 

The conclusion of the investigation conducted in this chapter 
is not very satisfactory : we have seen that with respect to the 
direct modifying effects of food everything in fact remains to 
be done. However, the few well-ascertained cases suffice to 
prove that the smallness of our stock of positive knowledge on 
the subject is probably due only to the fact that no systematically 
pursued investigations have been carried out.!® This may, it is 
true, be excused on the score that zoologists—on whom this 
task would principally devolve, owing to the position taken up 
by physiologists—have been prevented fulfilling it partly by 
the direction which the development of their science has taken, 
but above all by the absolute insufficiency of their institutes 
and laboratories. 


70 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


CHAPTER III. 


THE INFLUENCE OF LIGHT. 


Ir has been poetically said that the plants and trees of our time 
are the incorporate sunbeams of to-day, and that coal contains 
the sunshine of long past epochs, divided from the present by 
millions of years; and the saying is to a great extent true, as 
everyone knows, for the greater proportion of vegetable organ- 
isms depend entirely for life and growth on the direct influence 
of light. It is equally well known that animals are to a certain 
extent independent of this influence. At the same time even 
they are open to it, and the question might even be suggested : 
Whether animals are not in fact at least as dependent as 
plants on the direct influence of light, even though the nature 
of their relations may be altogether different? In discussing 
this point we will distinguish between the heat-giving rays and 
the light-giving rays, even when these are in the most intimate 
combination ; and we are justified in doing this, since we know 
that these two modes of motion act upon living organisms in 
different and often antagonistic ways. 

The difference between animals and piants.—If we except 
the lowest organisms, the relations between light and the organ- 
ism seem to be maintained by two very dissimilar organic 
structures —by the eye in the animal, and by the chloropbyll 
bodies in plants. These, nevertheless, have been occasionally 
compared.'® Each organ would seem to preclude the other. 
It is true we know of some highly organised animals that have 
no eyes, and true plants which are devoid of chlorophyll; but 
plants never have eyes at all, and such animals contain no 


CHLOROPHYLL IN ANIMALS. 71 


chlorophyll ; thus in these exceptional cases the influence of 
light appears to be almost or entirely excluded. We venture 
to assert that this contrast holds good in by far the larger 
number of animals and plants, and it is quite certain that 
true eyes are never found in plants, while it still remains doubt- 
ful whether chlorophyll does actually occur, as has often been 
asserted, even in the lowest animals. Theoretically its existence 
in animals is certainly not impossible; and this theoretical 
possibility has perhaps given rise to the assertion. The wide 
interest which attaches to this assumption may justify us in 
digressing here into a somewhat closer discussion of the data 
relating to the matter. 

The chlorophyll bodies of plants are, as is well known, 
microscopic and elementary bodies of peculiar structure and of 
definite function ; their principal property is that they decom- 
pose carbonic acid under the influence of light, and form organic 
compounds by the combination of three or four elements. 
This true chlorophyll has, besides, properties which allow the 
botanist to distinguish, when necessary, whether the green 
colour of a newly discovered plant is actually caused by the 
presence of chlorophyll, without any need of previously investi- 
gating whether the green particles decompose and assimilate 
carbonic acid. Among these properties are certain absorption- 
bands in the spectrum of solutions of chlorophyll, its direct 
dependence on the presence or absence of light, its reaction 
under certain chemical agents, and its peculiar microscopic 
structure. In most cases it is sufficient for the Lotanist to have 
detected any one of these features when the case in point is to 
prove whether the green colour of a plant depends on chloro- 
phyll. Besides this, so far as I know, ro exception worth 
meutioning has hitherto been admitted to the rule, that the 
green colour of all plants is occasioned not by any true pigment, 
but by the presence of chlorophyll. In animals, however, the 
case is quite different. We know that most animals are 
absolutely incapable of decomposing carbonic acid; but they 
are, nevertheless, frequently of a green colour. In by far the 
greater number of cases this green colour is undoubtedly due 
not to chlorophyll, but to a true pigment. Hence we cannot 


72 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


without further proof assert the presence of chlorophyll in any 
animal, even if we coulil prove the animal’s direct dependence 
on light, or the similarity of the spectrum of the solution of the 
green pigment with that of chlorophyll, or even a possible 
agreement in the microscopic structure. Positive proof of its 
existence can be derived only by evidence of the presence of all 
the characteristic properties of true chlorophyll in the green 
colouring matter of the animal. And it may at once be said 
that the decomposition of carbonic acid by green-coloured 
animals has never been proved by exact experiment.* 


Fic. 17.—Animals in which chlorophyll grains have been detected. a, Stentor viridis; 
i pigine viridis ; ¢, Vortex viridis. The first two are Infusoria, the last is a Tur- 
ellarian, 


The animals (fig. 17) in which it has been asserted that 
chlorophyll is present, belong exclusively to the Invertebrata. 
Among the Protozoa the following are the best known; Euglena, 
Stentor, many Radiolaria and Spongilla; the green fresh-water 
polyp Hydra among the Ccelenterata, and a few Turbellaria 
among the worms.!7 

The arguments for the statement that the green colour 
of these creatures is actually due to chlorophyll are many and 

* Such a decomposition has been recently proved by the experi- 


ments of Mr. Patrick Geddes on the green Turbellarian worm, Convo- 
luta Schultzit.— (TRANB.) 


TESTS OF CHLOROPILYLL. 73 


various. Mr. Sorby has shown that the green variety, or 
species, of our common fresh-water sponge (Spongilla fluviatilis) 
owes its colour to minute particles of colouring matter which 
seemed to be identical with chlorophyll, for he proved that their 
spectra were identical. The same method was followed by Mr. 
Ray Lankester, who, with regard to Spongilla, came, it is true, to 
a conclusion different from that of Mr. Sorby, but, on the other 
hand, recognised the presence of chlorophyll in Hydra viridis. 
The much-Jamented Max Sigismund Schultze, to whom we owe 
the earliest accurate observations on animal chlorophyll, endea- 
voured to prove its identity with vegetable chlorophyll by com- 
paring the chemical reactions of various solutions of each, as 
well as by observing that Vortex viridis loses its colour in the 
dark, and that the animal, exactly like plants, always seeks 
the lightest side of the aquarium. But the decomposition of 
carbonic acid by animal chlorophyll has never been demon- 
strated, although Sorby himself has pointed out that it would 
be very interesting to know whether such a process does 
actually take place in animals that contain chlorophyll; for if 
the decomposition of carbonic acid could be ascertained in 
these low forms of animals it would prove that they are able to 
elaborate and assimilate organic matters in the same way as 
plants, though they also, like all other animals, require ready 
elaborated organic nourishment, or they cannot thrive, But 
this would be a fact of very far-reaching significance, exhibiting 
a certain affinity to the instances known to us of carnivorous 
or insect-eating plants (Drosera, Dionza, and others). 

At the same time it must be pointed out, on the other hand, 
that—granting unconditionally that the pigment of the animals 
that appear to contain chlorophyll is truly of the nature of 
chlorophyll—its presence in animals may be explained in two 
different ways. In the first place, if the chlorophyll bodies of a 
Stentor, for instance, were really elements of the animal tissue, 
elaborated from its protoplasm by the direct influence of light, 
then—but only then—might we say that there were actually 
animals which assimilate in the same way as plants. But, in 
the second place, it might be possible that the green constituents 
were not integral elements of the animal, but foreign bodies 


74 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


living within is—commensals or ‘ messmates,’ as they are called. 
Kleinenberg’s observations on Hydra viridis are decidedly 
favourable to the former of these views; Schulze’s statements 
as to Vortex viridis are equally positive in favour of the 
second. For he expressly declares that the chlorophyll bodies 
of this worm are true cells, unlike those of plants; that they 
divide and multiply spontaneously, which the chlorophyll 
bodies do not; and finally that they are in some individuals 
wholly wanting. The importance of these arguments is increased 
by other facts. It is known that most of the Radiolaria in- 
variably bear in their body certain peculiar particles known as 


Fie. 18.—Cellozoum inerme (Haeckel), a Radiolarian forming colonies. a, a colony; 6, 
a solitary individual, or, more correctly, the internal vesicle of one (the shaded bodies 
are globules of fat, the outer spots indicate the numerous yellow cells). 


the yellow cells (fig. 18), in which a few starch-grains are always 
present. These yellow or sometimes green cells occur in many 
fresh-water Radio!arians which have lately been often made the 
subject of minute investigations. From these, above all from 
the very careful labours of Cienkowsky, it has recently been 
proved that these yellow cells in the Radiolarians are in fact 
nothing more than one-celled Alge living as messmates with 
the animal in the same sort of community as certain Fungi and 
Alge which, as is well known, combine to form the apparently 


ABSORPTION OF FOREIGN ELEMENTS. | 75 


simple vegetables known as Lichens, which, however, are still 
generally classed as a distinct group of plants. It may at 
first sight seem somewhat bold to assume that living plants, 
even of the simplest conceivable structure, could constantly, 
or almost constantly, be so associated with an animal as to 
seem one of its histological elements. But this hypothesis 
assumes a high degree of probability when we remember that 
numerous parasites occur with unfailing regularity in certain 
organs of every individual, or nearly every individual, of a 


Fic. 19.—Longitudinal section of Sphencpus Steenstrupii S. The skin of the creature, 
ep, which is thinnest above, has agglomerated grains of sand throughout its sub- 
stance. 

species—for instance, the larvee of certain Nematodes in the foot 

of the common snail; when, moreover, we take into considera- 

tion that different animals, more particularly Sponges and 

Polyps, frequently take up dead or living foreign bodies and 

utilise them as normal elements-of the tissues (fig. 19). 

Of course no decisive answer can be arrived at by this 
method, and only experiment can find one. But it seemed to me 
to be advisable to state both these possible solutiens, and to 
bring forward those facts which may perhaps soon require us, if 


76 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


we find true chlorophyll in animal tissues, to recognise in its 
presence a singular and interesting case either of parasitism or 
of the community of two organisms so different as an animal 
with true tissues and organs, and a one-celled plant.'§ 

The general relations between light and the vital activity 
of animals.—By far the larger number of animals are conscious 
of light by means of the eye only. This was directly proved 
by the interesting experiments of Lister and Pouchet, which will 
be more fully described further on. The commonest effects of 
light, of its different degrees of intensity, and of its total absence 
are familiar to all. They are exhibited every day in regular 
succession in every animal that lives; darkness induces sleep in 
diurnal animals, and with this are connected certain other 
effects on some of the organs and their functions ; for instance, 
the amount of carhonic acid exhaled by the Mammalia during 
sleep is different from that exhaled when they are awake. 
These proportions, however, are of no particular interest in this 
place. What is far more important is the observed and well- 
ascertained fact that all active diurnal creatures fall asleep 
promptly during an eclipse of the sun; the darkness deceives 
them as to the hour, and so interrupts the periodicity of their 
vital activity. But all animals do not react in the same way 
under the alternation of light and darkness; while some—the 
diurnal animals—go to rest at the approach of night, others, 
nocturnal animals, then rouse up, and we might be tempted by 
this to divide them into day and night animals. But such 
a division has merely a biological value, for we know that 
it is in no way cu-extensive with the conditions of affinity in 
animals. We are acquainted with diurnal and nocturnal species 
among the Mammalia as well as among Birds; some Butterflies, 
Beetles, and other Insects are nocturnal, though the greater 
number fly by day; nay, even within the limits of quite smatl 
families or even genera, there are some species which are lively 
by day and others by night. To give one example only : every 
entomologist knows that night-flying Lepidoptera, nocturnal 
as to their affinities and structure, as, for instance, the Sesic or 
Agha Tau and others, rest by night and fly gaily about by day 
to seek food or to seize the female. The causes of these 


BLIND CAVE-ANIMALS. 77 


differences in the mode of life of related forms are entirely 
unknown, and at the present time it seems impossible even to 
suggest any hypothesis which would refer such changes in their 
mode of existence to any sufficient causes. 

By far the larger proportion of nocturnal animals, although 
they are quite lively even in the darkest night, have eyes 
quite as good and perfect as those of the diurnal animals. 
Although, asa fact, here and there—as, for instance, in nocturnal 
birds—certain differences have been observed in the structure 
of the retina (M. 8. Schultze) which might be hypothetically 
connected with its exceptional functions or with the exceptional 
time at which they are exercised, yet these investigations 
supply us with no answer, not even a hypothetical one, to the 
question as to why certain animals, provided with organs of 
sight, fly exclusively by night. If we remember that even in 
the darkest night a certain amount of light always reaches the 
earth, we might certainly propound the hypothesis that this 
minute proportion of light suffices them for seeing clearly by. 
But this hypothesis would give no true explanation of the 
observed facts ; this could only be given if it were possible to 
compare the differences in the structure in the retina of 
diurnal and noctural animals with direct reference to the scale 
of intensity of light to which they are exposed. A circum- 
stance which is more important, because it is directly referable 
to certain vital relations of animals, is the occurrence of half- 
blind or wholly blind animals in spots where the light of day 
cannot penetrate, such as deep caverns, the internal parts of 
larger animals, and the deepest parts of the ocean or of large 
fresh-water lakes. The blind crayfish of the mammoth cave 
in Kentucky is well known, as are also the blind Fishes, 
Insects, Crabs, Amphibia, and Mammals (moles) of the old and 
new worlds, and it seems unnecessary to give a complete list of 
such casesin my text.!® These familiar facts have hitherto been, 
and must still be, regarded as so many instances, sufficiently 
proving the statement that total darkness gradually destroys 
the eyes of animals originally possessing them ; for, since these 
organs are absolutely useless in such circumstances, in the 
course of generations they must gradually disappear, according 


78 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


to the law of degeneration, in consequence of their disuse. This 
explanation, it is obvious, presupposes that such blind animals 

F are descended from a parent form that could 
see; and it cannot be denied that many of the 
facts hitherto ascertained seem to justify this 
view. Some of the so-called blind animals are 
not, accurately speaking, sightless; thus the 
blind Proteus (fig. 20, @), an Amphibian of 
the caves of Carniola, has an eye deeply 
seated in the head and entirely covered by the 
skin. The structure of this organ is very re- 
markable; it possesses all the characteristic 
parts of the eye, but they have been arrested at 
an almost embryonic stage, with the exception 
of the crystalline lens, of which every trace is 
absent (fig. 20, 6); the pigment-layer of the 
retina is scarcely coherent, and consists of only 


iD 


NESS 
FEN 


Fic, 20.—a, Proteus of the Adelsberg grotto, reduced; b, vertical 
section of the rudimentary eye; cpl, the optic nerve ; cc, corpus 
ciliare reting, the inner portions of which meet in front because 
the Jens is absent; cv, the internal cavity of the eye without 
any vitreous humour. The ccll-layers of the retina (set) are 
unusually thick ; the pigment-layer, p, very slightly developed. 


a few scattered pigment-cells. We may therefore be very doubt- 
ful as to whether this Proteus can receive a clear image of the 
objects that surround it even in a place where there is light ; 


THE PROTEUS. THE MOLE. 79 


but certain observations, which I have made ona family of 
Proteus that I have kept for four years, incontrovertibly prove 
that this creature is highly sensitive to diffused daylight. As 
this contains no heat-rays, the eye of the Proteus can receive 
no impression but that of light. Now it is impossible to 
suppose that the eyes are now first developing in an originally 
blind Amphibian which, like the Proteus, lives in total dark- 
ness ; for even if such an organ could originate under such 
circumstances it could never become permanent in the struggle 
for existence, because it could never be of any real use in that 
struggle. The contrary hypothesis, on the other hand, that 
the rudimentary eyes of the Proteus are a degenerate form of 
the more highly developed eyes of its progenitors, seems per- 
fectly natural when we remember that all the other amphi- 
bians have highly developed eyes, and that these, when they 
come to the light from time to time, use them to very good 
purpose. 

The Mole offers another fumiliar and even better example. 
This animal, whose peculiar habits are known to everyone, 
has true eyes, from which none of the essential parts of the 
eyes of the Vertebrata are absent, although these parts are all 
of the simplest, almost of embryonic structure. The whole eye is 
very small, deeply imbedded in muscles, and quite covered by the 
skin, so that it is quite invisible externally. The lens consists 
of a very small number of minute and little altered embryonic 
cells; the retina, in the same way, is much simpler than in the 
eyes of other Vertebrata. True degeneration, then, such as makes 
the eye incapable of seeing, has not taken place ; nevertheless 
the eye of the mole is reduced to almost total inefficiency even 
when by chance it has an opportunity for usingit. This almost 
total blindness in the mole is the result solely of complete de- 
generation of the optic nerve, so that the images which are 
probably formed in the eye itself can never be transmitted to 
the animal’s consciousness. Occasionally, however, the mole 
even can see a little, for it has been found that both optic 
nerves are not always degenerate in the same individual, so 
that one eye may remain in communication with the brain 
while the other has no connection with it. In the embryo of the 


80 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


mole, however, and without exception, both eyes are originally 
connected with the brain by well-developed optic nerves, and 
so theoretically efficient. This may indeed be regarded as a 
perfectly conclusive proof that the blind mole is descended from 
progenitors that could see; it would seem, too, to prove that 
the blindness of the fully grown animal is the result not of 
inheritance, but of the directly injurious effects of darkness on 
the optic nerve in each individual. 


Fia. 21.—a, Pinnotheres Holothurice of the natural size; b, degenerate water-lungs, with 
the distended portion, c, in which a small Pinnotheres is established. 

To these examples I will add one more which I myself have 
studied. There isa peculiar family among the crabs, the Pinno- 
theride, of which various species live in the branchial cavities 
of many Mollusca ; some live in Serpule, and others (fig. 21) 
which I have found off the Philippine Islands live in the water- 
lungs, as they have been called, of Holothurians. These are 
elongated branched tubes in direct communication with the 
terminal intestine or cloaca, so that parasites can enter them 


DEGENERATION OF ORGANS. 81 


only by the anus; but when the young larve of the Crustaceans 
have once found their way in—which is not difficult by reason of 
the strong and rhythmical indraught of water through the cloaca 
—they never seem to quit the situation of their own choice; at 
the same time they greatly irritate the organ, and as they grow 
they stop up the tubular vessel more and more till at last 
serious degeneration of the organ is induced (fig. 21, 6). The 
main trunk is greatly distended, while the lateral branches, 
which usually form a highly ramified structure, dwindle alto- 
gether, and are visible only as thin filaments, sometimes feebly 
branched. The young larve now produced are excluded, and 
become wandering bodies, in obedience to the law which 


Fic. 22.— Zoea stage of the larva of Pinnotheres Holcthuric. 


governs all Ento-parasites; this they do under the form 
of the larva, or Zoea (fig. 22), which is common to all crabs, 
and they have the well-developed eyes of the typical character. 
Even when they enter the animal, they still preserve these eyes ; 
but as they grow they gradually become blind or half-blind, the 
brow grows forward over the eyes, and finally covers them so 
completely that, in the oldest individuals, not the slightest 
trace of them, or of the pigment, is to be seen through the 
thick skin; while at the same time the eyes seem to undergo a 
more or less extensive retrogressive metamorphosis. 

The instances here adduced show very clearly that the 
absence of light sometimes occasions degeneration from disuse, 


5 


§2 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


and that this occurs to each individual separately within the 
period of its separate life. These, however, as every zoologist 
knows, are not the only cases. Most of the blind Parasitical 
Crustacea now extant have larve with well-developed eyes ; the 
young form of many worms, Parasites on Mollusca, &e. 
(Trematoda), can see, though the adult individuals are blind. 
In the greater number of these cases—as, for instance, in all 
interna] parasites—we must refer the loss of sight to the samo 
above-mentioned cause, namely, disuse of the organ. 

But though we are thus fully justificd in saying that dark- 
ness so complete as not to allow of the eyes being used at all 
has in most cases exercised an injurious effect on their existence 
and structure, it would nevertheless be wholly false to assume 
that the lack of light must necessarily lead to total or partial 
blindness. We know of a number of facts directly opposed to 
such a conclusion. Among the numerous cave-insects there 
are many which have well-developed eyes, and yet inhabit the 
game spot as blind species. In some caves in the Philippines 
and the Pelew Islands which I myself explored, I found, in 
spots where the most absolute and total darknegs reigned, only 
insects with eyes; Hadenwcus, a species of grasshopper which 
lives in the caves of Kentucky, has well-developed eyes like 
other animals found there at the same time.?? Why should 
not darkness have had the same effect on these animals as 
on others which have in fact become blind? It might be 
said—in fact it has been said—that the cave-animals which 
can see have migrated into the cave only within a short period, 
and have not been exposed to the influence of the darkness 
long enough to suffer; while the blind or half-blind, having 
entered the caves at a remote period, have lost the use of their 
eyes, wholly or partially, in consequence of long desuetude. 
But this explanation contradicts the fact previously mentioned, 
that every mole, Pinnotheres, &c., originally had eyes apparently 
capable of further development, and of perfectly fulfilling their 
normal function; and that the influence of darkness is proved 
to be direct in each individual, and not hereditary. This ex- 
planation is also quite decisively contradicted by a fact which is 
little known generally, and even among zoologists is familiar 


DARKNESS AND BLINDNESS. 83 


to none but entomologists. I owe my own knowledge of 
it to my friend Dr. Hagen of Cambridge, U.S. In all the 
species of the cave beetle, Wachwrites, the females only are 
blind, while the males have well-developed eyes; in spite of 
this they both live together in absolute darkness. This proves 
that the same result—total blindness—may come from dif- 
ferent causes; for we may fairly regard it as impossible that 
in the last-named case the darkness of the cave has affected 
the females alone, and been ineffective on the males; hence the 


Fic. 23.—Blind Cymothoe in fresh water (small pools) at Pelelew, Pelew Islands, 
About ten diameters. 


blindness of the former cannot be caused by the darkness. In 
confirmation of this statement J may also adduce the fact that 
there are many blind or half-blind animals which live in well- 
illuminated situations, where the moderate intensity of the light 
would allow them the full use of eyes; this is the case, for 
instance, with many Bivalves—all fresh-water bivalves and 
many sea bivalves—with various Annelida (Chetogaster), Crus- 
tacea (Cyclopite), and others. I myself have found a perfectly 
blind small species of Cymothoe (fig. 23) living in slightly 
brackish water in a basin overshadowed by limestone rock, but 


84 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


in spots where full daylight could penetrate. Thus we find 
ourselves driven, by the facts here adduced and numberless 
others, to this question: What are the various causes which can, 
or must, first occasion eyes to be developed, or conduce to. their 
preservation or destruction? A. precise answer to this ques- 
tion, unfortunately, is impossible in the absence of all experi- 
mental data ; but we, as zoologists, may allege the difficulty— 
indeed, the impossibility—of such experiments, as a sufficient 
excuse for their never having been hitherto carried out. 

In cases like this, where we are not in a position to treat a 
physiological question experimentally, we must be allowed to 
construct a hypothetical explanation of the observed phenomena. 
I therefore consider myself justified in mentioning a very preg- 
nant hypothesis, which was put forward some little time since 
to account for the presence of animals that can see in the deepest 
parts of the ocean, where positively not a ray of light can pene- 
trate from above. 

It is not very long since it was universally believed, in 
accordance with the too rapidly drawn inferences of Edward 
Forbes, that all animal life ceased on the floor of the ocean at 
the level where rays of light cease to penetrate (at a few hundred 
métres). But it is now well known that even highly developed 
animals live at the enormous depth of from two to three thou- 
sand fathoms in both the Pacific and Atlantic Oceans. We 
have become acquainted, principally through the incessant 
labours of English, American, and Norwegian naturalists, with 
a wonderful deep-sea fauna, showing the same striking mixture 
of blind and seeing animals as the fauna of the caverns.2!_ This 
case is all the more puzzling, because the chief part of such 
deep-sea animals as can see are extraordinarily unlike their 
nearest congeners living at the surface and in the light, so that 
we are forbidden to suppose that they may be species that have 
only lately migrated from the surface to great depths; indeed, 
it admits of scarcely a doubt that the deep-sea animals that can 
see are very ancient forms, the survivors of past geological 
periods. MacCulloch and Dr. Coldstream suggested a pleasing 
hypothesis in explanation of these striking facts, which was 
afterwards taken up and extended by the naturalists of the 


PHOSPHORESCENT ORGANS. 85 


‘Porcupine’ expedition (1869-70). This hypothesis, which is 
known as the Theory of Abyssal Light, consists essentially in the 
idea that the light diffused by phosphorescent creatures is 
capable of taking the place of sunlight in those depths which 
the rays of the sun cannot penetrate. It is evident that the 
correctness of this idea cannot possibly be experimentally tested 
and proved, but at the same time we cannot but admit that it 
is highly probable. For although it has been argued, as an 
objection to this idea, that phosphorescence is not an exclusive 
peculiarity of deep-sea creatures, but on the contrary, so far as 
we know, occurs more frequently among animals living on the 
surface, this objection must certainly be considered as anything 
but conclusive. We know from exact experiment in individual 
cases, particularly on glowworms, that phosphorescence is the 
product of a chemico-physiological process in the living body of 
the animal, exactly as carbonic acid is a natural product of 
respiration. What requires us to assume that this ought to 
occur in deep-sea animals only,?? supposing that theory to be 
accurate? The obvious ground of this objection is the tacit 
assumption that, if phosphoric light can really be of use to 
any creature, it must only occur in cases where it could be 
utilised. But this mode of argument offers an example of a 
very common but very gross error: the idea, namely, that the 
effect produced by the function of an organ, or that the function 
itself—in the present instance the production of light in the 
light-organ—can be brought into existence by reason of the 
usefulness of its results, when the use, in fact, makes its 
appearance at a later period. Phosphorescence, as it is 
developed in the living tissue of animals living at the surface, 
may perhaps never be of any use to the creatures that possess it, 
nor to the enemies that pursue them. But the same effect of a 
similar chemical process may nevertheless be advantageous to 
other creatures, which, like the deep-sea animals, would other- 
wise be condemned without exception to live in total darkness. 
We are not at present acquuinted either with the various che- 
migal processes by which phosphoric light is produced in dif- 
ferent animals, nor with the uses which these processes may 
subserve for the animals themselves; but we know of some 


86 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


animals, at least among the insects, to whom this light serves 
asa guide by which to find each other, as in the male and 
female Lampyris; and such a, light would undoubtedly be 
equally serviceable at the bottom of the sea to all the animals, 
those preying as well as those preyed upon; for without light, 
escape and pursuit must alike depend wholly on accident, and 
the remarkable fact that the eyes of deep-sea creatures are not 
always and completely abortive would thus be accounted for, as 
far as is possible perhaps on the whole. 

One pressing difficulty, however, remains. We know that 
blind animals, as well as those that can see, exist at the bottom 
of the ocean, while their nearest allies at the surface have well- 
developed eyes. Why have the deep-sea species lost their cyes ? 
The same question confronted us with regard to the cavern 
animals, and could not be answered even hypothetically. With 
regard to the deep-sea animals—more accurately deep-sea 
fishes—Dr. Giinther, in London, has lately made a remarkable 
attempt to explain the case, and although his views are as yet 
unpublished he has been so amiable as to communicate to me 
their most essential features. IIe has found, particularly 
among the deep-sea fishes brought back by the ‘ Challenger’ 
expedition, certain very peculiar forms, blind and not-blind; 
the latter have exceptionally large eyes, which seem espe- 
cially fitted to absorb pale phosphoric light in large quantities, 
while the blind fish, on the other hand, are distinguished by 
peculiar and sometimes colossal organs on the head, which have 
quite displaced the eyes, and which exhibit a very singular 
structure, that justifies us, according to Dr. Giinther, in assuming 
that they are peculiarly and strongly developed phosphorescent 
organs.23 Now these, in Dr. Gtinther’s opinion, may very 
possibly be used by their owner, as torches and other lights are 
used by fishermen, to entice and catch other fish. But, just as 
pivates are attracted by the hghts of fishermen and guided to 
their victims, so the light which these blind fish carry in the 
two lanterns on their head to attract their prey may be a 
beacon to their enemies, and at the same time be of assistance 
to such fish as can see, in their movements generally. Thus 
we can well understand that in the struggle for existence, which 


PIGMENT INDEPENDENT OF LIGHT. 87 


must of course have been carried on .among the various crea- 
tures on the floor of the ocean, every form having small eyes 
or small illuminating organs, being unable to see clearly or 
to give enough light, must soon have been exterminated, while 
none but the most extremely developed species could hold their 
own in the struggle. Newly introduced varieties must there- 
fore have been able to develope either larger eyes and keener 
vision, or else strongly illuminating organs, in order to escape 
annihilation. This evidently presupposes that the lantern 
fishes of the ocean-depths, being blind, must have other means 
for distinguishing and identifying the prey or the foes that 
approach them ; and this seems in fact to be the case, for from 
their proboscis or muzzle depend long feelers, beards, and the 
like, and at their tips or bulbous ends, organs of touch or of 
smell might easily be situated which could serve such a 
purpose. 

Special instances of the influence of light on animals. 
There are numerous special influences exercised by the different 
degrees of intensity of light or by its periodical changes on the 
different functions of the animal organism; but those only 
interest us which may now be regarded as directly connected with 
the fitness for life of a species under certain external conditions 
of existence. Thus we may entirely leave out of consideration 
the influence, for example, of red light on the formation of 
carbonic acid during respiration, the difference of the amount of 
carbonic acid exhaled by day and by night, and others; although 
these processes are of the utmost importance for the life both of 
the organs and of the animals. If we thus dispose of these and 
other similar effects of light, there remain two points which we 
must discuss ; the first being the presence or absence of pigment 
in the skin of the animal and the chromatic function, as it is 
termed. 

All animal pigments in the skin were formerly regarded as 
arising from the direct influence of light upon the skin, and, as 
a necessary corollary to this view, it was also asserted that the 
absence of light always prevented the formation of such pig- 
ments, or destroyed that which was already formed. The fact 
that the greater number of cavern animals and almost all 


88 THE INFLU“NCE OF INANIMATE SURROUNDINGS. 


ento-parasites are quite, or almost quite, white, appears a 
striking proof of the accuracy of this statement. Even as 
lately as 1870 it was asserted by the celebrated French député 
and physiologist, Paul Bert, that the larve of the well-known 
Axolotl (fig. 24) were incapable of forming pigment when they 
were brought up under the influence of yellow light, and he 
unhappily designated this absence of the epidermal pigment 
as ‘etiolation.’ This term, as is well known, has a fixed signi- 
fieation in the physiology of plants; it is exclusively used to 


Fig, 24.—Siredon pisciforme, the Mexican Axolotl. 


designate those cases of the absence of the green hue in plants 
which, having grown in the dark, have been checked in the 
formation of the chlorophyll-bodies, which are the organs by 
which they assimilate and elaborate their nutrition ; at the 
same time, as the light is no longer able to act asa check on 
their excessive growth, the leaves and stems become much 
elongated and acquire a yellowish-white hue, all of which phe- 
nomena can be easily observed in the shoots and leaves of 
potato tubers which have begun to sprout in a cellar. In the 


cases of so-called ‘ etiolation’ described hy Bert as occurring in 


CHLOROPHYLL AND PIGMENT. 89 


the larve of the Axolotl, on the contrary, in the first place, no 
abnormal growth was observed as a result; secondly, it must 
he strenuously disputed whether animal pigment is in fact 
capable of ‘ etiolation ;’ for it is certainly not, like chJorophyll 
in plants, an organ capable of decomposing carbonic acid 
under the influence of light. Thus the term was decidedly mis- 
applied by Bert to cases in which the pigment of the skin dis- 
appeared under any influence, whatever it might be; whether 
yellow light or the total absence of light was primarily the 
cause of the disappearance of the pigment which he mentions, 
is not clearly stated. We know that in plants all true pig- 
ment—not, that is to say, the green of chlorophyll nor the 
brown and red of xanthophyll, but the true yellow, red, and 
blue pigment of flowers—is formed just as well in perfect dark- 
ness as in broad daylight. Tulips, for instance, which are 
made to bloom in the dark, have a singular effect from the 
contrast between their brilliant colouring and the shapeless out- 
lines and pale yellow hue of their etiolated leaves. This holds 
good with regard to most, if not all, animals; they preserve 
their colour in spite of the more or less complete absence of 
light, as is proved by the undeveloped young of reptiles and 
butterflies, chicks, &c.; true deep-sea creatures which live at a 
depth of from 2,000 to 3,000 fathoms often exhibit?‘ colours 
quite as brilliant as those of animals living at the surface, and 
it is easily provéd by experiment that the larve of frogs or the 
tadpoles of newts develope their pigment quite as rapidly and 
perfectly whether they are brought up, from the time when 
they leave the egg, in full daylight or in absolute darkness. 
The earliest experiments on this subject with which I am 
acquainted are those of Mr. Higginbottom.*® Although he 
does not expressly declare that pigment is normally developed 
in the dark, it follows from the remarks he makes; and I can 
myself add the results of investigations pursued during two 
years, by which I have established that in the tadpoles of our 
common toads and frogs the pigment is equally well developed 
in yellow, blue, or red light and in absolute darkness. It is 
unnecessary to discuss these experiments in detail, for in every 
case where the other necessary conditions were at their optimum 


90 LM INFLUENCE OF INANIMATE SURROUNDINGS. 


the pigment of the skin of the tadpoles was normally developed 
in every kind of light, as also in the dark. 

Thus experiment here confronts experiment. It is not 
difficult to find an hypothesis to account for this. In none of 
the experiments hitherto conducted, not even those of Bert, 
were the heat-rays or the chemical rays excluded from the light 
falling on the young animals. It may have happened that in 
the darkness the little larvee were not supplied with supple- 
mentary or even requisite nourishment—in short, it would seem 
that the absence of pigment observed by Bert in the young 
Axolotl did not arise from the absence of light, but from the 
effects of some other cause as yet not ascertained, as insufficient 
or unsuitable food, the sinking or raising of the temperature, &e. : 
or it was perhaps a case of true albinism, and thus a form of 
disease. All who have bred the Mexican Axolotl are well 
aware that sometimes a white variety—not a true albino— 
suddenly occurs; but the cause of this variation is at present 
unknown. ‘Thus Professor Killiker of Wirzburg reared a 
whole family of these white Axolotls, which, with their blood- 
red gills, were very beautiful objects; while in my own labora- 
tory, where there is a much greater absence of light than in 
Kolliker’s, I have ag yet entirely failed in breeding even one 
white Axolotl, although during the last six years I have bred 
hundreds of individuals under the most various conditions of 
life. I am wholly unable to assign any plausible explanation 
for this difference, and it is the more striking because the six 
old specimens, from which I have now had at least six or seven 
broods, came originally from the same brood as those from 
which Kolliker has obtained so many white individuals. 
Finally I can but repeat my conviction, founded on these experi- 
ments, first, that we have as yet no suspicion even of the causes 
which sometimes determine the absence of the epidermal pig- 
ment in the Amphibia and other animals (as rats and mice, in 
which these unknown causes even become hereditary); and 
secondly, that this absence of colour is certainly not to be 
ascribed to the absence of light, since we know that animal 
pigment, like vegetable pigment, can be developed in total 


CHROMATIC FUNCTION. 91 


darkness, and in fact .is so developed normally in many 
animals.26 

In absolute antagonism to the old hypothesis which ascribes 
the origin of the pigment in the skin of animals to the direct 
influence of light, there is another which, under the almost 
supreme influence of Darwin’s theories, is now as generally 
accepted as the other was formerly. It is now almost uni- 
versally asserted that the colours of animals have arisen from 
either natural or sexual selection. We will postpone the dis- 
cussion of this view to a future chapter, in which the uses 
accruing to animals from their colours will be considered ; but, 
since it is proved by abundant evidence that at least one par- 
ticular kind of protecting resemblance—i.e. the adaptation of 
the colour of the skin of certain animals to the colours of the 
objects that surround them—depends on the influence of light 
through the medium of the eyes, it will be convenient to treat 
of it here. Pouchet applied the term ‘chromatic function’ to 
that adaptation of colour to the surroundings of the creature 
which is indirectly the result of sight, in order to distinguish it 
clearly from other cases in which—so far as we can at present 
tell—the distribution of colour is not influenced by light at all. 

The term ‘chromatic function’ refers neither to constant 
colouring, even when this causes a protective resemblance, nor 
yet to such variations in colour as are occasioned in Chame- 
leons and Cuttle-fish by physical irritation without any protec- 
tive resemblance being the result, The expression, which is not 
altogether a happy one, is new; but the fact it designates, that 
such protective changes do occur in many animals, has long been 
known. Inthe year 1830 Stark made a number of observations on 
the subject, on species of the genera Leuciscus, Gasterosteus 
(the fresh-water Stickleback), Cobitis barbatula, and the common 
Perch, Perca fluviatilis. All these fishes change colour with 
some rapidity, some in a few hours, others in from two to three 
minutes; and we know now that many splendidly coloured 
sea-fish have the same power, often in a quite extraordinary 
degree, as, for instance, species of Serranus. Shaw seems to 
have been the first to observe, in 1838, that such fish as are 
capable of changing their colour, apparently at will, must be 


92 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


more or less protected against their enemies by the resemblance 
thus caused between the colour of their skin and that of their 
surroundings. Similar observations were made by Agassiz, 
Ayres, and Storer, on the salmon of the United States, while 
European naturalists were the first to experiment on Amphibia 
which exhibit a similar power of assuming a protective hue. 
Finally, Heincke of Kiel has quite lately published a very 
careful description of the protective changes of colour?” in Gobius 
Ruthensparri, which exhibits the most conspicuous variations of 
colour that have as yet been described. 

Before entering on the discussion of those experiments by 
Lister and Pouchet on the chromatic function, by which we 
were first enabled to understand the observations above men- 
tioned, it will be advisable to describe the structure of the skin 


Fic. 25.—Section of afrog’s skin. ep, epidermis, including five pigment-cells; c, cutis 
with black star-shaped, deep-seated cells ; a and 6, a thick single layer of yellow pig- 
ment-cells close under the epidermis. 


and the mode of distribution of the pigments in it. One ex: 
ample—the skin of the frog—will suffice for all cases. The skin 
(fig. 25) consists of two distinct portions, the epidermis and 
the cutis. The former (fig. 25, e7) is entirely composed of cells, 
and the innermost layer contains cylindrical cells; the cutis is 
chiefly fibrous and encloses nerves, large cavities for glands and 
cell elements. These last are commonly filled with pigment, 
and the remarkable changes of colour in the frog’s skin depend 
entirely on the distribution of these highly ramified pigment- 
cells and their power of shrinking under certain kinds of irrita- 
tion. The pigment in these contractile cells—known as the 
chromatophores—is different in different individuals and in dif- 
ferent parts of the body, yellow, brown, black, sometimes even 
red or green. LBesices, the colour of the chroriatophores varies 


ACTION OF CHROMATOPIIORES. 93 


with the state that they happen to be in, and differs during 
contraction and expansion. Heincke, for instance, has shown 
that in Gobius Ruthensparri the chromatophores that are yellow 
or greenish yellow when distended become orange-coloured 
when contracted ; while the orange or red ones when shrunk 
become brown or even black. These (so to speak) active 
movements of the chromatophores were observed before by 
Lister, whose careful drawings of the chromatophores of a frog 
have been copied in the accompanying woodcut (fig. 26); it is 
hardly necessary to remark that the studies of the various 


Fic. 26.—Chromatophores from the skin of a frog, copied from Lister. a, wholly con- 
tracted ; band ¢, half relaxed ; d, wholly relaxed; e, wholly contracted, a capillary 
vessel ; f, g, k, expanded colour-cells or chromatophores. 

stages of contraction were made from the chromatophores of 

living animals, and in fact it is quite easy to repeat these obser- 

vations on the extended web skin of a frog’s foot. 

These chromatophores are distributed in the skin with a 
certain regularity ; in this particular, reptiles, fishes, and amphi- 
bians show hardly any —or no—difference. They usually occur 
in the cutis only, but sometimes they penetrate into the epider- 
mis, as, for instance, was the case in the section of the skin of a 
common frog, shown in fig. 25; butitis not known whether they 
then retain or lose their contractility. Sometimes the epidermis- 


9+ THE INFLUENCE OF INANIMATE SURROUNDINGS. 


cells are all supplied with pigment, as in many reptiles, but 
these certainly are not true chromatophores, and so are in- 
cipable of occasioning any change of colour in the skin ; but of 
course their constant hue must affect that of the skin gene- 
rally, as well as the marking produced by the more deeply 
seated chromatophores. The true chromatophores lie in dif- 
ferent layers in the cutis; close to the epidermis, light-coloured 
yellow cells oceur, beneath them the red or brown, and, in the 
deepest layer, the black. In some spots the pigment-cells of 
one kind or the other may be wholly wanting; sometimes the 
black ones form a close mass in one spot, while in others the 
red or yellow predominate, but very few spots are devoid of 
pigment altogether. It is on this distribution and stratification 
of the chromatophores and their alternate expansion and con- 
traction that the pattern (so to speak) depends which the frog’s 
skin displays at any given moment. If all the chromatophores 
are relaxed, brown or black will predominate, and in the spots 
where light-coloured chromatophores lie in patches their hue 
will be dulled; if they contract while the light ones are still 
extended, these latter will be more conspicuous. Heincke 
detected in Gobius Ruthensparrt yet another kind of chromato- 
phores, which were filled with iridescent crystals of marvellous 
delicacy ; they are visible, according to the degree of contrac- 
tion, as spots of metallic sheen, or are altogether invisible. 

The property on which the contractility of the chromato- 
phores depends is at present unknown, although various 
hypotheses have been suggested in explanation of it. 1t is of 
little importance for our purpose to learn which of all these 
antagonistic hypotheses will ultimately be proved to be the 
right one, since we know that all living protoplasm is essen- 
tially contractile, and moreover that all cells devoid of mem- 
brane—like the young ovum-cell, the white corpuscles of the 
blood, and others—sometimes possess this contractility in a very 
high degree. And the chromatophores belong to the class of 
cells without membrane ; hence we need not be surprised to 
find that they contract like other similar cells. 

It was formerly supposed that the exciting agent which gave 
rise to the contraction of the chroma!ophores must act upon 


IN THE PLAICE. 95 


them directly, so that variations in the intensity of the light, 
warmth, &c., could not produce contraction, or, on the other 
hand, expansion, unless they were under the direct influence 
of the rays. Some observations certainly have been made, 
particularly those of Wittich, which prove that in animals 
having the chromatic function (as the frog) the direct effect 
from light-rays is, in fact, perceptible in a small degree; but it 
is now definitively established that this is not generally the 
case, and that the changes of colour thus produced cannot be 
included under the term ‘chromatic function,’ since no adap- 
tation of colour to the surroundings is effected by them. Lister 
demonstrated, on the contrary, by his experiments on frogs, as 
long ago as 1858, that the activity of the chromatophores 
in cases of chromatic function depends solely on the healthy 
condition of the eye. So long as the eyes are in connection 
with the brain by means of the optic nerves, the light reflected 
from surrounding objects has a marked effect on the chro- 
matophores ; but, so soon as the eyes are destroyed, or the optic 
nerves are divided, the chromatophores also become totally in- 
capable of perceiving any variations in the intensity of light 
and colour ; thus the light reflected from objects can only affect 
the colour of the skin by the interposition of the eyes. 

These observations were subsequently repeated by Pouchet, 
who evidently was not aware of the preceding experiments, on 
Fishes and Crabs ; and he, like Lister, came to the conclusion that 
the irritation which excited the action of the chromatophores took 
effect only through the eyes and optic nerves, and not directly 
on the pigment-cells. Among the numerous new instances 
which he brought forward, some of them highly instructive, 
the case of a plaice observed by him is particularly interesting. 
These fish have, as is well known, a white side which constitutes 
the under surface, and a parti-coloured side which lies upper- 
most; this upper side exhibits the ‘chromatic function’ in a 
very igh degree. Among a great number of normal speci- 
mens of the species which, on a white sandy bottom, were 
also whitish or very pale-coloured, he met with one single 
dark-coloured fish in which, of course, the chromatophores 
must have been in a state of relaxation, and this specimen was 


96 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


as distinct from its companions as from the bottom of the 
aquarium. Closer investigation proved that the creature was 
totally blind, and thus incapable of assuming the colour of the 
objects around it, the eyes being unable to act as a medium of 
communication between them and the chromatophores of the 
skin. 

Up to this point Pouchet’s researches present nothing really 
new. But he proceeded to investigate the natural question : How 
and by what course is the impression received by the eye passed 
on from the optic nerve to the chromatophores located in the 
skin? Two modes of transmission are here possible: one by 
means of the spinal cord and the pairs of nerves distributed 
by it to certain sections of the muscles and skin—these are 
known as the spinal nerves; the other by two nerves running 
longitudinally close to the vertcbral column, the sympathetic 
nerves, as they are called, and which are closely connected 
with the spinal nerves and the brain. Pouchet detected and 
proved that the connection was not severed, and the chromatic 
function was not interfered with, if the spinal cord was completely 
divided close behind the brain, thus cutting off the first means 
of communication between the eye, the optic nerve, and the 
chromatophores. On the otber hand, the chromatophores lost 
their power of contraction completely if the two sympathetic 
nerves only were destroyed at the root. These, as before 
explained, are connected with the very finest nerves of the skin 
—which, it would seem, extend to the chromatophores-—by 
means of the spinal nerves which are given off from the spinal 
cord on each side at regular intervals. By severing the connec- 
tion of some of these with the sympathetic nerve of the same 
side, Pouchet succeeded morcover in limiting the chromatic 
function to those spots where the nerves remained in com- 
munication with the sympathetic; and he was thus enabled to 
produce at pleasure a zebra-like marking on one side of a fish, 
while the other side retained its natural hues and their normal 
variation according to the colours reflected from surrounding 
objects. In this way it was indisputably proved that the 
sympathetic nerve, and not the spinal cord, is the conductor of 
the optical stimulus which causes the motions of the chromato- 


THE RETINAL CURRENT. 97 


phores; and we may now venture to attempt to investigate 
how it is that an adaptation to the colour of surrounding 
objects can be etfected by these variations in the colour of the 
skin, that are only indirectly dependent on the light. : 
Professor Dewar has recently shown 8 that the different 
colours of the spectrum influence the eye and the retina in very 
different ways by producing an electric current which has been 
termed the ‘optic current.’ The intensity of this current, ac- 
cording to Dewar, is greatest under yellow light, weakest under 
purple light, and nil in total darkness. Of course we cannot 
directly compare the stimulus which is communicated from the 
rays of light through the optic nerves to the sympathetic nerve, 
and then by way of the spinal nerves to the nerves of the skin, 
and finally to the chromatophores, with this ‘optic or retinal 
current,’ because an electric current invariably takes the shortest 
road, which the nervous irritation above described certainly does 
not. But if we assume that the measure of the force exercised 
by the eye on the chromatophores may be approximately esti- 
mated by the force of the retinal current, an explanation of the 
phenomena of the chromatic function would be easily found. 
Every object reflects the light according to the nature of its 
colour ; black surfaces, when they are not too smooth, absorb 
the rays in the highest degree, red come next in order, and then 
yellow. White reflects nearly all the rays ; hence a black back- 
ground, reflecting but little light, will stimulate the eye in a 
very faint degree, and the excitation, analogously to the ascer- 
tained working of the retinal current, will apparently not be 
strong enough to occasion the contraction of the black chromato- 
phores ; these remain expanded, and give the skin a dark hue. 
If the light is reflected from a red or blue object, the somewhat 
stronger stimulation causes the black or brown chromatophores 
to contract while it does not affect the red or yellow ones; the 
animal then exhibits a reddish or bluish tint. The light 
reflected from green or yellow bodies produces a still stronger 
effect on the chromatophores, till a pure white light makes all 
the inmost layer of the chromotaphores contract, and the ani- 
moal is almost colourless. This explanation coincides perfectly 
with Pouchet’s observations, though Heincke certainly makes 


98 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


a few contradictory statements. He says that when a Gobius 
Ruthensparri is placed on a red bottom the yellow chroma- 
tophores shrink as well as the black ones, although the yellow 
contract less strongly than the latter; but, according to 
Pouchet’s explanation, the yellow chromatophores should hardly 
or never contract under a red light, since it is incapable of 
affecting even the red chromatophores. This indicates that 
there is still much to be done in this enquiry; and it is to be 
hoped that naturalists who take an interest in the subject and 
are in a position to make independent investigations will not 
suppose that it is exhausted even after the interesting and 
extended experiments of Lister and Pouchet. 

We must also guard against the idea that another question 
which is connected with this has been in any way answered : 
namely, that as to the first formation of the pigment in the 
chromatophores—a question which is often, but erroneously, 
regarded as identical with the other : How a particular mode of 
coloration, or rather of distribution of the pigments, is to be 
accounted for. This has, in fact, been fully explained by Lister 
and Pouchet in the case of chromatic function, but it is clear 
that the other question is not touched by it; for chromato- 
phores, t.e. dermal cells characterised by a rapid and peculiar 
contractility, must have existed before the contractions occa- 
sioned by the light reflected from surrounding objects could 
result in a useful function. The permanence and even the 
further development of the chromatic function in such animals 
as most required its protective effects is of course easily ex- 
plained by the principles of the Darwinian theory—by natural 
selection in the struggle for existence; but its first occurrence 
depends exclusively on the pre-existence of pigment in highly 
contractile cells. 

The contractile power of the chromatophores, however, offers 
no special difficulty, as has already been observed, since we 
know that protoplasmic cells, devoid of an enclosing membrane 
like those of the chromatophores, are universally endowed with 
this property ; any such cell, being a cell of the connective 
tissue of the cutis, might become a chromatophore, if pigment- 
granules were deposited in its protoplasm. Thus the only 


ORIGIN OF PIGMENT. 99 


final difficulty is the indispensable pre-existence of the pigment. 
Whence and how does pigment originate? Recent Darwinian 
views no more supply the answer to this question than the 
older theories of the origin of colouring-matter through the 
direct influence of light. It is incontestably certain that light 
alone cannot give rise to a pigment, as was formerly supposed ; 
and it is very probable that, even if the production of darker 
colouring sometimes seems to depend on the influence of light, 
it is to be attributed to the chemical rays or heat-rays which 
are always associated with light-rays. It is equally certain that 
all the peculiarities collectively which make animal pigments 
useful to the owner do not make their existence indispensable ; 
so that the chromatic function, in this special case, explains only 
the various arrangements and rearrangements of pigment already 
existing, but can throw no light on the obscurity which shrouds 
the existence of these chromatophores, however great the utility 
they may acquire, and undoubtedly possess, by the nature of 
the different pigments they may contain and by their distribu- 
tion and dependence on the eye and the optic nerve. 

The question remains equally unsolved with reference to 
all other kinds of animal colouring. They may, as in the 
chromatic function, be elicited and influenced by the indirect 
action of the light, or they may, as is now very generally as- 
sumed, have originated by natural or sexual selection ;?° but 
these causes are still inadequate to the production of the pig- 
ment itself, when we think of its origen irrespective of its 
distribution. The cye was not formed by the faculty of sight, 
although, when once it was formed, it was largely modified by 
the function; the eye must have existed before it could be 
used. The same is the case with regard to the pigment. I 
lay some stress on this comparison, because it is so common to 
find it stated in popular treatises, nay, often enough in scientific 
works, that this or that colour is the result of selection or of 
adaptation, the word ‘colour’ being no doubt used by many of 
the writers instead of the more correct expressions—colouring, 
pattern or arrangement of colour. The answer to this ultimate 
query—How the pigment was first formed—cannot at present 
be given; and although many experiments and observations 


100 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


have already been made which indicate the possibility of an 
early solution of it, they are at present far from being perfect, 
enough for us to discass them in this place. One thing only 
may here be briefly observed. If the Darwinian principles are 
indead the true ones, we must assume that the pigment itself— 
not by its variable distribution—together with its subsequently 
acquired utility in the maintenance of the species by the 
selective influence of the conditions of life, must either have 
some direct primary function in the normal life of the indi- 
vidual, or else be the inevitable secondary product of some in. 
dispensable physiological process. In a few rare cases this last 
is known to be the case, and they have been classed by 
Darwin as cases of correlational colouring, But we may hope 
that the time is not far off when the presence of every kind of 
pigment will be as intelligible to us—as easily referred, that is, 
to definite causes—as are certain variations of colour, which, 
under the chromatic function, are now recognised as being 
directly and absolutely dependent on the effects of light on the 
eye of the animal.*° 


CHAPTER IV. 


THE INFLUENCE OF TEMPERATURE. 


THE sun, the source of light, supplies other powerful stimuli 
to organic life on our globe. All the heat which influences 
the development and continuity of life either is now, or formerly 
was, derived from the sun, in whose rays light- and heat-rays 
exist in combination. The influence of the heat-rays only, on 
animal life and on its distribution on the globe, will form the 
subject of the present chapter. 

It must be almost superfluous to bring forward any special 
facts to prove that heat, or the degree of temperature at a given 
time, has a marked influence on the life of animals and on their 
vital functions. Everyone knows that perspiration, i.e. the 
action of the sweat-glands in the skin, increases as the tempera- 
ture rises ; and that a considerable heat must be kept up if a hen’s 
egg is to be perfectly developed or hatched. The heat which 
reigns during summer in the Eastern States of America, in 
Madrid, Naples, and other places, is often intimately connected 
with fatal epidemics, nay, sometimesis productive of them. Most 
Europeans become indolent and slothful if they are forced to 
pass the hot season between the tropics or even in Naples or 
Madrid. The approach of winter, on the other hand, is equally 
perceptible, and it may be confidently asserted that many millions 
of human beings make their livings and support themselves 
solely by the indirect results of this transition from summer 
warmth to winter cold, otherwise frequently so injurious. If, for 
instance, we suppose that the thirty or forty millions of human 
beings who pass through the severe cold of an American winter 
were by some means relieved of the necessity of buying, say every 


102 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


three or four years, a winter overcoat, the necessity for a certain 
annual expenditure of a sum of certainly not less than a hundred 
thousand dollars wouldalso be removed, and an enormous number 
of the population would be thereby deprived of the means of 
existence. Animals, however, under these circumstances behave 
very differently ; the lower animals, and above all those that 
live in the sea, are far less dependent on variations of tempera- 
ture than man and the warm-blooded animals; nevertheless 
the very simplest of all forms, the Protozoa, are dependent on 
warmth in a very remarkable manner. 

Many of my readers have no doubt been at some time 
obliged to experience some degree of acclimatisation to a tropical 
climate. Everyone who has passed some time between the 
tropics knows that sooner or later he got accustomed to the 
higher temperature, and at the same time probably lost the good 
appetite which he enjoyed in his colder native country. He 
will also have observed that the effect of a high temperature 
on the action of the sweat-glands and kidneys was different 
from that of a colder climate. A European sees the natives 
of a tropical country turn drowsy or shiver with cold at a 
degree of temperature which, though low in those regions, would, 
in his own country, have made him wish to fling off his gar- 
ments and plunge into icy-cold water ; but a prolonged residence 
in the hotter climate will gradually accustom him to the sensi- 
tiveness of the natives to small variations in temperature. The 
natives of Port Mahon in Minorca were excessively astonished 
at seeing me and two other Germans bathe regularly in the 
sea in the month of September, although the temperature of the 
sea-water of the harbour where we bathed was certainly not less 
than 18° centigrade, and very probably more. 

These few facts may suffice; I should not have mentioned 
them if it had not seemed desirable to make the reader familiar 
at once with the idea that the influence of temperature on 
animals depends not merely on the absolute degree of heat 
experienced, but on the variations of temperature to which every 
animal, almost without exception, is exposed in the course of its 
life. The above-mentioned facts prove too that animals are 
capable of enduring the effects of change of temperature and to 


MEAN TEMPERATURE. 103 


adapt themselves to it without any change of structure being 
the inevitable result, although such changes are clearly recognis- 
able in function ; and, in the third place, that the same degree 
or variation of temperature affects different organisms in 
different ways. We might perhaps be disposed to assume, on 
the ground of theoretical conjecture, that all the animals living 
together in the same climate must be affected in the same 
manner by the normal variations of its temperature; but such 
an assumption would be, as everyone knows, altogether false. 
On the contrary, well-known facts tend to show that there are 
enormous differences in this respect, and the same facts teach us 
at the same tine, that the well-being of animals living in associa- 
tion depends far more essentially on the variations and extremes 
of temperature than on the absolute degree of heat to which 
they may be simultaneously exposed at any given time. 

The results thus laid down, somewhat dogmatically perhaps— 
but the reasons on which they are founded will be given pre- 
sently—justify us (only hypothetically, it is true, for the pre- 
sent) in denying the value frequently attributed, even quite 
lately, to the curves of temperature as constructed by meteoro- 
logists. Annual isothermal (isochimenal or isotheral) curves 
are constructed by estimating the mean temperature for the 
days first, then for the weeks, months, seasons, or for the whole 
year; but these curves calculated from mean temperatures are 
in truth, if of any, only of very small importance to the matter 
in hand, Thus, for instance, it is certain that a given degree 
of temperature, conceived of as absolute in its effects, will have 
a favourable effect on one animal, while on another it is less 
favourable or even injurious. Now the mean temperature of 
a day, as calculated by the meteorologist and: assumed as the 
basis of all his curves, can afford no standard by which to 
measure the influence of the heat during that day, since it is 
not the same as any of the different temperatures observed 
during the day, and that mean of temperature may be the re- 
sult of very dissimilar extremes. A pond-snail is developed, 
lives, and feeds best in a mean temperature of about 20° centi- 
grade ; but this, as a daily mean of heat, might be the mean of 
two extremes lying far apart from each other. This water-snail 


104 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


does not assimilate its food—that is to say, digest and grow— 
till the water has attained a warmth of from 14° to 15° centi- 
grade, and it entirely ceases to do so when the water has 
reached 30° to 32°. Hence it is ylain that the pond-snail is 
for a long time less favourably circumstanced than a bird or 
mammal living near the pond or in it, and close to the snail ; 
for they, like all warm-blooded animals, can carry on the pro- 
cess of digestion even when the temperature falls below the 
freezing point or rises above 36° or even higher ; the Lymnza 
meanwhile is materially checked in its growth, or even killed. 
Nevertheless the calculated mean daily temperature may be 
identical with that which would afford the most favourable con- 
ditions for the pond-snail. ence it is evident that a classifi- 
cation of animals according to the climate in which they live 
in fortuitous community—as those of the tropical, temperate, 
and frigid zones—has no real value, and is simply an expression 
of the fact that different animals live in different climates. All 
such divisions founded on the terminology of meteorology serve 
only to conceal the true relations of animal life to the tempe- 
rature that influences it, and consequently cannot be regarded 
scientifically as either accurate or useful. I therefore shall 
suggest another method of classification which agrees better 
with the nature of the relations that can be proved to subsist 
between animals and those variations of temperature to which 
they are subjected.?! 

Almost all animals are exposed within the course of a day 
to more or less considerable changes of temperature. If we 
assume, as we are justified in doing by certain observations 
that shall presently be communicated to the reader, that there 
is a certain degree of heat (which need by no means be identical 
with the meteorological mean temperature of the day) which is 
most favourable to the well-being of one or of several species 
of animals, obviously every rise or fall of temperature above or 
below this favourable point must be toa certain extent injurious 
to the creature. The interval between the daily extremes may 
be great or small without any alteration in the daily meteoro- 
logical mean ; moreover, the favourable temperature—the opti- 
mum of temperature for the animal—may either coincide with 


EURYTHERMAL AND STENOTHERMAL. 105 


the meteorological mean or lie nearer to one of the extremes— 
the maximum or-the minimum—than the other. We have here 
assumed that the optimum is the same forall animals; but they 
may nevertheless be very differently affected by the variations in 
temperature according to the degree of the variations themselves. 
Then those animals which can endure the greatest variation in 
the direction of either extreme evidently exhibit a certain 
contrast to others which can only thrive under very small 
departures from the optimum, and their distribution must 
depend essentially on this characteristic. Certainly the distinc- 
tion thus indicated cannot be regarded as absolute; but we 
shall nevertheless do well to avail ourselves of it asa means 
of classification, and to designate animals, according to Mébius,** 
the former as ewrythermal, the latter as stenothermal. 

We also know that the optimum of temperature may be 
extremely different for different animals, since some exist near 
the poles and others on the equator, some live on ice and some 
in hot-water springs. Nevertheless, a rise of temperature above 
the optimum must in either locality influence the animal in an 
analogous manner, whether it dwell on the ice of the North Pole, 
or at the summit of a high peak, or on the scorching plains of the 
tropics ; and a fall of temperature below the optimum must 
likewise produce analogous, though not identical, phenomena. 
Hence we may, in a certain sense, assume the optimum of tem- 
perature as being the same for all animals as a basis for our 
discussion of the question: How do variations of temperature 
affect the animal? and thus we may divide this chapter into 
three sections, the first dealing with the effects of a falling 
temperature, the second with those of a rising temperature, 
and the third with those of an equal temperature, and at the 
same time with the dependence of animal life on these three 
conditions—the term ‘equal temperature’ excluding any great 
changes whether of rise or fall. 

I. The influence of a falling temperature on animal 
life.—This influence may exhibit itself in many different ways. 
A small fall in temperature may be as injurious to one animal 
as a great fall to another, while a third species may be wholly 
unaffected by either. Animal life is often destroyed before the 


6 


y 


106 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


freezing point is reached, while some animals can endure even 
to be actually frozen up without being killed, their vital powers 
merely becoming latent. Sometimes the effect of the reduced 
temperature induces modification of the functions only, some- 
times it leads to changes in organic structure. Unfortunately 


Intervals between two Contractions. 


In Sec. 2 4 6 8 1012 14 16 18 20 22 24 26 28 30 32 34 degrees centigrade. 
si iT Bi 
T? 


2 

4 

6 pe 
ae 


54 at Ht 


58 [tH <a 


co wh — 


62 l i ele 


Fig. 27.—The mean thermal curves detected by Rossbach for the contractile vesicle of 
Infusoria. I. For Euplotes Charon, II. Por Stylonychia pustulata, III, For Chilodon 
cucullulus, The animals are shown in fig. 28., 


very few experiments on individual cases have been made 
which elucidate this branch of the subject, but we will investi- 
gate these more closely. A reduction of temperature which 
never falls so low as the freezing point of fresh water may still, 
under some circumstances, lead to the suspension of various 


EFFECTS OF REDUCTION OF HEAT. 107 


animal functions. Rossbach proved that the rhythmical con- 
tractions of the contractile vesicle in Infusoria are very remark- 
ably affected by a low temperature. He found by very carefully 
conducted experiments (fig. 27) that the pulsations of these vesicles 
were generally carried on most regularly and rapidly in a tempe- 
rature varying between 15° and 25° centigrade. But the lower- 


Fic. 28.—The Infusoria observed by Rossbach. ua, Chilodon cucullulus ; b, Euplctes 
Charon; c, Stylonychia pustulata, Highly magnified. 


ing of the temperature to 5° ahove freezing point (5° centigrade) 
has a quite different effect on the different species ; thus, at this 
low temperature the vesicle of Chilodon cucullulus contracts 
seven times in a minute, that of Stylonychia pustulata only three 
times, while at 15° there are fifteen pulsations per minute in 
each. At 10° above zero* the length of one pulsation in 


* Zero, in the centigrade thermometer, is freezing point; 100° the 
boiling point of water at the sea-level. 


108 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


Chilodon cucullulus is seven seconds, in Euplotes Charon forty- 
eight. We see at once by this that the active life of the animal 
is not suspended even at so low a temperature as 5°; for although 
the pulsations succeed each other more slowly than at a higher 
temperature, they occur regularly, and the contents of the vesicle 
are discharged with the same regularity. But if the temperature 
is still further reduced to 3° or 2° above zero centigrade, the pul- 
sations of the vesicle, as well as all the movements of the various 
members of the creature—its cilia, bristles, and so forth—cease 
entirely ; a condition of the animal protoplasm supervenes— 
which must not be mistaken for death, though it frequently 
precedes it—to which Rossbach has given the appropriate name 
of ‘chill-coma.’ If this chill-coma of the Infusoria is not too 
long continued, the creature may be revived by raising the 
temperature ; but if it is long continued, or if the temperature 
is still further lowered, the animal finally dies. 

Our common pond-snail, Lymnea stagnalis, offers a very 
interesting example of the influence of cold on the organic 
functions. JI found by experiment that this animal, when 
young, first begins to assimilate food, and consequently to grow, 
when the water is at about 12° centigrade; at the same time a 
temperature much below that which induces chill-coma in the 
Infusoria has no injurious effect on the animal’s life, though it 
entirely prevents its growth. Indeed, observations have been 
made which seem to prove that the Lymnaea may be quite 
frozen up without being killed. This molluse grows at a very 
moderate rate; individuals brought up even under favourable 
circumstances take about three months to develope a shell 
twenty-four millimétres long, and they do not attain their 
full size under two years, although the whole life of the in- 
dividual can scarcely exceed three or four years at the utmost. 
Assuming that a young Lymnea were placed ina lake or stream, 
of which the temperature constantly exceeds the minimum at 
which the snail can begin to grow, during only two months of 
the year, while it never perhaps reaches the high optimum, 25°, 
the molluse will be unable to attain its due proportions during 
the first year, or to grow to its full size even during the second, 
and thus a dwarfed form will inevitably arise. This dwarfed 


ON INFUSORIA AND LYMNAA. 109 


form will still be able to reproduce and multiply itself; for the 
maturation of germinal matter—the ovum and sperm—takes 
place during the winter and early spring, at a time when the low 
temperature of the water hinders all growth, and the optimum 
of warmth for the sexual processes is much lower than that for 
growth. Thusa permanently diminutive race 3 might arise if the 
conditions of temperature above described remained constant for 
several successive years in the lake or stream where the young 
molluscs or the eggs have been deposited. Hence it has been 
supposed, and in many cases no doubt with justice, that the 
dwarfed races of animals which are found on high mountains or 
in the polar regions, where they must meet with the conditions 
of temperature just described, have originated directly from the 
low temperature hindering their growth. This assumption, as 
is quite evident, perfectly accords with my experiments on 
Lymnea ; still it must not be forgotten that other influences 
might have precisely the same effect, such as insufficient food 
and other circumstances, which will be discussed in the next 
chapter. 

Thus we have seen that a degree of cold nearly as low as 
the freezing-point of fresh water kills Infusoria, but not the 
Lymnea; also that their vital activity, as shown by the con- 
traction of the pulsating vesicle, begins at 4° above zero, while 
assimilation, and with it the other organic functions, only begin 
in the Lymnea at 12° above zero, although the optimum of 
temperature which favours the highest vital activity and de- 
velopment is almost identical for both, viz. 25°, The enormous 
difference in the powers of resistance to a fall in temperature, 
possessed by different creatures, which is proved by these ex- 
amples (and by many others which need not be particularly 
mentioned), shows that its influence is not absolute, 7.e. cannot 
equally affect all the animals exposed to it; but that the ex- 
tinction of certain animals under secular refrigeration in either 
hemisphere of the globe must have depended in part on the nature 
of the animal itself. 

This reaction against the influence of cold is in the highest 
degree dissimilar in different species of the same family. A few 
special phenomena illustrating this, particularly those of hyber- 


110 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


nation or winter-sleep, and of resuscitation after being frozen 
up, must be briefly examined. 

It is universally known that there are forms among the 
Vertebrata, as well as the Invertebrata, which are distinguished 
from all their congeners by the circumstance that they are 
lulled to sleep by the falling temperature at the beginning of 
winter; they sink into a state of rest of longer or shorter 
continuance, during which their active life, if not totally sus- 
pended, is reduced toa minimum. My two tame Prairie-dogs, 
Hans and Gretel, feel the effect of a falling temperature, when the 
thermometer is only down to 7°-10° centigrade; the Amphibia 
of our latitudes first perceive it when the temperature is very 
little above the freezing-point of water, while in Cuba they 
succumb at degrees lying between 7° and 24°; the common 
continental vineyard-snail, Helix Pomatia, does not throw off 
the calcareous lid which protects it during its four-months 
sleep till the temperature by day has reached about 10°-12° centi- 
grade. In tropical countries, various animals, as snakes, lizards, 
&c., fall into a state of chill-coma, precisely resembling a 
winter sleep, at a temperature far above that at which the hyber- 
nating animals of northern latitudes *4 are still quite active— 
another proof of the truth of the statement that it is the depar- 
ture from the optimum, not the absolute high or low tem- 
perature, that affectsanimals. In the Philippine Islands I have 
frequently found snakes under stones early in the morning, 
that were quite stiff, though the temperature of the soil under 
stones, or protected against radiation by the shade of forest 
trees, was never below 16°-18°. It may perhaps be here ob- 
jected that, for this very reason, the winter-sleep of our animals 
cannot be directly compared to the chill-coma of those living 
between the tropics; but I think this objection ill-founded. 
Certainly the temperature in tropical countries never remains 
for any length of time, for days or even weeks, so low as to 
induce an unbroken or prolonged chill-coma during such 
periods. But it is obviously of small consequence whether the 
effects produced hy the lowered temperature are of long or 
short continuance; even among our bybernating animals the 
daration of their sleep varies considerably, but the general 


WINTER-SLEEPERS. 111 


identity of the imfiuence exerted by a reduction of temperature, 
both in high latitudes and equatorial regions, is not to be dis- 
puted. This consists, as we have seen, in the fact that every 
reduction of temperature below the optimum, whether that 
optimum be high or low, so diminishes the vital energy of 
many, though not of all animals, that they gradually fall asleep, 
and remain in a condition resembling sleep as long as the low 
temperature, which induces that condition, lasts. 

In general, warm-blooded animals seem to be protected 
against the effects of a reduced temperature by their power of 
keeping up, by combustion, the warmth indispensable to their 
existence. But they do not all escape its influence nevertheless. 
Everyone who has lived for any length of time between the 
tropics, and has observed the people there, knows that the natives 
become excessively sleepy under a sudden and great diminution 
of warmth ; and that the most inveterate winter-sleepers belong 
to the warm-blooded mammals is equally well-known. It is in 
no way surprising to learn that a cold-blooded snail, molluse, or 
frog becomes lethargic during the winter, because the warmth of 
their bodies always exactly or very nearly corresponds with that 
of the surrounding medium, air or water, and because they are 
not fitted by their internal structure to produce such warmth as 
is indispensable for the energetic vital action of their organs ; 
hence they’ must gradually subside into sleep, ¢.¢. a torpid 
condition of life, as soon as the temperature of the air or water 
falls below the point where their powers of assimilation begin. 
But we may well feel surprise at seeing that a warm-blooded 
animal, whose body is constantly maintained by internal 
processes at the high temperature of from 36° to 38°,35 ig 
nevertheless incapable of resisting the lowering and soporific 
influence of cold. With regard to this point a fact of the 
greatest importance has lately been discovered. We have learned 
from tolerably numerous observations that in several hybernat- 
ing mammals, which have been examined with this view, the 
temperature of the body is very considerably lowered during 
their winter-sleep. The lowest temperature hitherto detected 
in such a creature is 2° centigrade in the Zizel, Spermophilus 
cisilius, according to Horvath’s researches. To this naturalist *¢ 


112 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


we owe, too, the far more interesting observation, that the Zizel, 
when lying in its winter sleep, always has the same, or nearly 
the same, degree of warmth as the surrounding air. In one case 
the temperature of the room was 2° above zero, and a thermo- 
meter inserted in the rectum marked exactly the same degree ; 
in another experiment the animal was sleeping in a room, at 
about 9° to 10°, for several days, and its body (in the rectum) 
was at 84°. This shows that during their winter sleep warm- 
blooded animals become truly cold-blooded: at any rate this 
is true of the Zizel, since its temperature corresponds with 
that of the surrounding atmosphere. Other facts proved by 
Horvath’s elegant experiments, and of which the interest is too 
specially physiological for mention here, I have quoted in 
Note 36, for they appear to me to deserve to be more widely 
known than they have hitherto been. 

A second point of more general interest is the great power 
of resistance to extreme cold manifested by many animals, and 
their power of enduring even to be frozen up without injury to 
their vital functions. In this respect the cold-blooded have 
certainly the advantage over the warm-blooded races. Man is 
compelled to supply his deficiency of natural protection against 
severe cold in the most various ways, or to borrow them of 
furred quadrupeds ; a rabbit will infallibly die if the heat of 
its body is reduced to 15° centigrade. Thus this rodent, whose 
normal body-temperature is about 31° to 32°, needs only the 
comparatively insignificant reduction of sixteen degrees to put 
an end to its life; and it is to be assumed that, with the 
exception of the winter-sleepers, the same would be the case 
with all mammals. But it is different with the cold-blooded 
animals, Their body-temperature, as we have seen, is always 
exactly or nearly the same as that of the surrounding medium, 
and rises and falls with its varlations.*7 But their powers of 
resisting a considerable amount of cold are nevertheless very 
various, and even the same individual, at different stages of 
its development, differs greatly in this respect. Frogs and 
toads can bear to be frozen up, or even endure a degree of cold 
approaching the freezing point, only when fully grown; many 
fishes, as the salmon, can do so, both as embryos in the egg 


EFFECTS OF FREEZING. 113 


and when fully grown; it is well known that salmon’s eggs, 
with the embryo in them, are conveyed in ice even to America 
and Australia; other animals, again, can bear such cold only 
in the egg state. Many insects perish in the winter, while their 
eggs survive, and the embryo within the egg of many insects 
cannot be frozen even at an extremely low temperature. The 
so-called winter eggs of many of the lower crustaceans— 
Daphnide for instance—and the germs or statoblasts of Bryozoa, 
or of the fresh-water sponges, resist any degree of cold, while 
the fully grown individuals regularly perish in the autumn, 
apparently from cold. So far as I know, no explanation has 
yet been given, or even sought for in the right way, of the 
fact that the soft contents of so minute a body as the egg or 
germ of an invertebrate animal cannot be frozen so long as 
it remains enclosed in its firm but always extremely delicate 
capsule. 

With regard to the capability manifested by many animals, 
or even certain separate organs, of enduring to be frozen up 
without having lost their vital powers in the smallest degree after 
they are thawed, many observations have been recorded, but 
hardly any, if any, thorough and complete series of experiments. 
The statements that have been made are often nothing short of 
astounding. Thus it has been said that frogs and toads do not 
die even when so completely frozen that their skin, muscles, and 
bones can be broken up into fragments. The extremely delicate 
marine Naked Mollusca are said, like many other Mollusca, to 
endure freezing in ice without injury, and excised portions, as 
the heart, muscles, or nerves, to undergo the same treatment 
without losing their functional powers. But, on the other side, 
Pouchet declares, on the ground of various experiments, that 
actual freezing infallibly kills separate members, and perfect 
individuals too; for according to him the corpuscles of the 
blood are in the first instance destroyed by freezing, and these 
dead elements act as a poison after the individual is thawed, 
killing the animals, or, as the case may be, the organs. This 
view, if not actually contradicted by Horvath’s researches, is 
rendered improbable, to say the least ; for from them we learn 
that a frog whose legs have been killed by freezing lives on all 


114 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


the same, and is not poisoned by the introduction into its cir- 
culation of the dead corpuscles from its frozen legs. From 
Pouchet’s point of view it might be answered that the amount 
of poison supplied by the freezing of the legs was insufficient to 
do permanent injury to the frog after they were thawed. But 
the question is not one of any present moment. 

At any rate, the results of Horvath’s experiments, which 
agree with Pouchet’s, as to the deadly effects of freezing, make 
the various observations as to the revival by thawing of frozen 
animals in the highest degree improbable. Horvath showed 
that a frog, or a portion of a frog, infallibly died when frozen in 
a temperature of the surrounding medium (water or quicksilver) 
of 5° or more below zero C. ; whereas nerves, muscles, and hearts, 
after being reduced to a temperature of from 0° to 4° below zero C. 
or even actually frozen, are said to be capable of renewed activity 
when thawed again. From this we may infer that the essential 
part of muscular fibre or nerve-tissue, on which their functional 
activity depends, is not brought to its freezing-poimt even when 
reduced to a temperature of 5° centigrade. But we cannot 
extend this conclusion to whole animals, for we do not know 
whether other parts of the body, as, for instance, the Llood or 
gland-cells, might not freeze at a still higher temperature and die 
in consequence, and then the whole animal would undoubtedly 
be killed. However, exact estimates of the internal heat of 
animals thus hard frozen are not at hand; it was concluded 
at once from their stiffness that they were frozen through 
and through, without the least consideration of the fact that 
the freezing-point of the different juices found in the body may 
vary greatly, and that consequently, even in apparently hard- 
frozen animals, those parts may not be actually frozen on whose 
properties their resuscitation by thawing essentially depends. 
Horvath’s experiments ought to have been repeated, in a com- 
prehensive way, on uninjured animals before a positive opinion 
could be formed on earlier observations.?8 

Meanwhile these as well as Horvath’s experiments show 
that there are animals which can bear to be frozen up, and even to 
be in part actually frozen. From this a latitude in their power 
cf resistance to variations of temperature is presumable, which 


WINTER COLOURING. 115 


must be of the greatest importance in considering the question 
as to how far secular variations in the distribution of tempera- 
ture on the surface of the globe may have acted as a selective 
influence on the animals peopling it. For instance, the selec- 
tion from species now living, which might possibly at some future 
time recur as the result of a considerable reduction in the tem- 
perature of our latitudes and the introduction of new species 
from high latitudes—the fauna of the ice period—might be 
accompanied by modifications in the structure of the individuals 
of such species as survived the change. 

Now, so far as I know, no single investigation proves 
that the coarser main structure of the different organs of 
animals can be modified by changes of temperature. On the 
other hand, many modifications in the fur and colouring of 
mammals, and in the colouring of birds and insects, may with 
good reason be referred to the direct or indirect influence of a 
reduced temperature ; as regards insects, indeed, this effict has 
been positively proved by experiment. (Another effect of a 
falling temperature, namely, its influence on the production of 
eggs, will be treated of later.) 

It has been recently asserted, on the contrary, that the 
change of colour in winter, which occurs regularly in many 
mammals and birds, is a result not of cold, but of selection. I 
must confess that I do not understand how such a conclusion is 
arrived at. It is self-evident that selection per se cannot 
possibly modify colour, ¢.e. the pigment itself, in the smallest 
degree ; the causes which result in a brown fur becoming white, 
more or less quickly, must undoubtedly be of a different cha- 
racter. It is well known that the whitening of the hair in man 
is usually a sign of advancing years, that sometimes it occurs 
in early life, and that this peculiarity is often hereditary, and 
that occasionally violent and sudden agitation of mind will 
cause it within a few hours; in all these cases the efficient 
causes are perfectly dissimilar. But of the nature of these 
causes we are wholly ignorant; and we may, though with 
caution and reserve, express the view that the fall of tempera- 
ture at the beginning of autumn may somehow produce an 
effect, direct or indirect, on the pigments situated in the skin. 


116 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


On the other hand, it is difficult to understand how a race of 
brown animals can be gradually transformed by selection into a 
variety which always turns snow-white in winter. Granting 
that a brown weasel could, by any external or internal cause, 
be changed during the winter into a brown and white spotted 
one, this weasel would not have the smallest advantage over 
the brown one in consequence of the white mixture in its fur, 
for it would be quite as conspicuous asa plain brown one in 
the pure white of the snow, perhaps even more so. That a 
white variety should arise from a gradual increase of the white 
patches in the piebald fur is not to be thought of. It might 
indeed be possible that a selection should be effected, if a pure 
white variety were at once and from the first produced from 
the animals which first exhibited this modification of their 
summer colouring, since these, like the nearly white ones, 
would in fact enjoy an essential advantage over the brown or 
spotted ones. But, even then, selection would not bave pro- 
duce this pure white winter colouring by the cumulation of 
small and useful variations, but have chosen between the two 
varieties offered, the white and the brown ; and the question as 
to the origin of the winter colouring is still unanswered. The 
same arguments hold equally good for all pure white varieties, 
whether in arctic regions or on the ice and snow of high peaks 
in mountain chains; these, like all other species that turn 
white in winter—it would be superfluous to enumerate them 
here 39—cannot have preserved their winter colouring by means 
of selection ; whereas, no doubt, after other causes, unknown 
to us, had in the first instance given rise to a constantly white 
variety, or to one white in winter and brown in summer, this 
may have been secured by the rapid extirpation of the less 
well-protected brown or spotted varieties. It is thus that 
Wallace accounts for the occurrence of white species in northern 
regions, but he does not even refer to the question as to how 
the white hue originated. 

The nature of these causes is in fact unknown; it was pro- 
bably an error to assume that in all the above-mentioned cases 
this whiteness, 2.e. the absence of the pigment, was the direct 
effect of the winter cold, or of the low temperature of the polar 


EFFECTS OF A RISING TEMPERATURE. 117 


regions or of the eternal snow-fields of high mountain peaks. 
In one single case only can we assert with certain y that the 
winter colouring of un animal may be referred to the direct in- 
fluence of the reduced temperature in autumn. Professor Weis- 
mann in Freiburg, to whom we are indebted already for many 
facts established by bis elegant experiments, has proved that 
two varieties of butterfly, long regarded by entomologists as 
distinct species, are in fact only the summer and winter forms 
of the same species of Vanessa (Vanessa prorsa-levana), for 
he succeeded in rearing the winter variety (Vanessa levana) in 
the summer season, and from a summer brood, by keep- 
ing the air in which the caterpillars and pupe lived at an 
artificially lowered and regular temperature.4° It is much to 
be wished that zoologists would more frequently carry out 


Fic. 29.—Desoria glacialis, the Glacier Flea. 


similar experiments; for if this were done, I have no doubt that 
a far more extensive influence of cold on animals would soon be 
recognised, and its limits more accurately defined than is at 
present possible. 

II. The influence of a rising temperature on animal life.— 
Variations in temperature below the freezing-point of the fluids 
contained in the body can obviously have no effect on those 
animals, or on their peculiarities, which either die when the 
cold reaches that point, or whose vitality becomes latent. It 
will matter little to a perfectly frozen frog whether it was 
frozen at 5° below zero or at 10° or 12° above. It is not until 
the water surrounding the creature, and the fluids contained in 
its tissues, recover their fluidity under a thaw, that any further 
rise of temperature can affect its vital activities. For although 
many animals can live on snow and ice, or even in ice—as the 


118 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


e 


glacier flea (fig. 29), Degeeria nivalis, two species of Sminthurus, 
Chionea araneoides, &sc,—their internal temperature is not that 
of the ice; nay, most of these creatures belong to the warm- 
blooded races, who themselves can supply the deficient warmth. 
Besides, the temperature of the air at the surface of the ice or 
snow is by no means always below the freezing-point ; on the 
contrary, often considerably aboveit. The fact that animals live 
on or in the ice must not be taken as proving that an active 
animal life is generally possible at temperatures below zero, or 
that a rise in the temperature of the air or water from —10° to 
—5° can be of any importance to animal life. A rise of tempera- 
ture above the freezing-point of the water and of the juices of the 
body must, on the contrary, have much effect on the animals ; 
but, as we have seen, it has by no means the same effect on all 
the anima's that live together in one spot and exposed to the 
same rise in temperature. 

The effect of a rising temperature on animals depends of 
course on their nature; as this differs, so will they be differently 
affected. This is familiar to all, and easily observed. Some 
animals rouse from their winter sleep earlier than others ; some 
at a low temperature remain rigid in morning torpor, while 
others, roused to sexual activity, display their charms, and are 
already busy, laying their eggs or bringing forth their living 
young. In short, from the very freezing-point, a rising tem- 
perature begins to exert its stimulating influence on the vital 
functions of every living thing up to the point where these 
functions are at the highest possible stress under the optimum of 
temperature, which, as is proved by the above-mentioned facts, 
is different for every animal. If the heat rises above this 
optimum, the effects are reversed; functional activity is more 
and more reduced, till at last a sleep-like condition or heat- 
coma precedes death, which ensues wnder too great heat. 

The optimum of temperature varies not only according to the 
species, but in every individual, nay, in every organ of every 
individual, The best, or at any rate the best known, example 
of this is offered by the Infusoria. Rosshach, in the work 
already referred to, showed that the rhythmical contractions of 
the contractile vesicle always grew more rapid under a rising 


ON INFUSORIA. 119 


temperature until they reached the maximum of rapidity, which 
varies in different species, at the temperature of 30° centigrade. If 
the heat increased beyond that point, the rapidity of the con- 
tractions diminished, while at the same time that of the ciliary 
movements increased ; if, finally, a temperature of 35° was 
attained, a striking difference was seen in the reaction of two 
kinds of cilia; those which effect the rotatory movement move 
without being in any way affected by the animal’s will, while 
the others, by which the creature is enabled to move backwards 
or forwards, depend entirely on its will. The rapidity of both 
kinds of motion increases equally so long as the warmth is rising 
up to 35°, but after that the backward or forward motion of 
the animal ceases to be under the contro! of its will, and a 
peculiar combination of aimless direct and rotatory motions is 
the result. When the heat has risen to 40°, the direct motion 
which became involuntary at 35° ceases entirely, while rotation 
continues with undiminished vigour, till at last between 42° 
and 45° heat-coma supervenes and death ensues. We see by 
this that even two so closely allied functions as contractility 
and ciliary motion, or even two very slightly different kinds of 
cilia—those that work voluntarily and involuntarily—are in- 
fluenced in widely different ways by the same increase of tem- 
perature, exactly as individual animals react differently under 
a rising temperature. Hence an increase of temperature, in 
any district, resulting from secular variation, or an alteration 
in the distribution of heat as to the seasons, must exert 
widely different influences on animals previously living to- 
gether; and the view so frequently expressed in the statement 
that animals may be classed as tropical, sub-tropical, temperate, 
and so forth—as if all the animals living together in the same 
place must be equally well adapted to the climate of it, and as 
if they must all react in the same way under any variations in 
temperature—is evidently devoid of any solid foundation. 

We will now examine a few particular instances, to illustrate 
more exactly what has been said by some extreme examples. 

We know that the optimum of temperature is not the same 
for all animals,‘! so we shall not be surprised to see that cer- 
tain species can live in a temperature which to others ia 


120 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


promptly fatal. Animals of the North Sea, when exposed to a 
temperature of 30° under the direct rays of the sun, die, it 
would seem, at once, and invariably, while this appears to be 
the optimum of warmth for the Branchipus stagnals, living in 
our pools. Animals living within the tropics are generally 
exposed to a much higher and more equable temperature than 
our northern or boreal forms, and few species can long survive 
atransfer from one region to the other. I shall presently return 
to this subject. It certainly appears surprising, though it is an 
indisputable fact, that certain animals and even plants are capable 
of enduring a higher temperature 4? than is generally fatal, as it 
would seem, to protoplasm, which is the fundamental element of 
all organic life. It is known that animal protoplasm usually 
coagulates and dies at 40° centigrade, and always at 50°, and this 
agrees with what is known of vegetable protoplasm. Hence 
we should fecl inclined to conclude that animals could not 
exist in spots—as, for instance, in many hot springs---where 
the temperature exczeds the maximum limit in which proto- 
plasm can live. But this assumption is in contradiction to 
ascertained facts. It is superfluous to give these in detail; it 
will suffice to state that animals of tolerably high grade, as 
Crustacea, the larvae of insects, c&c., live in springs having a 
temperature of 50°-60° or even more. In view of the numerous 
data on this subject, and the trustworthiness of most of the 
observers, we are not justified in doubting the facts, although 
the physiological puzzle which they offer cannot at present be 
solved—indeed, no attempt has ever been made to solve it. 
‘We might have recourse to the supposition that the capability 
of many animals for living and multiplying in such heat must 
result from a capability in their protoplasm for resisting the 
injurious effects of a temperature of 50° centigrade and more. 
But this would prove far greater powers of adaptation in animal 
protoplasm than have yet been considered possible. 

It might perhaps seem plausible to many readers to assume 
that immunity from the injurious effects of too great heat 
bore some analogy to immunity from injury from too great 
cold. But such a parallel is not admissible from a closer and 
more exact point of view. Every fall of temprrature below 


SUMMER SLEEP. 121 


the optimum diminishes the vital energy of the creature more 
and more, till at last, at the minimum, life becomes latent; in 
this condition the animal can live for a long time without 
danger, since the consumption of organic nutritious matter is 
at its minimum, or, as it would seem in eggs which survive the 
winter, almost entirely suspended during long periods. If, on 
the other hand, the temperature rises above the optimum, the 
vital energies are more and more increased, often accompanied, 
no doubt, by phenomena which seem to indicate a certain 
deterioration in them; but a condition of latent vitality never 
supervenes ; on the contrary, death always follows as the result 
of a too rapid consumption of the organic matter of the body. 
Thus the power possessed by many animals, both warm- and 
cold-blooded, of enduring without injury a higher temperature 
than that which kills protoplasm, remains as yet unexplained 
even by the parallel above suggested, and the solution of the 
problem would undoubtedly be of the highest interest. 
However, the experiments and observations that have been 
made suffice at any rate to provethat the optimum of temperature 
differs for individuals, and even for the different organs of one 
individual. We have seen that the control which an Infusorial 
has over its progressive motion is lost at a degree of heat 
above which the actual movement itself continues to increase 
in rapidity, and we saw too that the rotatory motion con- 
tinues to be accelerated wp to the very point at which death 
ensues, while progressive motion ceases at a much lower degree. 
Hence we need not be surprised to see that a rising temperature 
sometimes produces phenomena which apparently resemble those 
produced by increased cold. Everyone knows that Europeans 
in tropical countries, or even in a hot summer’s day in our own 
latitudes, become very sleepy, exactly as natives of the tropics 
do under the influence of a cold climate. In these cases we are 
justified in assuming that the too great heat deprives us of the 
stimulus which usually keeps us awake during the daytime. 
An attempt has been made to refer the ‘snmmer sleep’ 43 of many 
animals in hot countries, as analogous to ‘ winter sleep,’ to a long 
continuance of such soporific influences; but this is certainly 
not. correct ; in all these cases it is most probable that the 


122 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


drought which prevails during the hot season is the direct 
cause of it, as will be more fully shown in a future chapter. 

One more effect of rising temperature must here be men- 
tioned as influencing one particular function, on whose normal 
action the existence of every species primarily depends—the 
multiplication of individuals, or reproduction, either by eggs, or 
by fission and budding. Every modification of this function 
must exert a direct and considerable influence, either selective 
or transforming, on every separate species. If, for instance, we 
suppose that the animal temperature in any country were to be 
so modified that the optimum temperature for the production 
of eggs did not occur at the proper season nor last a sufficiently 
long time, all the animals affected by the change would be 
infallibly exterminated, and the fauna would be utterly trans- 
formed ata single blow. The influence of arising temperature on 
the eggs already laid, and about to develope after their winter rest, 
would be almost equally important, as affecting the composition 
of the fauna of a country. Before going into particulars, it will 
perhaps be well once more to remind the reader that by the 
words ‘rising iemperature’ I mean, not an increase of heat 
from a fixed minimum toa certain uniform maximum, but a 
rise from any degree above zero to any other, the optinmum— 
two points which, as we have seen, have no direct reference to 
the degree of temperature, but only to the mode in which they 
may affect animals. 

It seems to me superfluous to adduce any special instances 
in support of the well-known statement that egg-formation 
undoubtedly begins—in other words, the sexual powers attain 
maturity—under a rising temperature. No experiments, how- 
ever, have hitherto proved how this effect is produced by heat, 
whether heat alone can have that effect, or whether it requires 
other concurrent circumstances; nor at what degree of rising 
temperature the result is produced in differentanimals. Several 
observations accidentally made‘! appear to be most happily 
explained by this hypothesis of the effect of warmth, but no 
attempt has been made, on any comprehensive method at any 
rate, to supply an exact answer to the questions to which those 
very observations first gave rise. Thus, for instance, and to go 


SUMMER AND WINTER EGGS. 123 


no further, the fact that many animals lay different eggs in the 
summer from those they lay in the autumn may be directly re- 
ferred to the difference of temperature at the two seasons, and 
they are accordingly known as summer eggs and winter eggs. 
But the formation of eggs certainly does not only depend on 
warmth, but also on food, both as to quantity and quality, as 
well as on the chemical conditions of the surrounding medium, 
the moisture of the air, and other conditions ; so that it must not 
be assumed out of hand and as certain that this classification 
(according to the seasoas) of the two forms of eggs, which occur 
among Crustacea, Insects, Rotatoria, and others, is invariably 
appropriate; on the contrary, it needs direct experimental 


Tic. 30.—Aphis Beecubunge, a Plant-Louse. To the left the wingless, and to the right the 
winged, form of the female. 


proof in each separate case, and this, as I have said, has not 
hitherto been obtained on any comprehensive method. 

In a few cases only have we experimental evidence that the 
formation of a particular form of eggs (and mode of reproduction) 
directly vesults from variations in temperature. Thus, for 
instance, we know that the Aphis (fig. 30), a family of the 
Hemiptera well known as the Plant-Louse, sometimes in a 
favourable summer produces as many as fourteen generations 
by parthenogenesis, as the eggs then produced do not need ferti- 
lisation by the male. At the beginning of the cold season males 
first make their appearance, and the eggs fertilised by them live 
through the winter, being known as winter eggs, till the next 
spring, when the rising temperature developes and hatches the 
embryo. Now, if the summer Aphides that multiply by par- 


124 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


thenogenesis were not exposed to the effect of cold, but kept in 
a constant summer heat, and at the same time supplied with 
suitable food, no males would occur, and the young would be 
uninterruptedly produced by the parthenogenetic mode. In 
fact Réaumur did succeed in this way in producing artificially 
above fifty parthenogenetic generations in the course of three or 
four years, all descended from one mother. The converse ex- 
periment has never, so far as I know, been made—namely, 
whether it would be possible to produce males in the spring by 


¥ 
OF 


‘' 
Ig, 
| ye 


oq 


Fic. 31.—a, Diplozoon paradoxum, from the gill of a fresh-water fish ; 6, Polystomum 
integerrimum, from the bladder of a frog. 


artificially lowering the temperature, although they do not pro- 
perly appear till later in the year, and thus to diminish the 
normal number of successive parthenogenetic generations in a 
summer. It is highly probable that it might succeed. 

The facts lately communicated by Zeller are no less interest- 
ing. He found that certain parasites, in precise opposition to 
the Aphides we have been speaking of, Diplozoon paradoxum 
and Polystomum inteyerriimum (fig. 31), produce true eggs in 
the summer only, which must be fertiliscd before they can 


THE PRODUCTION OF EGGS. 125 


develope, and that the formation of eggs ceases entirely at the 
approach of the cold season. But the production of eggs can 
be artificially prolonged throughout the winter if the fishes in 
whose gills the Diplozoon lives are kept in an aquarium in a 
room where the temperature is kept up to summer heat. It 
must be assumed that in this case, as in that of the Aphides, all 
the other conditions of existence, and the supply of food in par- 
ticular, were kept up to the optimum, for the effect of bad food 
would apparently have made the results of all the experiments 
in question very doubtful. Unfortunately we learn nothing 
from either Réaumur or Zeller on this point, nor are the exact 
limits of temperature given, nor any thermal curves by which 
we may gain some idea of how the production of eggs is de- 
pendent on the variation of the temperature between two fixed 
limits. From the fact that in the brooks and streams, at the 
bottom of which the fish live to which the parasites are at- 
tached, the temperature is considerably lower than that of the 
atmosphere in summer, we may certainly infer that these 
curves of temperature are quite dissimilar for the Aphides and 
the Diplozoa, and that the optimum of temperature for the 
production of eggs fit for fertilisation is probably also widely 
different. And thus, again, a selective influence might arise 
from any general change of temperature that might take place, 
and affect the animals of the region where it occurred, since 
the existence of a species largely depends on the normal succes- 
sion of generations. 

We saw a little way back (p. 108) that the growth of an 
animal is indirectly affected even by the temperature of the 
surrounding medium, since the assimilation of the amount and 
kind of food which is indispensable for growth can only be 
carried on to full advantage under a very different optimum of 
temperature for different animals. In the second place, we are 
also justified in supposing that this optimum for the animal’s 
growth is not identical with that for the production of eggs.45 
If now we recollect that the maturation of the eggs usually 
requires but a short time,*® while the growth of even very small 
animals often takes a long time—in Polystomum, for instance, 
several years—it plainly follows that sexual maturity does not, 


126 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


as is often assumed, necessarily indicate the completion of in- 
dividual growth. They may no doubt coincide, but they need 
not ; and instead of being surprised, as is frequently the case, 
at finding that larve, ¢.¢. animals not yet fully grown, are sexu- 
ally mature—e.g. Salamandra, Siredon, Blatta, and others—we 
ought rather to wonder that it is only quite recently that such 
cases have been investigated and considered worthy of record. 
An example of this kind, recently observed by me, is offered by 
the land-snails of the Mediterranean province. It is known 
that on high mountains or in high latitudes land-snails are fre- 
quently hindered by the low temperature from assimilating 
within a given time as much nourishment as others of the same 
species living on the plain or in a warmer climate, so that they 
never attain the same size, though they are capable of reproduc- 
tion. In this case it would be possible that the first sexual 
maturity and the last stage of growth might coincide, although 
the mature animal was of a small size. It is quite otherwise 
with the snails of the warm Mediterranean region. These, as I 
know from my own observations, are brought to sexual matu- 
rity by the time they are six months old by the intense heat 
combined with the sufficient moisture of the spring, though 
they are not fully grown; after a summer’s rest of three 
months, occasioned by the drought, a second period of egg 
deposition occurs at the beginning of winter, although the full 
size of the animal, as indicated by the completion of the margin 
of the mouth of the shell, is not attained till the second period 
of sexual activity. Thus species of the same genera, perhaps 
even the very same species, in our damp and cold climate, do 
not produce a new generation till they are fully grown, while 
in the dry warm region of the Mediterranean they have pro- 
duced two generations before they are fully grown. This is 
probably the true explanation of what are known in the ter- 
minology of the zoologist as Larva-forms,‘7 a nacre given to all 
animals which possess the characters of the larve of other 
species and are nevertheless capable of sexual reproduction, 
Such larva-forms occur in almost every group, in Vertebrata, 
Mollusca, Ascidians, Worms, &c. A very striking example 
occurs, among creatures of such high physiological development 


ORIGIN OF LARVA-FORMS. 127 


as insects, in the well-known walking-stick insects, allies of the 
grasshoppers (fig. 32), which resemble a more or less dried-up 
twig or shoot. Many of these species are wingless, so that they 
bear a wonderful resemblance to the larve of other, winged 
forms. If then we assume that the wingless forms are the 
progenitors of winged forms, the origin of the winged forms 
may be very well explained by the further assumption that the 
optimum temperature necessary for maturing the eggs may 
have been raised so much as to afford more time for assimila- 


Fic. 32.—Phasma sp., a wingless orthopterous insect, 


tion and growth, and consequently for more extensive modifica- 
tions of structure. If, on the other hand, we regard the wing- 
less species as the later form and derived from the winged 
species, their origin may be easily explained by assuming that 
the optimum of temperature for the maturing of the eggs was 
lowered, while the optimum for growth remained the same. 
For we know that in many cases, particularly among insects, 
the life of the individual ends as soon as the eggs are mature 
and deposited, and that the performance of this function seems 


128 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


to absorb their whole vital energy ; thus an animal which was 
forced by such external conditions as we have been supposing 
to lay its eggs while still in the larva stage must always, if it 
dies soon after from exhaustion, produce only such progeny as 
are sexually mature in the larva-form so long as the conditions 
remain the same—in this instance, namely, a low optimum of 
temperature for the maturation of the eggs. 

We will here interrupt this chain of hypotheses, since they 
are introduced merely to explain the possible origin of such an 
apparently paradoxical sexual larva-form. 


Fic. 33,--Apus. 


We must, however, briefly touch on another circumstance 
which plainly demonstrates the direct influence which may be 
exercised by variations of temperature on the growth and de- 
velopment of individuals. 

The general effect of a rise of temperature on the develop- 
ment of the embryo and young animal is well known. We know 
that a hen’s egg requires for its quickest normal development a 
warmth kept as constantly as possible at 40° centigrade; ata 
lower temperature the development will be delayed, while at the 
ordinary temperature of a sitting-room it ceases altogether. The 
eggs of certain Crustacea, on the other hand, as Apus (Lepidurus, 


OF HEAT ON DEVELOPMENT. 129 


fig. 33) and Branchipus, can endure much greater variations 
and develope equally well at any point between zero and 30°. 
The difference between the extremes of temperature which, on 
the whole, favour the development of these eurythermal eggs 
(having, that is, a wide range of temperature) is apparently even 
much greater, although even with them the rapidity of their 
development depends on a rapid rise of temperature to a tole- 
rably high degree.48 Thus I myself have observed the larve of 
Branchipus and Apus, known as Nauplius, hatched out in 
less than twenty-four hours at a temperature of 30° cent., but 
at 16°-20° they required some weeks. The same effects of 
variation of temperature were observed by Higginbottom in 
the larve of frogs; he found that at a temperature of 51° F. 
(=10-5° cent.) they hatched in twenty-one days, but at 60° F. 
(=15-5° cent.) they hatched in ten days. Hence a rise in tem- 
perature of only 9° F. (=5° cent.) doubled the rapidity of 
development of these embryos. The difference is still more 
conspicuous if we compare the time required in the two cases 
for the whole process of development from the time the egg was 
laid till the metamorphosis was complete. At the lower tem- 
perature of 51° F. 235 days were needed, at the higher, 60° F., 
only seventy-three days. Many other examples might be added 
to these, all proving the same effect of a rising temperature ; 
but unfortunately, so far'as I know, none give any exactly deter- 
mined thermal curve for particular species, and all that is clearly 
shown by these observations, often merely incidentally made, 
is the fact that the eggs of different, very closely related species 
behave in a totally dissimilar manner, An increase of tem- 
perature in a district, occasioned by no matter what cosmical 
causes, may thus affect some animals particularly favourably, 
while it injures others, since for the former it may merely raise 
it to the optimum, while for the latter it exceeds the maximum 
they can on the whole endure. In this way, by acting on the 
sexual activity and the rapidity of growth in the young animals 
and in the embryo, a transforming influence may be closely 
connected with the selective influence exerted upon the various 
species generally. It would certainly be an interesting and 
valuable task to investigate this point with accuracy. 


7 


130 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


III. The influence of constant temperatures, but at 
different degrees.— We have seen that a fall below the optimum 
of temperature or a rise above it is more or less injurious to the 
animal, according as it ranges more or less near to the fatal 
maximum or minimum. If there were a spot where the heat 
was absolutely invariable, and if the degree of heat there exactly 
corresponded to the optimum for the animal, if also by this 
nieans the injurious influence of variation were excluded, abso- 
lutely favourable conditions for the life and growth of the crea- 

‘ture would prevail, so far as warmth was concerned. It is 
evident, however, that very few animals live in such a peren- 
nially equable climate; none in fact but the parasites living 
inside warm-blooded animals and the creatures at the bottom of 
deep seas and lakes enjoy such an advantage. All others are 
exposed more or less to the effects of great periodical variations. 
Among these, however, those are evidently the most favoured 
which are exposed to the smallest deviations above or below the 
optimum. Hence a climate where the mean annual tempera- 
ture differs in only a trifling degree from the winter or summer 
mean temperature, or from the extremes of heat and cold, will 
be the most fivourable ; such an equable climate may occur in 
high latitudes as well as between the tropics, since its existence 
depends less on latitude than on the configuration of the land, 
the vicinity of the sea, and the predominant direction of winds 
and currents. For example, the eastern half of both the old 
and new continents are distinguished from the western by 
havirg what is called an ‘extreme’ climate, in which the ex- 
tremes of heat and cold lie far asunder, while England, and still 
more Ireland, are characterised by an extraordinarily equable 
climate as compared with many more southerly parts of the 
continent, which, like the countries of south-eastern Europe, have 
a continental, i.e. an ‘extreme,’ climate, with great heat in the 
summer and severe cold in the winter. 

Before entering on a discussion of the question as to how 
far the theoretically assumed advantage of an equable climate is 
practically real, I must anticipate one view which might other- 
wise be regarded as the inevitable consequence of the foregoing 
statement. It might seem as though it stood in trenchant 


EXTREME CLIMATES. 131 


antagonism to another which is frequently repeated and sup- 
ported by numerous examples, namely, this: That animal life, 
and more particularly the culture of the human race, can only 
attain their highest development in cold climates, although as 
a rule, not without exceptions, these manifest greater varia- 
tions between the two extremes of temperature than troptcal 
or subtropical regions. The high degree of culture that obtains 
among Kuropean nations is only attainable, so it is said, in the 
region of the north temperate zone. No contradictory evi- 
dence, it is true, is forthcoming. It might indeed be asked why 
humanity has not attained to so high a degree of culture, nor 
the animal world even reached such perfect development, in 
those parts of the southern hemisphere which have a cold or 
excessive climate, as they have done in Europe. But quite apart 
from this objection, which is certainly open to discussion, the 
opportunity afforded by favourable circumstances to some parti- 
cular species of developing every individual to an average degree 
of perfection and fitness on the one hand, and the value which 
greater contrasts in temperature—that is to say, actually un- 
favourable conditions of existence—may have in the modification 
and progressive development of a species still capable of such 
modification, are by no means identical. for it is self-evident 
that the necessity for enduring abrupt alternations of heat and 
cold can only be met by greater powers of resistance in in- 
dividuals and by enlarged powers of adaptation; hence, in 
extreme climates, the selection which will necessarily be effected 
by the violent contrasts of temperature, must, by eliminating 
the weaker individuals, improve the race; while in equable 
climates, where this mode of selection is absent, the weaker in- 
dividuals (weaker only in this one point) have as good a pro- 
spect as the stronger ones of a long life and of transmitting their 
characters. The two propositions, indeed, do not contradict 
each other, but on the contrary complete each other. A species 
whose individuals are at an end of their capability for under- 
going modification will succeed best in an equable climate, since 
there every individual, without exception, will find an equally 
favourable opportunity for reproduction—supposing of course 
that no other selective agency is at work. But so long as a 


132 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


species is capable of modification, the greatest alternations in 
the conditions of existence will offer the most favourable oppor- 
tunities for its transformation and improvement—thus, in the 
case in question, the utmost variations of temperature—since 
it is in this way that the most stringent selection is effected 
between the weak and the strong. 

Among the numerous instances which here offer themselves 
for discussion, want of room compels me to select only a few of 
the most significant. One of the first in importance seems to 
me to be a case of which the full value was first recognised by 
Mobius, who also, and apparently with perfect accuracy, referred 
it to its efficient cause. He mentions ‘9 the fact that the same 
species of Molluse, living on the coast of Greenland or in the 
Baltic Sea, was in the former instance very large, and in the 
latter small and thin-shelled ; and he attempted to explain this 
difference by saying that the animals in the Baltic are exposed 
to very considerable variations in temperature Letween the 
summer and winter, and are sometimes even frozen up, while 
on the coast of Greenland they live in a temperature which, 
though of course low, varies but little from winter to summer. 
Hence they are enabled to carry on the assimilation of food at 
an equable rate all the year round, never being disturbed by 
too great a heat in summer or too severe a cold in winter, while 
those in the Baltic are every year exposed to the liability of 
being checked in their growth by a too high or too low tempera- 
ture in constant alternation. Although the mean temperature 
of the Baltic is higher than that of the sea off Greenland, the con- 
stant low temperature, i.e. the equable climate, of the latter is 
infinitely more favourable to the growth of the animal than 
the higher mean temperature with constant variations of the 
Baltic. 

The numerous cases of successful acclimatisation lead us to 
the same conclusion. So far as I know, the old system is now 
given up in every zoological garden, by which foreign animals 
were kept in houses or cages in which an attempt was made to 
keep up by artificial means such a climate as they were used 
to; a contrary plan is now introduced, by which the animals 
are accustomed as quickly and thoroughly as possible to the 


MR. BUXTON’S PARROTS. 133 


new climate and to an open-air life. Unfortunately it would 
be impossible, without an enormous amount of labour, to arrive 
at any general views as to the results obtained in all the differ- 
ent zoological gardens in Europe; but in general it seems 
beyond a doubt that these attempts have succeeded best where 
the animals were introduced to an equable climate, and least 
well in gardens in the eastern region, where the climate is 
essentially continental, i.e. extreme. The first statement, 
that the acclimatisation of even tropical animals may be suc- 
cessful in an equable climate, though much colder than that of 
which they are natives, has found a striking and in every way 
interesting proof. Mr. Charles Buxton,* a rich member of Parlia- 
ment, made such an experiment ona vast scale. He kept during 
many years numerous individuals of eleven species of cockatoos 
and tropical parrots in a large glass house; he then all at once 
gave them their liberty, putting them out into a wood con- 
tiguous to his garden. Being accustomed to be fed at fixed 
hours in the glass house, they came regularly down into the 
courtyard ; but though the house was open to them, with nesting 
boxes to breed or to winter in, they built nests of their own 
accord in hollow trunks, and bred and wintered there, and, 
though exposed to a temperature of 7° below zero centigrade, 
not one died. It must have been a beautiful sight when the 
flock of gaudy parrots came crowding down in midwinter to 
pick up their food on the snow-covered courtyard. Many 
species were propagated, and even a hybrid race was produced 
between a scarlet and a white cockatoo; the young of these 
were distinguished from their parents by a fine orange-coloured 
tuft on the head. In view of the well-known effects of hybridi- 
sation, it would of course be absurd to ascribe this result to the 
effect on the parents of a climate to which they were not 
accustomed ; hut it proves that animals which we are wont to 
regard as incapable of living in freedom in our climate, because 
they are tropical species, are not only able to do so, but can 
propagate their kind and effect a voluntary cross-breeding 
which, if it were attempted artificially, would probably not 
succeed even in their native country. However, the climate of 
England is very equable in temperature, and it is more than 


134 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


doubtful whether such an experiment would succeed in central 
Germany, for instance, or in the east of Europe. 

Everyone knows that in European countries animal life 
exhibits a very marked periodicity. The greater number of our 
birds quit us in the winter; many Mammals, Insects, Molluscs, 
&c., hybernate_in a condition resembling winter sleep ; others die 
off altogether, as Sponges, Bryozoa, many Crustaceans, Insects, 
&e., after laying eges which survive the winter and are developed 
in the spring. The creatures that live in streams and lakes, 
nay, even on the sea-shore, breed only once, or at most twice, in 
the year, some in the spring or summer, others late in the 
autumn or at the beginning of winter, as the salmon. This 
periodicity depends apparently on the direct effects of the severe 
extremes of summer and winter temperature to which animals 
are exposed when living in temperate continental climates, 
This conclusion is easily proved by the following considerations. 

Every individual requires a certain duration of life to achieve 
its individual development from the egg to sexual maturity 
and full growth; the length of time requisite for this is very 
various, and, above all, bears no proportion to the size attained. 
Animals grow at very different rates ; the minute Polystomum 
which‘is parasitic in the bladder of the frog (fig 31) requires, 
according to Zeller’s latest investigations, about five years to 
attain its full growth, while the Apus (fig. 33), a Crustacean of 
much larger dimensions, and Dranchipus reach their full size 
in a few weeks of a warm summer. This length of time, which 
we may generally designate as the period of individual growth, 
is not alike even for all the individuals of the same species ; 
on the contrary, it depends on the co-operation of so many 
different factors, that it must necessarily vary considerably. 
Now, if from any cause the period of individual growth, say of 
the salmon, became changed in consequence of the slower 
development of the embryo in the egg or of the young larva, 
most or all of the young salmon thus affected would die in our 
climate, because the greater heat of spring is injurious to them 
at that stage; or, on the other hand, an animal which is 
usually hatched during the summer, so as to attain by the 
autumn such a size as may enable it to resist the cold of the 


PERIODICITY. 135 


winter, would probably die in the winter if its growth during 
the summer were checked by any cause. Thus a climate that 
varies between two remote extremes of temperature must neces- 
sarily give rise to a sharply defined periodicity, by excluding 
those forms which have by nature a somewhat prolonged period 
of individual growth, since these, as the conditions requisite for 
their development are not fully satisfied, must have to contend 
with the temperature which checks their growth. 

The opposite state of things must prevail in equable, and 
above al]. in tropical climates, where the variations in tempera- 
ture are reduced to a minimum ; here the periodicity of animal 
life must be to a great extent obliterated, at any rate in so far 
as it depends on temperature. For every individual of a species, 
even if its period of individual growth is longer or shorter than 
the average of the species, can live and multiply if it is never 
exposed to an absolutely fatal degree of heat; according to the 
laws of inheritance, its progeny will probably have a tendency 
to develope more or less slowly, aud so by degrees one individual, 
whose period of individual development is short, may produce, 
let us say, six generations in a year, while another of the same 
species, but of slower growth, may produce only four. In this 
way all periodicity, as regards summer and winter, must be 
entirely lost, and at last fully grown individuals and young ones, 
larvee and freshly laid eggs will all be found together at every 
season and in every month of the year. Such cases, in fact, are 
not uncommon, although they have been but little heeded 
hitherto. Nothing in the Philippine Islands struck me so much 
as to observe that there all true periodicity had disappeared even 
from insects, land molluscs, and other land animals; I could at 
all times find eggs, larve, and propagating individuals, in winter 
as well as in summer. It is true that the drought occasions a 
certain periodicity, which is chiefly perceptible by the reduced 
number of individuals in the dry months and the greater number 
in the wet ones; it would seem that a much smaller number of 
eggs are hatched under great drought than wher the air is very 
moist. Even in January, the coldest and driest month, I found 
land snails which require much moisture, and at every stage of 
their development, but only in shady spots, in woods, or by the 


136 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


banks of streams. But what was far more striking in thesa 
islands was the total absence of all periodicity in the life of the 
sca animals, particularly the Invertebrata; among these I could 
not detect a single species of which I could not at all seasons 
find fully grown specimens, young ones, and freshly deposited 
eggs. Even in cold seas periodicity is far more often eliminated 
than is commonly supposed. Nordmann tells us that he has 
found the eggs of a sea molluse (Lergipes) at all seasons, even 
in midwinter, when the temperature of the water was only at 
a few degrees above freezing-point ; Mibius *! states that eggs 
of Mollusca and Worms were found at all seasons of the year 
in the Baltic, and I hope that Dohrn’s zoological station at 
Naples may ere long furnish us with a long list of species 
which may be observed the whole year round in all the stages at 
once of growth, larve, and eggs. So far as I can judge from my 
own observations, which of course are by no means conclusive, 
the Mollusca manifest the least periodicity in their reproductive 
powers. 

The influence of an equable temperature on the inhabitants 
of the sea is displayed, too, in another manner. It is known 
that in our northern seas the daily variations of temperature 
on the shore, or even at some depth, are not inconsiderabie, while 
in tropical seas the variation at the surface even between winter 
and summer heat is not neatly so great—at the Philippines, for 
instance, not more than 2° centigrade ; so small a variation, as 
has been shown by Meyer and Mobius in a remarkable work,5? 
occurs only at a great depth in northern seas. It is to this 
apparently that we must refer the fact that many genera of sea- 
creatures which are known as boreal forms live in the north at 
great depths, while in tropical seas they live very near the 
surface.5> In my monograph on the Holothuride I have shown 
that a great number of genera which we had been accustomed 
to regard as typically boreal were found also in the Philippine 
seas, and lived there at a moderate depth, while in the northern 
seas they were found only at very considerable depths. The 
same seems to be the case with regard to many animal forms 
which are now found at the bottom of the Atlantic, and which 
may be regarded as survivors from a long past geological period ; 


PALHONTOLOGICAL ARGUMENT. 137 


an inconceivable variety of these forms was brought to light by 
the ‘ Challenger’ expedition. A woodcut is here given of one of 
the most beautiful of these species, the Luplectella, which belongs 
to the group of Sponges (fig. 34). Although the results of the 
‘Challenger’ expedition have not yet been fully published, so 
that it 1s impossible to give a complete list of the various deep- 
sea forms and their distribution vertically in depth, it seems to 
be tolerably certain that they havea much wider vertical distvri- 
bution in tropical seas than in northern oceans; in the north, 
for instance, no Euplectella nor any allied form of sponge—six- 
rayed Siliceous Sponges—has been found ina less depth than 
300 fathoms, while in the Indian Ocean they are common in 
100 fathoms or less. Thus the higher temperature of the water 
to which these cold-water animals *4 are exposed in tropical 


Fic. 84.—Euplectella Aspergillum, a siliceous sponge of a group which consists mostly 
of fossil forms. 

seas is in no way prejudicial to their existence; and this can 

only result from the fact that these animals are better able to 

bear a difference of temperature, so long as it remains equable, 

than variations between two extremes lying far apart, and to 

which they are more or less suddenly exposed. ' 

Here, in conclusion, we must briefly discuss an application 
of the foregoing statements and arguments to Paleontology. 

It is generally assumed that we are justified in attributing 
to extinct animals a mode of life analogous to that of the nearest 
related surviving forms. But, in the first place, it is often 
extremely difficult to decide what may have been the nature of 
the affinities between extinct and living animals, and it cannot 
be disputed that, in instituting such comparisons, we are often 
obliged to judge by characters which in no way warrant our 


138 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


forming any decisive conclusion. For instance, far too much 
value has been attributedin this way tothe in-operculated Terres- 
trial Mollusca (Pulmonifera) ; for, in my opinion, it is absolutely 
impossible to form any opinion as to the affinities of extinct 
animals by comparing their shells, which are all that remains of 
them, since recent investigations as to living Pulmonifera show 
that very often species of the same genera have quite dissimilar 
shells, while, on the other hand, the shells of many species belong- 
ing to quite distinct genera, or even to different families, are so 
much alike, that until quite recently they have been considered 
as species of the same genera. I shall enter more minutely into 
this subject somewhat later on. 

Even if we were prepared or obliged to admit that the fossil 
remains in. every instance allowed us to determine the affinities 
of the species to living animals with absolute certainty—and 
not in the case of Vertebrata only, but in the Invertebrata also 
—still we might assert, and defend the position with success, 
that the extinct species need by no means necessarily have 
lived under the same climatic conditions as those forms which 
are now regarded as their nearest living allies. For we have 
seen that animals which in separate spots are stenothermal 
(enduring but a small range of temperature) are able to exist in 
very dissimilar temperatures when the whole extent of their 
distribution is taken into consideration; thus Hwplrctella and 
Semperedla live in a constant warmth of about 15° in the 
Philippine seas, while their nearest congeners can thrive in other 
localities in so low a temperature as 1° above zero, which is the 
temperature of great depths in the Atlantic. We have seen, 
moreover, that animals, as parrots, which live almost exclu- 
sively in the tropics under a mean annual temperature of 26° 
to 28° with a variation of from 6° to 8° at most, can never- 
theless subsist in the open air in England, multiply, and even 
produce new ‘sports’ or varieties, although living in a mean 
annual temperature of only 12° to 13°, witha variation between 
the extremes of as much as 17°. Thus the occurrence of a 
parrot or of Siliceous Sponges and Crinoids in any geological 
stratum in high latitudes is not a convincing proof that a 
tropical climate prevailed during the deposition of that formation. 


AS RESTING ON CLIMATE. 139 


Eurythermal animals are of even less value in forming an 
opinion on the subject, since it is well known that they are 
especially characterised by the extraordinary adaptability that 
they sometimes display to very different and remote extremes 
of temperature. If we are bent on reconstructing the mode 
of life of fossil forms, and their climatic conditions of existence, 
by comparison with allied living forms, land-animals must be 
decidedly preferred to water-animals ; but even these, as it seems 
to me, offer absolutely no certain evidence. At most this mode 
of comparison can only apply when the fossil and living ani- 
mals are so closely similar that we are forced to regard them 
as identical. This is known to be the case with the animals of 
what is known as the Glacial Period ; but, as soon as we reach 
the deeper strata, and the identity of the species with those now 
living ceases, our right to construct a theory uf the climate of 
past epochs by a comparison of fossil and living species entirely 
disappears. The very generally received opinion that sucha 
reconstruction is possible rests in part on the old, but abso- 
lutely false, idea that certain absolute degrees of warmth, and 
particularly the mean annual temperature, have a definite effect 
on the life of animals ; and, secondly, on the indisputable fact 
that the climatic difference of two countries always goes hand 
in hand with a dissimilarity in their fauna. But it ought not 
to have been forgotten that the daily and annual variations of 
temperature are not the only means which Nature has had at 
her disposal for the selection of species and the geographical 
limitation or distribution of particular forms in successive 
geological periods ; it ought to have been duly considered that, 
when a change of temperature is introduced in any locality, the 
influence, whether favourable or unfavourable, that it may have 
on the mode of life or even on the existence of the animals 
may often be completely neutralised by the effects®> of other 
conditions of existence in no way depending on the temperature 
and its variations. 


140 THE INFLUBNCE OF INANIMATE SURROUNDINGS. 


CHAPTER V. 


THE INFLUENCE OF STAGNANT WATER. 


TuE media surrounding the animal, and in which it lives, are 
sometimes gaseous, as the atmosphere, sometimes fluid, as the 
water of the sea or of rivers, sometimes even solid; these last, 
as earth, wood, stone, &c., may be considered as absolutely 
motionless with regard to the animal, since they can only influ- 
ence the creatures that live in them by their varying hardness 
or their chemical changes. Gaseous or fluid media cannot be 
regarded as perfectly inactive ; they are capable of certain swift 
modes of motion, known as currents or as winds. Hence we 
are compelled to investigate the influences of water and air on 
the animals that live in them under two separate heads, ac- 
cording to whether the air or water is stagnant or in motion, 
since they influence animals quite differently in these two dif- 
ferent states. Moreover, we must separate our enquiries as to 
the effects of air from those as to the effects of water, for they 
affect the animal world very differently. I shall begin the dis- 
cussion of the whole subject with such facts and experiments as 
illustrate the selective or transforming influence of stagnant 
water. 

I. General preliminary remarks——Water is an indispen- 
sable condition of animal life. A frozen-up frog, fish, or egg of 
an. insect is leading only a latent, not an active life. In pro- 
toplasm, the essential living constituent of every animal cell, 
there is a great quantity of water; if it is all extracted by 
drying, the cell ceases to live. The old statement is well known, 
‘Corpora non agunt nisi fluida.’ But the universal effects of 
a condition of life which is equally indispensable to a single cell 


COMPOSITION OF SEA-WATER. 141 


and to a whole organism are of no particularly prominent in- 
terest to the question we are discussing; on the other hand, it 
will be necessary to investigate a number of special instances 
of the effects of water more closely, in order to be able to 
contrast those cases in which it gives rise merely to a selection 
between different forms with others which prove that its in- 
fluence is also capable of causing a true transformation. Few 
as these latter cases are, for that very reason they have special 
claim on our interest. 

IT. Effects of the chemical composition of the water.—In 
proceeding to investigate these, it will be well in the first place 
to direct our attention to two extreme cases—to the influence, 
that is to say, of fresh and salt water. 

The salt savour of sea-water is occasioned, as is well known, 
by the presence of a tolerably large proportion of sodic chloride, 
generally known as common salt. When we speak of the 
effects of sea-water on animal life, they are usually ascribed to 
this salt. Still, as many other substances are found in the sea 
besides sodic chloride, such as calcium salts, magnesium salts, 
broniine, iodine, and other metals, carbonic acid, &c., it is to 
be supposed that these are not wholly without importance in 
the economy of animal life in the sea. But we do not know how 
great their effects may be; and as we are now perfectly accus- 
tomed to attribute all the differences which have been observed 
in the effects of sea-water, as distinguished from fresh water, 
simply to its saltness, and to express the difference between 
them in fractions per cent. of the amount of sodic chloride 
held in solution, we will follow the usual custom in our enquiry, 
without forgetting that at the same time numerous other 
matters add their effects, though these are unknown, to those 
of the pure sodic chloride. 

At the first glance we might feel inclined to explain the 
fact, that a much greater multiplicity of forms, prevails in the 
sea than in fresh water, by the supposition that the salt in the 
ocean favours the production of variety in animals. It is 
known that in recent times whole groups of animals are wholly 
excluded from fresh waters, as the Echinodermata, Sipunculide, 
polychetous Annelida, Tunicata, Brachiopoda, and Cephalopoda, 


142 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


while other groups include very few fresh-water species; to 
these last belong the Sponges, of which only one genus, and the 
Polyps (Ceelenterata), of which only two genera, inhabit fresh 
water ; besides these, two families of Bryozoa and two of Annelida 
are fresh-water forms. Other groups, again, have as many repre- 
sentatives in salt asin fresh water. On the other hand, only 
one single class inhabits fresh water exclusively, the Amphibia. 
If we consider the smaller groups, families, or genera, we cer- 
tainly meet with several typical fresh-water forms; to these 
belong Melania, Neritina, the Planorbide, Lymneide, Unio- 
nide, and Anodontide, among the Mollusca; the Astacide and 
Asellide among the Crustaceans ; Phylactolemata among the 
Bryozoa; the true leeches, Vaidide, Tubifex, and Chetogaster, 
among the Annelida; the Cyprinoide among the fishes, &c. But 
their number is insignificant compared with the enormous number 
of families typical of the sea and living in it exclusively. 

Thus the fact that a much greater abundance of different 
forms is to be found in the sea than in fresh water is abso- 
lutely beyond dispute. But it is doubtful whether this wealth 
of forms does actually result, as has been supposed, from the 
great quantity of salt contained in sea-water. The possibility 
that it is so must of course be conceded, and I shall even 
adduce certain facts which tend to prove the justness or the pro- 
bability of this view; but it must not be forgotten, in the first 
place, that the sea covers three-fourths of the surface of the 
globe, and so offers an infinitely wider surface for the develop- 
ment of animal forms than is offered by fresh-water lakes and 
streams. This circumstance alone would account for the greater 
multiplicity of the marine forms. In the second place it must 
not be forgotten that the animal life on our globe apparently 
originated in the sea, and that therefore the oceanic world of 
animal life has had a history of development of much longer 
(geological) duration than the fresh-water fauna; thirdly, that 
the influence of natural selection in fresh water is much stronger 
than in the sea, if only by reason of the abrupt variations of 
temperature; and finally—if we assume that animal life origi- 
nally took its rise in the sea—that only such sea-creatures 
could accustom themselves to living in fresh water as were 


MIGRATION OF AQUATIC ANIMALS. 143 


good swimmers, and as were eurythermal, and that they 
must not have been subjected to the injurious influence of a 
sudden change of food and sudden transfer to the salt elements 
of sea-water. 1t is commonly said—to give one special in- 
stance—that the rich variety of forms in the fauna of the Red 
Sea and the Mediterranean is caused by their high degree of 
saltness. The former is 4°31 per cent., and the second 3°79 per 
cent., at the surface; but this leaves out of account the fact 
that a merely superficial current pours in incessantly through 
the narrow straits which, in each case, divide the sea from the 
adjoining ocean, while a contrary current at the bottom of the 
sea carries the waters of the inland sea back to the ocean. 
Since, therefore, most swimming creatures, and particularly the 
larvee of non-migratory animals, swim close to the surface, many 
more creatures in both seas must be brought in than are 
carried out, and thus the rich varicty of forms in them may 
certainly quite as likely be caused by the direction of these cur- 
rents as by their greater saltness. 

In the total absence of all expcriments directed to these 
points, we may set aside stich vague speculations and pass on 
to the discussion of those facts which seem to prove that no 
perfectly hard and fast line of demarcation exists between 
fresh-water and marine animals, and that it is not absolutely 
impossible to accustom them to live in the element to which 
respectively they are strangers. The general importance of 
this question requires that we should enter into particulars. 

A. Fresh-water animals that live in the sea.—We are ac- 
customed to give the name simply of fresh-water animals to 
such groups, species, genera, families, or orders, as live exclu- 
sively or almost exclusively in fresh water. It is evident that 
if they were to migrate into the sea they would be exposed to a 
certain effect from the salt, and it may even be supposed that 
this effect might be injurious and strong enough to make it 
quite impossible for a fresh-water animal that had migrated into 
the sea to continue to live. However, there are a great number 
of so-called fresh-water forms which do actually live in the sea, 
some as visitors and some as constant inhabitants. It can 
hardly be necessary here to remind the reader of the well-known 


144 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


migratory fish—the salmon, the eels, many herrings, p'aice, and 
others. More interesting, because less generally known, are 
the cases of marine Insects or insect larvee. Slabber has de- 
scribed the larva of a fly which lives in the sea, and I myself 
frequently met with a similar one in the Philippine and China 
seas; Audouin studied the habits of a beetle (Blemus flavescens) 
which lives in the sea like the fresh-water spider, Argyroneta 
aquatica; Packard has given a list of the insects which occur in 
the salt waters of North America, and he enumerates as_be- 
longing to them not less than ten different species of beetles, 
flies, and bugs. In the Pacific Ocean and Philippine Sea, I have 
myself often found various Insects and even Spiders in the sea, 


Tic. 35.—Halobates sp., caught by me far from land in the China Sca. 


sometimes swimming in great numbers on the surface, some- 
times creeping between rocks under water by the shore. A bug 
of the genus Halobates (fig. 35) is particularly common in these 
seas, besides the above-mentioned larve of flies. This genus was 
discovered by Eschscholtz, and now includes fourteen species 
living in seas the most remote from each other. The species in 
question runs about like our Water-Bug, //ydrometra, in great 
numbers and in every stage of development, on the high seas 
hundreds of miles from land. Among Mollusca a species of 
Unio lives in the Brisbane River within reach of the flood-tide, 
Dr. Carpenter found Planorbis gluber (Jeffreys) at a depth of 
1,415 fathoms at Cape Teneriffe. Nerttina viridis, in the West 


ANIMALS IN BRINE SPRINGS. 145 


Indies, has long been known, which, like MNeritina Matonia 
(Risso) at Nice, lives in the sea. I brought a great number of 
marine Neritine from the Philippines, the Pelew Islands and 
China, which from their variations are of the highest interest. 
I have also found a few species of Melania in brackish water ; 
several species of Lymnea and a Neritina live at Bornholm in 
the Baltic, in spots where the water contains from 1 to 1-5 per 
cent. of salt. The Oligochetous Annelida, to which the earth- 
worm belongs, are typical fresh-water or land forms; neverthe- 
less, at’ least nine or ten species are known which live on the 
sea-shore in salt water; they belong to the genera Senuris, 


Fie, 36.—Pachydrilus sp., living in the Salines of Kissingen. It belongs to a group of 
worms, Oligocheta, which is principally confined to fresh water. 


Enchytreus, Tubifex, and others. Marion, at Marseilles, has 
discovered a new genus nearly allied to the common earth- 
worm, which he has called Pontodrilus; this worm lives there 
under stones and decaying tangle, far from all fresh water, and 
below high-water mark, so that it is apparently alternately 
moistened by salt water and fresh (rain) water. In the very 
strong brine springs of Kissingen, I myself have found a new 
species of the genus Pachydrilus (fig. 36), of which Claparéde 
found another species—on which he founded the genus—in the 
brine of Kreuznach ; they are remarkably near to the fresh- 
water form, Tubdifex. Finally, I will mention that the common 
stickleback, Gasterosteus aculeatus, which usually lives in 


146 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


fresh water, lives and thrives perfectly in the Bay of Kiel as 
well as in the North Sea, and specimens of this fish, caught at 
Wiirzburg in the month of May, were even placed at once in 
sea-water without sustaining any injury.°° 

B. Marine animals in fresh water.—C'ases of this sort are 
just as frequent as those we have just been discussing, and 
occur among both the Vertebrata and Invertebrata. Among 
the Vertebrata we must first mention the American Manatus, 
which lives in the great rivers of South America, hundreds of 
miles from the sea; then a true dolphin of the genus Gobioce- 
phealus, which is found far inland in the Irawady river, 600 
miles from the sea, and which is quite different from Globioce- 


Fie. 57.—Platurus vulcanicus, a water-snake living in the fresh-water lake of Taal (Luzon), 
and having a paddle-like tail. 
phalus indicus, which lives in the Indian Ocean. Among the 
reptiles the family of J/ydrophide contains only sea-snakes, 
which are very common in the seas of the eastern hemisphere, 
and are often found there swimming in the high seas ; it is only 
at breeding-time that they go to land.5” The only exception to 
this rule is found in a new species—here represented for the 
first time—of the genus Platurus (fig. 37), which I myself dis- 
covered in the fresh-water Jake of Taal in Luzon, which is 
famous for its still active volcano; it is true that this lake is 
connected with the sea hy a not very long river. Together 
with this snake and associated with typical fresh-water forms— 
as Neritine, Melania, Palemon, &c.—other mavine animals are 
found, such as Pristis Perrotteti (the siw-fish), which is very 


MARINE FORMS IN FRESILT WATER. 147 


common also in the magnificent Laguna de Bay near Manila. 
Sea-fishes, which normally live also in fresh water, or which 
thrive well when introduced into it, are hy no means rare ; 
thus Peters found Rays deep in the heart of East Africa; the 
Lake of Acqua, near Padua, which is of pure fresh water, has 
become famous by the success of an attempt made there to breed 
sea-fish— Magi (the grey mullet) and Labraz (the basse)—in 
great numbers for the market. Among the Invertebrata such 
cases are yet more common. Palemon, a genus of Crustaceans 
which inhabit fresh water almost exclusively, belongs to a 
family which generally includes none but marine animals ; 
various species of this genus live in rushing mountain streams 
in the Philippines, and are found at an elevation of 4,000 feet or 
more above the sea. In the branchial cavity of this Crustacean 


a 6 


TOPS 


VA. 


= 


Si 


Ftc. 38.—Bopyrus ascendens. a, the lower ; 6, the upper side. It lives in the gill cavity 
of Palemon ornatus (Oliv.), and is found with it ascending fresh-water streams at a 
height of 4,000 feet above the sea, All the other known species are marine. 


lives a species, as yet undescribed, of the genus Bopyrus (fig. 38), 
which J have named Bopyrus ascendens. It is the only fresh- 
water form hitherto known, while the other very numerous 
species live exclusively in the branchial cavities of sea crabs. 
Aucapitaine states that a true Cyprea—the species known as 
the money-cowry—is caught in the interior of Africa, near 
Timbuctoo, in quantities by the natives; various molluscs of 
the family of ship-borers—Nausitara Dunlopi (Wright) and 
Teredo senegalensis (Blain.)—and of the Pholadide—Martesia 
rivicola—live in the rivers of India and Java, while all the 
other species of these families are true marine creatures. I ate 
oysters (fig. 39) at Basilan in the south of Mindanao, which, 
although they had a salt flavour and were. indeed bathed 
by brackish water at high tide, yet at ebb tide were surrounded 


148 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


by a rapid stream of pure drinkable fresh-water, and opened their 
shells to it. Many marine Bryozoa occur also in fresh water. 
Among the Annelida the case seems to be rarer, and I have 
only been able to find one instance mentioned in books by 
Leidy, who discovered a worm, Afanayunkia, belonging to the 
Cephalobranchiata, in the Schuylkill River, near Philadelphia. 
The Nemertine worms, so common in the sea, have only one 
representative in fresh water of certainly a very divergent form ; 
of Sponges we find only one genus, Spongilla ; of the Hydroids 
only two, Hydra and Cordylophora (fig. 40), which, in the course 
of time, have become true fresh-water animals.°8 


Fus. 39.—Oyster from the Cumalaran River at Basilan (south of Mindanao) ; it lives in 
spots where the water is quite fresh. 


C. The effect of the different percentage of salt in the 
water.—The cases adduced above prove that it is often impos- 
sible to distinguish, by systematic characters alone, whether an 
animal is fresh-water or marine, since there are many species in 
fresh water whose nearest allies live in the sea, and vice versa. 
Theoretically, then, we must admit that there is no general 
and insuperable impossibility that they should exchange their 
life in one medium for that in the other. But this theoretical 
possibility is not, so far as we know, universally practical ; 
for whole groups—as the Brachiopoda, Sipunculide, and 
Echinodermata—have hitherto been found only in the sea. 
The question now is: What causes have prevented or still pre- 
vent a transfer of marine animals from sea-water to fresh 


MIGRATION OF SPECIES. 149 


water, or vice versa, from actually taking place much more fre- 
quently ? 

I have already indicated that very often the strength of the 
current in a river, or the surf at its mouth, its temperature or 
the kind of food it affords, must cause quite as great hindrances 
to the passage of a marine animal into the fresh water as the 
necessity for subsequently living in water devoid of salt. Thus, 
for instance, the remarkably tender bodies of the larve of the 
Echinodermata, Ascidiz, sea-anemones, Hydroid polyps, and 
others, are scarcely fitted to overcome such impediments ; so that, 
even under the assumption that they might be capable of living 
in water without salt, their transfer into fresh water seems to be 
almost impossible; and this is still more probably the case 
when the fully grown creatures—such as Ascidians, Corals, 
Polyps, and others—do not move freely on the sea-bottom, but 
are permanently attached to it. But if we now leave out of 
the question the other influences which are often combined with 
the variable amount of salt in the water, and which shall be 
discussed in another place, we have in the first place to deter- 
mine the optimum as well as the extreme proportion of salt in 
the water which may be advantageous to different animals, so as 
to be able to estimate how far variations in its saltness may have 
a selective influence on those living in it or migrating into it. 
Secondly, we must deal with the question whether and how 
far an alteration in the salt contents of the water is capable 
of directly modifying the morphological characters of a species. 
But first of all we must ascertain the mode by which the 
salt held in solution in the water penetrates to the interior of 
the body, where alone it can produce any effect. 

Claude Bernard has proved that salt, when in solution in 
water, can penetrate the body of an animal without the 
creature’s agency, merely by the endosmotic action of the skin. 
If a frog is placed in a vessel in salt water, in such a posi- 
tion that it cannot swallow any salt, it will nevertheless be 
found that its body soon contains salt. If it absorbs more 
than it can bear, it will die, and its death will ensue all the 
sooner, the stronger the solution is in the first instance. In 
order to determine what is the minimum percentage of salt 


150 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


that is, on the whole, injurious to the frog, I made a variety 
of experiments in the following manner. To prevent the 
creature from swallowing, and so dying of suffocation, I tied 
it to weighted sticks in such a way that it was unable to dip 
its nose and mouth into the water, even when its head began 
to sink from weakness of the muscles. A great number of 
frogs were placed in different vessels, each containing the same 
quantity of water with various, but known, amounts of salt in 
solution ; death was assumed to have taken place when the eye- 
lids of the frogs no longer reacted under irritation, and did not 
recover their sensibility after the creature was taken out of the 
salt water and washed in fresh water. By this I found that a 
frog commonly died, on an average, in about two hours and a 
half in a solution of five per cent. of salt, in three hours in 
three and a half per cent., in almost seven hours in two per cent., 
and not before more than twenty-four hours had elapsed in one 
and a half per cent. They all, without exception, endured a solu- 
tion of one per cent. withont sustaining any injury ; that is to say, 
they lived as long in their very uncomfortable position as other 
frogs which were fastened up in the same way in pure fresh 
water—namely, from three to four days. It remains still doubt- 
ful, therefore, whether a frog cannot really live just as well in 
water with one per cent. of salt in it as in fresh water. I have 
not made any experiments on this point. But near Greifswald, 
on the Baltic, frogs live and spawn, as I have learned from my 
assistant, Dr. Braun; so it is highly probable that a solu- 
tion of one per cent. of salt in the water is about the limit of 
where it begins to be injurious to frogs. Similar experiments 
have been made by Plateau on aquatic Articulata, and he seems 
not to entertain the slightest doubt that in this caso also the 
salt penetrates through the skin ; although, when the animals 
are completely immersed in the water, imbibition through the 
mouth does not seem to be excluded. But as aquatic Articulata 
cannot die of suffocation so long as the water contains a sufi- 
cient quantity of air, or as the animal is allowed to rise to the 
surface to breathe, this question is of no practical importance to 
us. The most important result established by the above-men- 
tioned experiments, and by Plateau’s, is this : that the behaviour 


ABSORPTION OF SALT. 151 


of different animals under the effects of the same degree of con- 
centration in the salt solution is by no means identical; the 
maximum of strength which is perfectly innocuous to the 
frog is about one per cent., while the stickleback can bear from 
two to two and a half per cent.; migratory fish, as the shad, 
salmon, eel, &c., have still greater powers of resistance, as they 
can bear as much as from three and a half to four per cent. of 
salt in the water.®9 

It results from this, that the difference in the osmotic action 
of the skin in different animals, and the various degrees of re- 
sistance to the amount of salt absorbed into their tissues, con- 
nected with such a difference, do, in a certain sense, cause and 
maintain the distinction which prevails between the fauna of the 
ocean on the one hand, and that of rivers and fresh-water lakes on 
the other. We may assume that the absorption of salt is most 
rapid in animals with a soft skin; we are not surprised when 
we find that the soft gelatinous Medusa is almost instantane- 
ously killed on coming into contact with fresh water, while 
crocodiles with their strong and horny scaly covering, through 
which salt, as it would seem, cannot penetrate, can live equally 
wellin the sea and in fresh water. Between these two extremes 
the gradations are infinite. Every variation in the amount of 
salt in fresh or salt water must therefore influence the animals 
living in it in a different manner; some will be killed, others 
checked in depositing their eggs or hindered in their growth, 
while others will bear the change without any injury. It would 
be a very interesting problem to determine by exact experi- 
ments a curve showing the resistance of different species to the 
absorption of salt by the osmotic action of the skin. 

Unfortunately hardly any such experiments are on record ; 
but the few that are before us offer so much that is of interest, 
even under the scarcely exhaustive treatment they have met 
with, that we must here go into them somewhat more closely. 

In the first place an experiment must be mentioned which 
Nature herself has made on a certain Polyp. It is, so far as I 
know, the only example of an animal that can be proved to 
have originally lived in the sea, or in brackish water, and 
which, within our own time, has gradually accustomed itself to 


152 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


live in pure fresh water. When I was still a student, Cor- 
dylophora lacustris (fig. 40) was found only in estuaries and at 
the mouths of rivers where the water was at any rate occa- 
sionally salt or brackish; it was discovered almost simul- 
taneously in England and Belgium, and somewhat later I 
found it in the Schlei, near Schleswig. ince that time, 1854, 
the animal has in many places migrated into rivers; it has 
already reached the Seine at Paris, and has got into the fresh- 


Fic, 40.—Cordylophora lacustris (from F. B. Schultze), a brackish-water polyp which 
within the last ten years has gradually migrated into pure fresh water. 


water aquarium of the Jardin des Plantes there, where it is 
said to be very common. Its migrations in the Elbe were still 
more remarkable. After reaching Hamburg, and even, if I am 
not mistaken, finding its way into the Alster, it took possession 
at the same time of the great water-pipes of the city, in which 
it lived, associated with the well-known bivalve, Dreissena 
polymorpha, in such enormous quantities as to impede the flow 
of water through the pipes. This case is the more interesting 


BEUDANT’S EXPERIMENTS. 153 


because the Cordylophora is a quite soft animal of the Polyp 
group, and yet it could quickly become accustomed to a 
diminution of salt in the water which would, beyond a doubt, 
entirely destroy many apparently stronger animals. It would 
probably be of much assistance and interest to compare ex- 
amples of Cordylophora from different localities, to see whether, 
perhaps, the variations in the mode of life have not given rise 
to some variation in the structure of the animals living under 
different conditions. This point has not, so far as I know, 
hitherto been closely investigated. 

Only three series of experiments are known to me, which were 
made under artificial conditions, with the express purpose of 
determining what animals could bear a transfer from salt to 
fresh water and vice versa. The experiments made long ago by 
Beudant have never hitherto been repeated. He found that 
various fresh-water molluscs were quickly killed if they were 
suddenly transferred from fresh water to the concentrated salt 
water of the Mediterranean ; but when he increased the amount 
of salt very gradually he obtained very different results. He 
began in April by putting animals into water which contained 
only one per cent. of salt, and by September, by gradual addi- 
tions of salt, he had brought it to a solution of about four per 
cent. Species of the genera Lymnaea, Physa, Planorlis, and 
Ancylus, lived in this salt water as well as in pure fresh water, 
while of Paludina vivipara, Bythinia tentaculata, and Neritina 
fluviatilis, a much greater number of individuals had died in 
the salt water than in the fresh water. Of bivalves—Unio, 
Anodonta, Cyclus—every specimen had perished before the 
water had reached its highest strength of four per cent. He 
subsequently conducted the experiments in the inverse order 
at Marseilles, placing true marine animals in fresh water. He 
then found that a sudden transfer killed almost every species, 
while gradual additions of fresh water to the salt were borne 
by many species, till in the course of a few mouths it had become 
perfectly fresh, so that finally true marine animals were living 
with Lymnea and Planorbis. The edible mussel seemed par- 
ticularly resistent, for not one single specimen perished through- 
out the whole duration of the experiments. Of 610 indivi- 


8 


154 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


duals of various species which were gradually accustomed to 
fresh water, only 37 per cent. died, while of a corresponding 
number which were kept at the same time constantly in sea- 
water 34 per cent. died. Thus the percentage of mortality in 
the group of animals that were gradually accustomed to a 
foreign element was only three per cent. higher than in those 
which remained in their natural element. Certainly it must be 
considered that this result was due to the circumstance that 
certain species—as Mytilus—remained altogether unaffected, 
while others died ont entirely. For further details I refer the 
reader to the note. 

We perceive from these experiments of Beudant’s that some 
species of Molluscs can live equally well in fresh and salt water, 
although they may be exclusively fresh-water or marine forms. 
Unfortunately the experiments have not been carried out far 
enough for us to be able to draw any far-reaching conclusions 
from them. SBeudant, it is true, proved that a fully grown 
Mytilus could be accustomed to fresh water, but not that it 
could multiply in it. Granting that a gradual transformation 
of the salt water in the Baltic into fresh water could take place, 
according to Beudant’s experiments a number of full-grown 
or half-grown animals might become accustomed to the fresh 
water ; but the species might nevertheless very possibly die out, 
particularly if their eggs and larve were not equally capable of 
surviving in fresh water. In the quaternary period numerous 
oyster-beds existed in the Baltic which have since then entirely 
disappeared ; ©! and yet the oyster belongs, according to Beudant’s 
tables, to those forms which are able to live almost as well in 
pure fresh water as in salt water. The extinction of the oyster 
in the Baltic may have resulted, as must certainly be admitted, 
from a variety of causes ; but in view of the total absence of 
all means of proof we must not reject as unfounded the 
assumption that it was caused by the incapacity of the young 
oyster-larvee to withstand the injurious effects of the diminution 
of salt in the Baltic. 

Plateau went somewhat further than Beudant in hig re- 
searches on the aquatic Articulata. His experiments on the 
common Water-Louse(Asellus aquaticus) ave particularly interest- 


PLATEAUS EXPERIMENTS. 155 


ing. By accustoming fully grown specimens of this species to 
water to which he constantly added salt, he brought them to 
live and lay eggs in pure sea-water. The young sea-lice born 
in fresh water died much sooner, according to him, than the old 
ones, when both together were suddenly transferred to sea-water. 
While the young fresh-water lice lived only five hours when 
put into sea-water, the young ones which had been born in water 
already salt lived about 108 hours. Whether they died for 
lack of food or from the effects of the salt is not determined. 
But even if we arbitrarily assume that the salt was in this case 
really the cause of death, it nevertheless results from the data 
above given that at any rate the injurious effects of the salt are 
different at the two different ages of the same animal; and, 
secondly, that the injurious effect on young individuals can 
be materially diminished when the older and sexually mature 
individuals are accustomed to the strange element and breed 
in it. These experiments, as well as those of Beudant, ought to 
be repeated in a methodical manner; but, imperfect as they 
are, they teach us that many aquatic animals can be accustomed 
to a foreign medium, and can even propagate in it. Now, 
although, in consequence of the imperfection of these experi- 
ments, no extensive application of this conclusion is possible, 
they still allow of our propounding the view that it can no 
longer be said to be impossible to accustom certain fresh-water 
species perfectly to live in the sea, or, on the other hand, 
marine species to live in rivers or lakes. 

A still higher interest attaches to the recent experiments of 
Schmankewitsch. The fresh-water Crustacean, Branchipus stag- 
nalis (fig. 41, a) is remarkably like the Artemia salina (fig. 41, 
b), one of a genus otherwise found exclusively in the salt lakes 
of America, Europe, and Africa. Nevertheless the differences 
between them have always seemed sufficiently conspicuous to 
justify their separation into two different genera; these are cer- 
tain dissimilarities in the shape of the antenne of the male, and 
the number and form of the posterior segments of the body, of 
which Artemia has but eight while Branchipus has nine.®? There 
are numerous species of Artemia in Europe. The most unlike are 
Artemia salina and A. Milhausenit; the latter is distinguished 


156 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


by the absence of spines on the lobes of the tail, the small size 
of these lobes, and the relatively large size of the branchial 
appendages of the legs. Schmankewitsch showed that it was 
possible to raise a brood of Artemia Mithausenri from Artemia 
salina, which lived in salt water of 4° Beaumé, by gradually 
raising the percentage of salt to 25° B. This transformation 
occurs very gradually, and only in the course of several gene- 
rations. He observed the same process also in a free state of 
nature. A dam which divided a lake containing salt water of 
4° B. from another where the water marked 25° B. gave way 
in the year 1871, so that the density of the water in the lower 
lake fell to 8° B. At the same time numborless individuals of 


Fic. 41.—a, Branchipus stagnalis ; b, Artemia salina. 


Artemia salina were carried through to the lower lake by the 
flood, and there they soon settled and propagated. After the 
dam was repaired the saltness of the water in the lower lake 
naturally increased again; in 1872 it had risen to 14° B., in 
1873 to 18° B., and by the end of September 1874 it had reached 
its old mark of 25° B. During this period the Artemia salina 
that had migrated had gradually become transformed into 
Artemia Milhausenit. The stages of transformation, as they were 
actually successively observed one after another by Schmanke- 
witsch, are here given in a woodeut (fig. 42) copied from 
Schmankewitsch’s drawing. 

He also conducted the converse experiment with perfect 


TRANSFORMATION OF A CRUSTACEAN. 157 


success, for he brought Artemia Milhausenii back to Artemia 
salina by breeding successive generations in salt water which 
he made weaker and weaker. Now the differences between the 
two species are so great that no zoologist had previously cast 
any doubt on the accuracy of classing them as two species, 


3. 


Tic. 42.—Transformation of Artemia salinu to A. Milhausenii. 1, tail-lobe of A. salina 
and its transition through 2, 3, 4, 5, to 6, into that of A. Milhausenit. 7, postabdomeun 
of A, salina; 8, postabdomen of a form bred in slightly salt water; 9, gill of A. 
Milhausenii ; 10, gill of A. salina. From Schmankewitsch. 


and with all the more reason because each seemed to exclude 
the other; Schmankewitsch’s experiment has nevertheless 
proved their relationship, and also explains very simply the fact 
that they never occur together. It is merely the constancy of 
the external conditions of life—the greater or less saltness of the 


158 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


water—which in one case determines the character of Artemia 
Mithausenii and in another that of Artemia salina. But 
Schmankewitsch was so fortunate as to be able to carry the 
experiment still further. He kept Artemia salina in salt water, 
which he constantly diluted by adding fresh water, till at last 
it was perfectly fresh; the Crustaceans bad meanwhile gone 
through several generations, and bad gradually so completely 
changed their character that finally they had acquired those of 
the genus Branchipus. 

These discoveries are certainly of the greatest interest; for 
they afford a proof we can scarcely doubt, that a change in the 
amount of salt contained in the water can produce a regularly 
recurring and very conspicuous modification of the specific and 
even of the generic characters of certain animals. Darwin’s 
opponents will probably say that in this case those zoologists 
were in error who attributed to the differences between Artemia 
Mithausentt, A. salina,and Branchipus stagnalis, a specific and 
even a generic value, and that all these forms must now be 
regarded merely as varieties of one single species, since proof has 
been given that they pass into each other. It is no part of my 
purpose here to oppose such a view of the case; it will suffice to 
observe, on the other hand, that, logically speaking, writers 
on Crustaceans must then cease to have any justification for 
separating or describing species at all, since those differences 
between Branchipus and Artemia which, according to this view, 
have neither specific nor generic value are precisely those of 
which they constantly avail themselves for distinguishing the 
species and genera when describing other Crustaceans. 

Thus evidence has been given in this chapter that changes in 
the degree of saltness of the water exert not merely a selective 
influence on the animals exposed to them, but also sometimes 
effect a remarkable modification of them; and it is probable 
that other soluble elements in the water besides simply sodic 
chloride may be able to exert a similar influence. We are only 
at the beginning of our knowledge on this point. A careful 
repetition of the experiments here briefly described, with as 
great a variety as possible of animals and with as much 
thoroughness as Schmankewitsch exercised, would, beyond 


EFFECTS OF VOLUME. 159 


doubt, contribute many important facts. But they would 
certainly confirm the result obtained already: That there can 
be no idea that a uniform change in one definite condition of 
existence will produce a uniform effect on different animals.® 
This conclusion is self-evident when we reflect that the result 
of any influence must be the resultant of a reciprocal action of 
the external efficient force and of the inherent plasticity of 
the organism which is influenced. 

III. Influence of the volume of water.—It is well known 
that the volume of water has a marked influence on the 
growth of an animal, and on the size it finally attains. Every 
lover of the ‘gentle craft’ of fishing—for salmon, trout, or 
other fresh-water fish—knows that these fish are usually small 
m small streams and lakes, and attain their full size only in 
large ones. This fact has often been proved in America as well 
as Europe. All experimental zoologists know moreover that 
it is often difficult, or even impossible, to rear fresh-water 
animals in a small aquarium to the size which they grow 
to under the normal conditions of a free life in rivers, ponds, 
or even small pools.°4 This is attributed, if not without 
exception, ‘at least very generally, to a deficient food-supply. 
Without any experimental enquiry, and under the tacit assump- 
tion that all the other conditions—such as the temperature, 
the composition of the water, the amount of the oxygen it con- 
tained, and the number of individuals—were the same in the 
aquarium as in small ponds or large lakes, it was asserted 
that the smaller size of creatures in a small body of water was 
due solely to the circumstance that the absolute quantity of 
food at the disposal of each individual must necessarily be 
smaller ina small volume of water than in a great one, and 
hence be insufficient for the development of the animal’s full 
size. Of course it cannot be disputed that a fish must remain 
small if the food within its reach does not attain the daily 
optimum. But it has never been investigated whether the 
small size of the creatures in a small body of water is due, 
without exception, to the small amount of food within reach, 
either by proving that this actually was less than was indis- 
pensable for the full growth of the animal, or by attempting to 


160 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


show that any other influence was impossible. Observations 
do exist, on the contrary, which are calculated to warn us to be 
cautious in this matter. I will here only refer to the fact 1 
myself observed that some Water-Lice (Asellus) which were 
kept in an air-tight closed glass vessel for nearly two years, 
and produced three or four generations, were, in the last 
generation, abnormally small, though food, in the form of alge 
and other plants, was constantly abundant, and the air above 
the water, on opening the vessel, was found to be perfectly pure. 
In this case lack of food was assuredly not the cause of the 
small size of the Aselli; perhaps it was a result of constant 
inbreeding, although in so small a number of generations—only 
four—this is hardly probable. Hence it is a quite unfounded 
assertion to say that the small size of animals in a small body 
of water is always the result of a consequent deficiency of food, 
since if this were so, whenever a more than sufficient supply of 
food is at hand in the small body of water, the full growth 
ought to be attained. But this is not always the case, which 
proves that the often-observed effect of the volume of water 
on the size of the creatures living in it is not, up to the present 
date, understood, and still awaits an explanation. 

In order to solve this problem if possible, I carried out an 
extensive series of experiments on the common pond-snail, 
Lymnea stagnalis. I selected this creature because its growth 
is tolerably rapid in comparison with others, and because its long 
spiral shell offers an excellent test, of which it is easy to avail 
oneself in estimating its rate of growth. Moreover, this animal, 
as I had learned from an accidental observation, is so remark- 
ably sensitive to the effects of the volume of the water, that, 
in the space of six days, the difference in the length of those 
living in different volumes of water could be easily and 
accurately determined. 

It will be understood that I can in this place give only the 
general results of experiments carried on for more than two 
years. 

I instituted two series of experiments—one hy separating the 
animals from the same mass of eggs immediately they were 
hatched, and placing them simultaneously in unequal bodies 


EXPERIMENTS ON LYMNZA. 161 


of water; the other by placing two different quantities of 
animals, from the same mass of eggs, in two aquaria of equal 
size. All the conditions of existence, and above all the supply 
of food, were kept at the optimum. Consequently all the 
animals were under equally favourable conditions, irrespective 
only of the volume of water which fell to each animal’s share ; 
this varied at most between 100 and 2,000 cubic centimétres. 
In both experiments the results were similar (fig. 43): the 
smaller the volume of water which fell to the share of each 
animal, the shorter its shell remained; and, moreover, it made 
no difference, with regard to the length the shell attained in the 
different groups of animals, whether each isolated individual 
had from the first a definite quantity of water allowed to it, 
as in the first series of experiments, or whether several indi- 


6 6 


Fia. 43.—Four equally old shells of Lymncea stagnalis, hatched from the same mass of ova 
but reared in different volumes of water; a, in 100 cubic centimétres ; 6, in 250 cubic 
centimetres ; c, in 600 cubic centimetres ; and d, in 2,000 cubic centimetres. 


viduals living together had a larger volume of water to share 
among them in the same proportion. Thus I succeeded, under 
conditions of existence otherwise identical, in establishing a 
curve of growth for the Lymnea corresponding to the volume 
of water. This curve (fig. 44) shows that the favourable effect 
of an increase of volume of water is highest between 100 and 500 
cubic centimétres for each individual, and that it then gradually 
decreases, till, at 5,000 cubic centimetres, it would seem to cease 
entirely ; i.e. an increase of volume above this maximum has, 
as it appears, no further effect whatever upon the rapidity of 
growth. Thus the optimum of the volume of water which allows 
the greatest possible length of shell to be attained by a Lymnza 
within a given time lies approximately between 4,000 and 5,000 
cubic centimétres; to determine the point exactly was impos- 
sible for various reasons. The woodcut (fig. 43) exhibits the 


162 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


sheJls corresponding to this curve. The first of the shells, formed 
in 100 cubic centimétres of water, attained a length of only 6 
millimétres ; the second, in 250 cubic centimétres, was 9 milli- 
métres long; the third, in 600 cubic centimétres, was 12 
millimétres ; finally the fourth grew to 18 millimétres in 2,000 
cubic centimétres of water. It scarcely need be repeated 
that these animals, with such immense differences in length, were 
all the offspring of one mass of eggs simultaneously transferred, 
and had all reached the same age of sixty-five days. 

My experiments also allowed of my constructing wu curve of 
time for the rate of growth of the Lymnea, The reader may 
have observed, with reference to the foregoing statements, 
that according to this volume-curve it ought to be possible to 


Length of shell in 
millimetres. 
~ 
S 


Be 


0 | 
LTT 

0 200 400 600 800 1000 1200 1400 1600 1800 2000 cubic cen- 
timétres of water. 


Fia, 44.—Volume-curve for Lymncea stagnatis. 


enable a Lymnea to attain its full length of about 24 milli- 
meétres (for the first year’s growth) even in a volume of 100 
cubic centimetres, if only it were left there for a longer time 
than was requisite for acquiring that length in 2,000 cubic 
centimétres. Still, this would only be possible if the rate of 
growth, as determined by the volume of water, were at all times 
equal. This, however, is not the case. A# first the growth is 
very slow ; then succeeds a period of quickest growth, until the 
older the animal is, the more slowly it grows. ‘The curve ex- 
hibited in the subjoined woodcut (fig. 45) was constructed from 
experiments in a volume of water of from 1,000 to 2,000 eubic 
ceutimétres per individual, and it shows that, during the first 
three weeks after escaping from the egg, the growth of the young 
animal was, on an average, only 5 millimétres; then followed 


RATE OF GROWTH. 163 


a period, lasting about three weeks, of very rapid growth, 
during which it attained a length of shell of about 10-2 
millimétres; in the third period—40th to the 60th day— 
it grew only 6 millimétres more; and then ensued a period of 
very slow growth, only about 1:6 millimétre in nearly four 
weeks. It is thus proved that the same law obtains for 
Lymnea as for all other animals, namely, that the maximum of 
growth can only be attained, when all the other conditions are 
equally favourable, exactly within the period of quickest growth ; 
the opportunity once missed never recurs. A Lymneza which 
in the course of the first year has not attained a length of shell 


—— 


~ 
PN 


Age 
indays. 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 


Fig. 45.—Curve of time for the growth of Lymnca stagnalis. The growth is most 
rapid from the fourth to the fifth week. 


of at least 20 millimétres must become the parent of a race of 
dwarfs, if the causes which have checked its growth are regu- 
larly repeated in the succeeding years. 

A direct influence is hereby proved to be exerted by the 
volume of the water, irrespective of the supply of food and 
other influences.6° A short discussion of the details will show 
whether we are as yet in a pos‘tion to determine the special 
causes of this effect of volume. 

It is self-evident that a great variety of influences might 
have produced the same result—z.e. the dwarfing of the animals 
—such as food, temperature, injurious gases, or the absence of 


164 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


those that were necessary, &c. In some of my experiments 
such influences showed themselves very conspicuously. In one, 
for instance, in order to supply the young animals with the 
maximum of air required for respiration, I kept up a constant 
current in the vessel; but the experiment failed totally, 
because the young animals in the current were unable to cling 
to the plants they fed on. On another occasion a sudden and 
very considerable fall of temperature set in, almost down to 
13° centigrade. Now this is the degree at which the powers 
of assimilation of the young Lymnza cease, and consequently 
its growth. The effect on the curve of volume was very 
striking (fig. 46). The vessels, of unequal size and containing 
unequal bodies of water, stood side by side at the same distance 


iS} 
i) 


| ] 


H 
a 


= 
— 


ae 4 


Length of shell in 
millimetres, 


a 


= 


0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 cubic centi- 
miétres of water. 


Fic. 46.—A curve of growth totally altered by a change of temperature. It continues to 
rise as usual up to 500 cubic centimetres of water. There it suddenly falls, because the 
temperature of the large body of water is insufficient to allow the Lymneide living 
in it to assimilate. 


from a window, where the sun shone in the afternoon for two 
hours at the most; thus the temperature favourable to assimi- 
lation was attained in the smaller vessels, but not in the 
larger ones. Hence it followed that snails which lived in 2,000 
cubic centimétres of water, and consequently ought already to 
have been 10 millimétres long when 25 days old, were 
scarcely longer—about 3 millimétres—than those which had 
lived in water which, though less in volume, was sufficiently 
warm. At the same time the nourishment provided in each 
vessel was so abundant and wholesome that neither bad air nor 
a lack of food could have occasioned the striking break in the 
normal volume-curve; besides, in that case the effects must 
have been visible in those in the small as well as those in the 
larger body of water. 


AN UNKNOWN STIMULANT. 165 


A. superabundance of food was purposely supplied through- 
out all the experiments, and all the glasses in which the food- 
plants—Alye, Elodea canadensis, and Lemna—did not grow 
luxuriantly were emptied, so that from the very first, in all the 
experiments on which the curves were founded, the injurious 
effects of any kind of dearth of food were excluded. 

The temperature of the water, so long as it oscillates within 
an insignificant range near the optimum, also has no effect on 
the volume-curve, or at any rate a very trifling one; if the in- 
fluence of volume had been affected by variations in the tempe- 
rature of the water, all the animals in the various experiments 
growing up in different bodies of water, so long as they were at 
a similar temperature, must have attained the same or nearly 
the same size. Temperature does not exercise a really decisive 
influence till it approaches one of the two utmost endurable 
extremes. 

It might also be supposed that the different proportions of 
oxygenated air or carbonic acid contained in the water were the 
efficient cause; but this is easily disproved. In the first place, 
it is not very easy to see how, in that case, such regular curves of 
volume could arise, since the deficiency of air in each vessel must 
then have been always in the same proportion as the body of 
water; and this can scarcely be assumed as probable. On the 
other hand, this influence was already excluded by the fact that 
the superabundance of plants growing in the water disengaged 
so much oxygen that the water in all the glasses must have 
been absolutely saturated with it, and the stratum of air in 
contact with the surface, which, as is well known, is breathed 
by the Lymnza, must have been equally and perfectly pure. 
For the same reason the carbonic acid disengaged by the animals 
must always have been entirely reabsorbed by the plants. 

The salts which can be proved to be present in water can 
just as little be regarded as the cause of the dwarfed growth of 
the animals. With the assistance of a chemist, my friend 
Professor Hilger of Erlangen, I repeated my experiments with 
distilled water, and with water which was saturated with the 
constituents proved normally to occur in water (such as mag- 
nesic sulphate, calvic carbonate, &c.), and the normal course 


166 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


which regularly ensued showed that the salts present and dis- 
coverable by chemical tests had really no influence that could be 
detected. 

Even injurious gases which might be formed, in a certain 
proportion to the body of water and the number of animals, 
from the feeces and other decomposing organic matter, cannot be 
regarded as causing the effects referable to the volume of water. 
Suppose we take two vessels containing equal volumes of water, 
but place only one animal in one and twenty in the other, these 
last of course will disengage the larger amount of injurious gases, 
and consequently, in the first instance, a certain retardation in 
their growth might be caused. But since the one isolated 
individual grows immensely faster than those that live together, 
this one will very soon yield as much, and at last more, fecal 
matter than the twenty; so that growth must cease with it at 
least as soon as with the others. But since this is precisely 
not the case—for the curve of time remains the same for each, 
while it is only the size attained within a determined period 
which varies—it is, it seems to me, thereby proved that the 
effects of the injurious gases produced by the animals them- 
selves cannot be the cause of the effects of volume. 

What, then, is the real cause? I regret to say that I cannot 
give any answer to this question. With the assistance of my 
friend Hilger I have long been trying, but altogether in vain, 
to find anything present whatever, even in the smallest quantity, 
to which this effect of volume could be ascribed. I think, 
however, that we are justified in the following hypothesis. It 
would seem to follow from my experiments that some substance 
—as yet unknown—must be present in the water, probably in 
a very minute quantity, which, by its relations to the water that 
holds it in solution and by its osmotic affinity to the skin of 
the animal, can be absorbed only in a determined and extremely 
small quantity, and also within a definite period and in a definite 
amount of water. Now, if this substance were simply a stimu- 
lant, and thus, without actually contributing to growth, -were 
nevertheless essential to it—like the oil to the steam-engine— 
it must be absorbed in the quantity which is most favourable 
if the normal growth is to be accomplished within a fixed time, 


MODES OF RESPIRATION. 167 


And since, according to this hypothesis, the amount of the 
substance absorbable in a given time depends on the volume of 
the water, and increases or diminishes with it, growth would 
cease entirely if the body of water were so small that it had a 
stronger affinity to the unknown matter than the skin of the 
animal has. On the other hand, the attainment of the full size 
within the corresponding period would only be possible if the 
volume of water were so great that the Lymnza could at all 
times absorb this unknown stimulant from the water.§7 

IV. Influence of oxygen or air in the water.—We have 
seen in the foregoing sections that the effect of the substances 
held in solution in the water, and also apparently that of the 
volume of the water, depend’on the osmotic action of the 
animal's skin. Another substance held in solution in the 
water must take effect in a precisely similar manner—namely, 
the air used up in breathing by a number of animals. It is 
known that every animal, even the simplest Infusorium that 
lives in water, requires air, or rather oxygen, for respiration ; and 
as most aquatic animals have no special organs for breathing 
in the air itself—like most of the Vertebrata, as well as 
Insects, Arachnoide, Myriapoda, and many other creatures— 
their efficient respiration depends exclusively on the absorption 
of the air (oxygen) contained in solution in the water through 
the skin, or through the membranes of some internal organs 
through which water flows in and out in a constant stream. 

It is self-evident that every growing or young cell must be 
capable of breathing, ¢.e. of taking in oxygen and disengaging 
carbonic acid, if this respiration is a function of protoplasm 
itself. All growing parts which are in contact with media rich 
in oxygen, such as air and water, must consequently breathe,°* 
taking it for granted that their surface offers no resistance to 
the absorption of oxygen. But the disposition to absorb it 
may be very different in different parts of the body ; and we are 
accustomed to call such parts as seem better fitted to absorb 
oxygen, as compared with the others, simply ‘organs of 
respiration.’ 

Such specialised organs of most of the animals that live in 
water and breathe water ® are, in the first place, the skin, then 


168 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


appendages of the skin, known as gills or branchiz, and finally 
the interior of the intestinal canal. In very many Invertebrate 
animals—as in Holothuria, Annelida, Planarians, Water-Insects, 
and others—a constant stream of water enters by the anus, and 


Fic. 47.—a, Anabas scandens; head, with & the gill-cavity laid open, and/ the conti- 
guous cavity containing the foliated labyrinthine structure. 6, Tadpole; c, young 
Polypteru from the Nile; d, embryo of the Shark. All these have external gills, br. 


in a few cases, as in Holothuria, a very easily demonstrable 
stream passes out from it also, Thus, in the simplest condition, 
the mucous membrane of the intestine serves for respiration, 
like the skin of an animal; and in this respect the well-known 


FORMS OF BRANCHLE. 169 


French physiologist, Paul Bert, is perfectly right when he says 
that any dispute as to whether this or that portion of the body 
of an animal is its respiratory organ is fundamentally and 
perfectly superfluous. But when special appendages are de- 
veloped from the skin in a foliated or arborescent form—known 
as gills—which seem specially adapted by their structure and 
the delicacy of their walls to absorb more air from a given 
body of water than the skin can, and to transmit it directly to 
the blood or to the fluid of the body-cavity which circulates in 
those gills, we are certainly quite justified in designating these 
appendages as special organs of respiration. 

Such gills or branchie occur in the intestine as well as on 
the skin of the most dissimilar animals living in water. 

The gills of the outer skin bear so striking a relation to 
the animal’s mode of life that they must here be briefly dis- 
cussed. In Fishes (fig. 47, a) and in many Amphibia the gills 
are placed at the side of the head or partially under it, where 
they are concealed beneath larger or smaller folds of the skin, 
which, with the flat bones that support them, are known as the 
gill-covering. In the embryo of the Shark (fig. 47, b, d) and of 
Amphibia, external ramified gills appear before these internal 
gills; these, in the fishes, subsequently disappear, but in the 
Amphibia persist throughout life (Perennibranchiata). In 
Crustaceans we often find gills in places analogous to their posi- - 
tion in fishes, that is to say, by the side of the cephalothorax, 
and covered by a large skin-fold attached to it; this is the case 
in Orabs (Brachyura), and in many of the Macroura, Lobsters, 
Prawns, &c. In other Crustaceans, on the contrary, as in our 
Water-lice (Asel/us) or the Sea-louse (dotea), they are situated 
at the end of the abdomen, and in yet other species they are 
appendages of the legs, whatever part of the body these may 
be attached to. In the class of Mollusca we find not less than 
five forms of gills morphologically different—first, the usual gills 
of bivalves (fig. 48, b); secondly, those borne on the back of 
many of the naked marine Mollusca, as the #olide, Doris, and 
others (fig. 48, d) ; thirdly, the dorsal branchial cavities of such 
genera as Neritina and Melania; fourthly, lateral gills, as in 
Phyllidia; and lastly, the highly interesting mantle-gills of 


170 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


some species of Lucina (fig. 48, «), situated on the ventral 
margin of the mantle. In the Annelida the gills are usually 
an appendage of the legs, and sometimes are placed directly on 
the body or at the fore end, as in Sabella, Serpula, Terebella, 
&c. Finally, the number of Invertebrata is by no means small 
which dispense entirely with such distinct, conspicuous organg 


Fic. 48.-Gills of Mollusca. a, Lucina philippensis, with four mantle gills behind the 
muscle m; b, Mytilus, with & the gills, and 7 the laminated lip; c, Elysia grandis 
(Bergh.), destitute of gills; d, Doris sp., with a tall tuft of dorsal branchise. 


of respiration, and consequently breathe only through the skin ; 
among the Mollusca there ave the Llysiadee (fig. 48, ¢) and their 
allies; among the Annelida the common leech and the Oligo- 
cheetz (the earth-worm, «&c.); many of the lower Crustaceans— 
parasitical as well as independent—all Infusoria, the Ceelente- 
rata, and even many Echinodermata, &c. 


RESPIRATION BY THE INTESTINE. 171 
a 


Less variety is found among the internal gills, which some- 
times are situated in the intestinal canal of water-animals. In 
the larve of the Libellulide, for instance, leaf-shaped organs 
are found inside the rectum, which apparently serve for respira= 
tion. I myself have described a system of foliated processes 
on the mucous membrane of the stomach of the Holothuride 
(fig. 49) which have all the attributes of true gills—as an 
extensive surface, delicate membrane, and abundant blood- 
vessels, with a constant renewal of the water that bathes the 
lamine. In most Annelida and many other Invertebrata, no 


Fig, 49.—Part of the stomach of a Holothurian (Stichopus variegatus) split open length- 
wise and laid flat. a, the dorsal furrow between the two series of gill-foliations; b, 
the broad tumid ventral surface which divides them ; c, the foliaceous gills, 


doubt a regular current of constantly renewed water passes 
through the intestine, which nevertheless hears no special gills ; 
the more or less extensive folds of the mucous membrane 
here take the place of the absent organs. It may here be inci- 
dentally mentioned that even a fish (Cobitis fossiles, a species of 
Loach, fig. 50) breathes through the intestines ; but in this case 
the conditions are slightly different, inasmuch as it takes in air- 
bubbles at the surface of the water through its mouth, and 
swallows them, so that here the air comes into direct contact 
with the respiratory surface of the intestine.’® 

All these different organs of respiration’! must act in 


172 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


essentially the same manner—namely, by absorption of oxygen 
(air) from the water by means of the osmotic action of their 
membranes. This action varies with the different animals. 
Bert 7? has shown that the powers of absorption even in nearly 
related species of fresh-water fish are remarkably various. 
Hence we need not be surprised to find that occasionally the 
amount of oxygen which is conveyed to the blood by the typical 
organs of respiration is not perfectly sufficient, so that the defi- 
ciency has to be made up in other ways. Nor are we more 
astonished to learn that the general respiration of the skin can 
be so inereased that under some circumstances it suffices per- 
fectly for the requirements of the animal, and renders the 
employment of the special organs of respiration quite super- 
fluous. 

The former case has been proved in Crustacea and in Fishes. 


Fic, &0.—Cobitis fossilis. It swallows air-bubbles which pass through the intestine, 
where the mucous membrane takes up the oxygen for respiration, 


Fritz Miiller observed that crabs of various species (Grapsus, 
Sesarma, &e.) often raised the hinder portion of the cephalo- 
thorax, so as to let the air directly into the branchial cavity, as 
the amount of oxygen absorbed from the water through the 
gills was insufficient to supply their requirements. Many of our 
fresh-water fish, particularly all the species of Bleak (Cyprinoide), 
regularly swallow air in order to saturate the water that 
bathes their gills with oxygen, since the oxygen derived 
directly from the water is usually insufficient. The amount of 
oxygen needed by these fishes must be considerable, for it is 
much more easy to drown a fish than a frog if both are pre- 
vented from coming to the surface to swallow air; and yet the 
frog breathes by lungs, while the fish, on the contrary, is a true, 
gilled water-breather.”* 

An instance of the second case—i.e. that general respiration 


THE AIR-BLADDER IN FISHES. 173 


through the skin may perfectly supply the place of respiration 
through any special organ—is offered by frogs, which usually 
breathe through lungs. Milne-Edwards the elder showed long 
since that frogs, when prevented coming to the surface, were 
able to live under water so long as they were not cut off from 
the possibility of obtaining food and were freely supplied with 
fresh water. In such a case general skin respiration must 
necessarily take the place of lung respiration. Since then, 
Paul Bert 4 has shown that skin respiration can only take the 
place of lung respiration when, in the cold season, the tempe- 
rature of the water varies between zero and 13° centigrade. 
The instances here adduced prove at once that the absolute 
amount of oxygen needed for respiration and absorbed from the 
water variex according to the peculiarities of the different species, 
and perhaps even in individuals; and moreover that its ab- 
sorption depends on certain external conditions, above all on 
the temperature. From this it further follows that there must 
be for every individual animal an optimum quantity of oxygen 
needed in a given time; if this optimum is not attainable by 
the ordinary organs of respiration, either the animal dies of 
suffocation, or else the deficiency must be supplied by some 
other means, as, for instance, in Milne-Edwards’ experiment 
on the frog, by general skin respiration. To this category 
belongs too, in a certain sense, the air-bladder of fishes, which, 
according to the most recent investigations, may under some 
circumstances be regarded as an organ auxiliary to respiration. 
A body of gas is deposited in it from the blood which also 
contains oxygen, and this is rapidly used up if the fish is in 
water which holds but little oxygen. Now, although generally 
no air can be transmitted to the air-bladder from outside, still, 
as it would seem, it serves as a reservoir for the superabundance 
of oxygen which is introduced into the blood by the absorbent 
action of the gills and the skin. Very numerous experiments 
have been made on this matter, but they have yielded so many 
contradictory results that it is superfluous to go here into any 
closer discussion of them ; and I refer those who are interested 
in the matter to Milne-Edwards’ well-known work, ‘ J.egons 
d’Anatomie et de Physiologie comparée.’ In a note 75 I have 


. 


174 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


given the titles of some new works not mentioned by Milne- 
Edwards. 

V. Power of enduring desiccation.—All water animals 
need a very high degree of moisture in the air or the direct con- 
tact of water to enable them to live; if a frog is transferred to 
dry air, it wili quickly part with all its water to the atmosphere 
and perish of desiccation. 

It has, however, been frequently stated of many water 
animals that they can endure perfect desiccation without dying. 
The experiments of Spallanzani, Dugés, Doyére, and others are 
generally known. Infusoria and certain worms of low type, 
the Rotatoria, the somewhat high-typed Tardigrada, and various 
kinds of Crustaceans, are said, according to these observers, to 
revive after being completely desiccated. The fact that when 
perfectly dry moss or hay is wetted all sorts of creatures are 
brought out of it is undoubted; but Pouchet’s experiments 
account for this in a very simple manner, while, as it seems to 
me, they strikingly prove that true and complete desiccation 
infallibly destroys fully grown creatures. He showed that 
Infasoria, Rotatoria, and Tardigrada, when dried up in the 
nidus, always die if they are actually and truly desiccated, but 
that sometimes germs, or it may be eggs, contained in them, 
and which are protected from utter desiccation by their enve- 
lopes, on being moistened again develope rapidly, though still 
enclosed in the desiccated matrix, and creep out. These young 
animals creeping out from the eggs and germs have apparently 
been mistaken for the dried-up creatures resuscitated. The ob- 
servations recorded as to the capability of many animals of the 
higher orders, even of Vertebrata, for enduring drought are not 
quite so erroneous; for it is not asserted that they themselves 
were desiccated. In tropical countries or in the Mediterranean 
province, where there is a sharp distinction between the wet 
and dry season, many animals fall during the latter into what 
is known as summer sleep. The Protopterus in Africa buries 
itself in mud, and surrounds itself with a thick perfectly 
desiccated crust, in which it is able to pass a latent life for 
months together, till the rain softens the crust and releases the 
fish. Many land snails attach themselves, during the day or 


RESISTANCE TO DROUGHT. 175 


during prolonged drought, to plants, stones, &c., or bury them- 
selves in the soil and close the mouth of their shell with a 
calcareous deposit known as the diaphragm ; thus they await 
the next rainy season to recommence an active life. Here it is 
easy to prove that the animals are not truly desiccated; for if 
we break into the shell of a suail thus found in its summer 
sleep, we see at once that the creature has preserved a very 
considerable amount of moisture, which the hygroscopically dry 
air has not been able to evaporate from the animal, protected as 
it is by its shell and diaphragm. 

It is to this property possessed by living animals of retaining 
a certain amount, however small, of moisture for a long time 
in their tissues, and consequently of escaping total desiccation, 
that the power is evidently due which enables the eggs and 
germs of the above-mentioned animals to continue all the year 
round’® in an apparently diy condition without being deprived 
of their vitality. It is certainly very striking that encysted 
Infusoria, and the ova or reproductive bodies of Cru taceans, 
Tardigrades, Worms, Sponges, &c., are capable of withstanding 
the long-continued desiccating effects of the air ; but if at the same 
time we remember that it is extremely difficult to desiccate albu- 
minous matter completely, even when dead, the fact loses some- 
thing of its astonishing character. Living plants, too, often 
retain the last remnant of their moisture with much obstinacy. 

Of the truth of these facts there certainly cannot be the 
smallest doubt. I have had for now six years a chest full of 
dried mud with the eggs of Apus and Cypris, which were sent 
to me in the spring of 1872 by Ehlers from Erlangen. Up to 
the present time every attempt to hatch out some of the 
Apus larve by softening a part of the mud has succeeded 
equally well in summer and in winter; the rapidity of their 
development is different, but this is due, as we have seen in a 
former chapter, to the degree of temperature at the time. 

Now, remarkable as is this long resistance of eggs to 
drought, we are acquainted with a much more wonderful, 
und, in fact, hitherto inexplicable, fact connected with it. The 
eggs of various Crustaceans—as, for instance, of species of 
Apus—never develope” if they have not first lain some time in 


176 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


the dry mud. Living specimens of Apus caught by me in 
Wiirzburg deposited a large number of eggs in the water of 
my aquarium; not one developed. Mud full of eggs from 
the same pool from which I had taken the fully-grown Apus, 
after it had lain by for a full year, still yielded no larve on 
being wetted, and it was not till the second year that I ob- 
tained a few; but from that time I succeeded regularly in 
making them develope in great numbers, and whenever I chose. 
The statements of Brauer and others coincide with this. The 
eggs of the Branchipus, so nearly allied to Apus, do not share 
this peculiarity, but develope equally well whether they have 
been dried or kept constantly in damp mud. Brauer points 
out indeed, in his interesting notes on his experiments in breed- 
ing, that it would be very easy to rear animals of these groups 
from different parts of the world in our laboratories, and to 
study them at our convenience ; since nothing would be needed 
but to obtain some dried mud from the localities where they 
live. In this way, for instance, Professor Claus was recently 
enabled carefully to investigate, in Vienna, the anatomy of the 
beautiful Daphnia Atkinsoni from Jerusalem. It would cer- 
tainly be a grateful task to determine exactly what species of 
animals have eggs which can endure desiccation, or even abso- 
lutely require it, like Apus, to qualify them for development, 
and to find out also what the maximum period is during which 
they can endure to lie dry without injury to their vitality. A 
fundamental investigation of these questions would undoubtedly 
contribute much to « satisfactory explanation of many peculiar 
facts in the geographical distribution of the lower animals.’8 
Concluding remarks.—If we now compare the facts 
established in the different sections of this chapter with those 
previously ascertained, we obtain again the same general laws. 
Animals living in the same places, and apparently under the 
same external conditions of existence, nevertheless behave in 
quite different ways under the influence of the various sub- 
stances held in solution in the water, as salt, oxygen, carbonic 
acid, &c. The ova of different and yet very closely related 
forms can endure a long period of drought, or even require it 
to enable them to develope. Hence every change, as, for in- 


MODIFICATION OR SELECTION. 177 


stance, in the composition of the water of a lake or a river, will 
not affect the fauna inhabiting it equally and as a whole, but 
will act on individuals; some will bear the change without 
being in any way affected by it; others will die, while others 
again will survive, but their habits of life will be changed, and 
at the same time their structure will be modified, as in the 
case of Branchipus and Artemia. Thus the constancy of the 
aquatic fauna of any spot depends on the constancy of all the 
external conditions of life prevailing there, and every change, 
however small, in these will effect a selection among the old 
forms, facilitate the introduction of new ones, and sometimes 
even determine the transformation of one species into another. 
On this last and most important point we at present certainly 
know very little; but the old experiments of Beudant, the 
more recent ones of Plateau, and, above all, those of Schman- 
kewitsch, show that this absence of information cannot be 
adduced as a convincing argument against the assumption that 
careful experiments directed to this question must tend to 
prove that stagnant water and the substances contained in it 
can exercise a far more direct transforming influence than has 
hitherto been considered possible. 


9 


178 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


CHAPTER VI. 


THE INFLUENCE OF A STILL ATMOSPHERE. 


THE most important influence of stagnant air on the animals 
living in it is strikingly exhibited by those organs which are 
intended to respire air and convey it to the interior of the 
body. The physiological action of these air-breathing organs is 
exactly the same as that of the skin and gills in water-breathing 
animals. They bring the blood into the closest possible contact 
with the oxygenated medium. But as regards structure no greater 
contrast can be conceived of than that between the gills ofa fish 
and the lungs (fig. 51) of the higher Vertebrata, or the trachez, 
as they are termed, of Insects, Myriapoda, and Arachnoide. 
_ These last (fig. 52) are usually fine tubes, with elastic walls 
with spiral] thickening, which ramify in all directions, and 
which allow of the alternate inspiration of fresh air and 
expiration of foul air—charged, that is, with carbonic acid. 
This is effected by’the opening and closing of the stigmata, or 
openings of the tuhes, by the act of respiration, and by the 
expanding and contracting of the tubes themselves. These 
trachese thus convey the air, in extraordinarily fine particles, to 
all the organs,’® so that their finest living portions certainly 
and abundantly absorb the oxygen they require direct from 
the air which is so brought to them. It is otherwise with 
the Vertebrata. Here the air is taken into sacs of a spongy 
structure (see fig. 51), of which the walls contain an exces- 
sively intricate network of blood-vessels; here, exactly as 
the skin or gills of fishes absorb oxygen from the water, the 
oxygen from the air passes by endosmosis through the mucous 


RESPIRATION IN AIR. 179 


membrane of the lungs into the blood; this oxygenated blood 
is then carried to all the organs, of which the living portions 
take up the oxygen from it, precisely as the corresponding parts 
of insects take it up directly from the air by means of the 
trachee. In all animals that breathe thus through the lungs, 
there is a strongly marked contrast ®° in the blood contained in 
different parts of the vascular system. That which is carried 
back from the lungs to the heart is rich in oxygen and known 
as arterial blood, and that which circulates in the vessels which 
convey it from the organs to the heart, or from the heart to the 
lungs, is poor in oxygen, and is called venous blood. 

We need not, however, in this place, investigate more closely 
the relations of the vascular system to the respiratory organs, 


Fig. 51,—Section of the lung of the embryo of the Pig, showing the spongy texture. 


nor the physiological distinctions which are based on the dif- 
ferent organs of respiration and their structure. On the other 
hand, it is essential that we should in the first instance deter- 
mine which of the constituents of the air are advantageous or 
injurious to animal life. 

Air contains, when it is pure, almost 21 per cent. of oxygen, 
with about 79 per cent. of nitrogen, and a variable trace of 
carbonic acid, besides water which it holds in solution in the 
form of vapour in a quantity varying according to the tempera- 
ture. All the other kinds of gas which are occasionally present 
in the atmosphere are of no importance. They are either irre- 
spirable or actually injurious, while the above-mentioned mix- 
ture is the normal one, and thus is the most favourable for 
animal life. Certainly we must make this statement with 


180 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


some reservations ; for, in the first place, we know that various 
animals are, as a rule, influenced in different ways by the 
medium in which they live; and besides, we cannot assert, on 
the basis of any experimental research, that certain gases which 
are injurious to men or to birds may not be indifferent or even 


Fig. 62.—Tracheal system indicated within the outline a of a Water-bug, d of the larva of 
an Eschna, The trachex are shaded. 

advantageous to other animals. For instance, it is well known 

that many larve of insects live in situations, as in decaying 

matter, where the air is undoubtedly mixed with gases which 

the higher Vertebrata could not breathe without injury ; also 

that the capability for resisting the effects of irrespirable or 


DRY AND MOIST AIR. 181 


poisonous gases is extremely different in different animals, 
Exhaustive experiments on this subject are not before us; 
hence the only general conclusion, as applicable to all animals, 
that we can draw from the experiments made on certain 
animals by physiologists, is, that every gas contained in the air 
affects the animals that breathe it according to its relative 
proportion, and to the peculiarities of each individual animal. 
Thus, for instance, carbonic acid, which is highly poisonous to 
man, ceases to be injurious when it is contained in the atmo- 
sphere in a proportion of only 1 to 2,000 (of volume); but even 
then it is not directly advantageous to animal life, unless, 
indeed, its stimulating action may perhaps be recognised as a 
not unimportant factor in the life of animals. At present, how- 
ever, we know little on this point. On the other hand, we 
know positively that no air-breathing animal is capable of de- 
composing and assimilating carbonic acid as plants do;®! nay, 
it may be doubted, as we have seen in a former chapter, 
whether even those aquatic animals— Sponges, Infusoria, 
Worms, &c.—which are said to have true chlorophyll in their 
tissues, do in fact make use of this constituent as an organ for 
the decomposition of carbonic acid ; nor do we know whether 
the maximum of carbonic acid which can be endured by the few 
animals experimented on—which is perhaps even advantageous 
to some—is equally endurable by all air-breathing animals, or 
whether, for many of them, it may not lie even higher. Pro- 
bably in this respect the various species, and perhaps even 
different individuals of the same species, may behave quite 
differently.8? 

What is of more importance to our enquiry, at any rate in 
this place, than the admixture of different gases in air, is the 
proportion of water contained in any given volume of air at a 
given time. Our personal experience teaches us that a dry ora 
damp wind has a totally different effect on different indivi- 
duals; phthisical patients are sent in North America to the 
driest mountain regions of the Union, as Colorado, while in 
Europe they are frequently sent to very damp places, as 
Madeira, &c. Moisture in the air frequently induces rheuma- 
tism, but in this respect also different individuals are differently 


182 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


affected. We will now go more fully into a few particularly 
interesting cases of resistance to extreme dryness of the atino- 
sphere, or to perfect saturation of the air with moisture. 

I. The power of resistance to a dry atmosphere.—The 
atmosphere that les near the surface of the earth is never 
perfectly dry ; but we usually call it so when it is drier than is 
good for our health or agreeable to our feelings. This occurs, 
for instance, with tolerable regularity every summer in Wiiz- 
burg ; * many tropical regions are distinguished by a very dry 
climate ; this is the case with the Sahara, the desert plains of 
Australia, and the desert coast of South America, where it 
never or very rarely rains; and even in tropical countries 
which are justly regarded as having a very damp climate—as 
Java or the Philippines—we nevertheless speak of a dry season, 
and everyone who has lived for any length of time in these 
islands knows that the dryness of the atmosphere there has a 
very unpleasant effect, although at least 50 per cent. of moisture 
is always present in the atmosphere. Hence all animals living 
in such localities must be able to withstand the desiccating 
effects of the atmosphere if they are to survive; and-if an 
originally damp climate were suddenly, or even by gradual 
change through local disturbance or secular variation, to 
become a dry one, a great number of species would infallibly 
fall victims to this change in the conditions of existence. 

Nevertheless animals live even in the driest places on the 
earth; many of these, indeed, belong to groups of animals of 
which the greater number require a very high degree of mois- 
ture in the air to enable them to live. This is the case, for 
instance, with many land Mollusca. Our common Road- 
snail and those Snails that creep about on trees require a very 
considerable amount of moisture in the atmosphere, or they 
cannot eat, digest, and grow. During the dry summer of the 
Mediterranean regions, even on islands, as the Balearic Isles, 
the active life and growth of these creatures is interrupted : 
they bury themse!ves deep in the dry earth or between slabs of 


* The place where the Author writes; but the case is the same, of 
course, with many places on the Continent. 


EVAPORATION. 183 


rock, and close the opening of their shells with a lid (operculum 
or epiphragm)—often of many layers, and membranous or cal- 
careous—which evidently contributes to prevent the utter 
desiccation of the creature. Other species again cling firmly to 
stones or plants, where they remain for weeks, and seem to be 
protected against the drought. Their powers of resistance, 
however, are not perfect ; every collector knows that a certain 
number perish annually from desiccation, and these are by no 
means old and enfeebled individuals, which in any case were 
approaching the end of their life, but for the most part young 
ones, not fully grown. From this it would appear that the 
young individuals are less able to resist desiccation than older 
or fully grown specimens. The same phenomenon is observ- 
able in tropical regions with an insular climate; here the dry 
season generally affects the land-snails iu the same way as in 
the Mediterranean province. Sometimes, however, local causes 
counteract the influence of the dry season. Thus, for instance, 
in a garden at Manilla in the Philippines, in the driest and 
coldest season, I found land-snails coupling, as well as their eggs 
and young, while in other spots the same or allied species were 
sunk in summer sleep. This was naturally the result of the 
increased local moisture of the air in this spot, under the thick 
leafy shade of large trees; nevertheless, even there, the abso- 
lute amount of vapour in the air was considerably less than 
during the wet season. Precisely analogous is the behaviour of 
Land-snails in deserts, where no one would expect to see animals 
living which part with the moisture from their bodies to a dry 
atmosphere so readily as snails do. These Desert-snails, as it 
appears, lead an active life only during the night or early 
in the morning, when a heavy dew moistens the soil; the 
moisture induced by the presence of vapour in the atmosphere 
is, however, very soon absorbed again, and during the dry day- 
hours the snails attach themselves somewhere where they are 
protected against desiccaticn. Thus the time during which 
they can imbibe the necessary moisture is about—or scarcely— 
as great for these animals as for the land-snails of the Philip- 
pines in the dry season. Still we must not overlook, in the 
first place, that probably they may be able to obtain a greater 


184 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


quantity of water from their food, consisting of the fleshy-leaved 
succulent desert-plants, than other snails; and, in the second 
place, that they may also be capable of absorbing a larger 
amount of water endosmotically through the skin than the 
snails living in our damp climates. Unfortunately, so far as I 
know, no experiments and observations exist as to the rapidity 
and period of growth of land-snails in countries where the 
moisture of the air differs widely or varies much. No series of 
systematic and accurate experiments are known to me, even 
with reference to our commonest snails, which collect in thou- 
sands every year, but only a few accidental observations ; §? so 
that the well-known statement of Agassiz—that in the shell of 
a Heliw a ridge corresponds to each year’s growth, like the 
annual ring in a tree—cannot at present be tested as to its 
general or partial accuracy. Researches in this direction would 
certainly be productive of results of universal value, as I am 
justified in concluding from a few general observations. Mean- 
while from the few materials at hand as to the behaviour of 
land-snails under various degrees of moisture in the atmosphere, * 
only one conclusion may be drawn which seems highly pro- 
bable: That the various species behave very differently in this 
respect, so that an alteration in the moisture of the air in any 
region must fundamentally alter the Snail-fauna inhabiting it. 
Other animals perish from desiccation in quite other ways. 
For instance, in the tropics, as well as in North America, very 
many insects die out almost completely during the dry season, 
which by no means always corresponds with the hottest season, 
as it does in America. On the western side of Luzon, January, 
the driest month, is also the coldest. Certainly even at this 
season a number of insects are always to be found, chiefly 
individuals of the commoner species; but these are for 
the most part old and worn-out specimens, and it may be 
reasonably doubted whether they would live long enough to 
secure the permanence of the species by reproduction at the 
advent of the following damp, warm season in the month of 
May. This, on the contrary, probably takes place exclusively 
or principally by eggs which have been dormant during the 
dry season, as we may infer from the fact that immediately on 


A SATURATED ATMOSPHERE. 185 


the commencement of the wet season a multitude of young 
larvee are to be found, which could not be the case if these 
old individuals had not till then coupled and laid eggs. Thus 
the eggs, though minute and only enclosed ina thin integument, 
show an especial resistance to drought. A parallel case is 
that of the eggs of many aquatic creatures which exhibit a 
power of enduring drought. I refer the reader on this point 
to what has been said above. But that the eggs of insects 
laid in the air, although perfectly protected by their envelopes, 
are not wholly impermeable to the air—that, on the contrary, 
they require that it should find access to the protoplasm of 
the ovum-cell—is proved by the following easily conducted 
experiment. If the eggs of insects are covered with a very 
thin film of resin or of oil, which prevents the passage of any 
air whatever through the pores of the integument, the embryos 
perish without exception, since the oxygen requisite for their 
respiration cannot penetrate to the protoplasm. Hence it 
follows that even though the ovum-cell may be partly pro- 
tected against desiccation by the envelope surrounding it, yet 
the perfect immunity shown by the eggs of most insects must 
be partly due to the properties of living protoplasm. 

II. The influence of a saturated atmosphere.—In many 
cases the moisture of the air reaches the maximum attainable 
under the existing temperature. This is not unfrequently the 
case, for instance, in European countries in the winter, and in 
the tropics during the rainy season, or under the leafy shade and 
protection of the primeval forests. 

Unfortunately we know next to nothing as to the influence 
of such absolutely damp air on the animals living in it; we 
can only say that it is highly injurious to some, and to others 
again particularly advantageous. An extremely remarkable 
fact, depending on this, in the geographical distribution and 
habits of life of certain animals needs a closer discussion. 

We should at first sight be naturally disposed to assume 
that species of animals whose organisation indicates adapta- 
tion to breathing water and to moving in water would be 
incapable of living in the air, since their skin must soon dry up 
in the air, so that it could no longer fulfil the functions proper 


186 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


to it, nor would they in many cases be in a position to use 
their organs of locomotion as suck. The observed facts, how- 
ever, do not correspond to this view, which was formerly 
somewhat hastily adopted. We know, on the contrary, that 
there are a tolerably large number of true aquatic animals 
which constantly or occasionally live on land. To these, for 
instance, belong the true Land-leeches, as they are called (fig. 
53, /), which live in the forests of India and the Indian islands, 
sometimes in such enormous numbevs that it is quite impossible 
for men to exist in them even for a few hours. I myself 
have often been driven out of the woods of Luzon and Min- 
danao, which are very favourable spots for insects and land- 
shells, by the myriads of leeches living on the trees and 
shrubs, from which they fall like a drop of dew on any human 
passer-by ; and I once read that a whole English battalion had 
to beat a retreat during the Sikh rebellion because they were 
attacked in a wood by these small blood-suckers in such numbers 
that passing through the wood was not to be thought of. They 
dry up with particular facility ; but as the air in these forests 
is constantly saturated with moisture, even in the driest season, 
they live in India in the open air on trees quite as well as their 
nearest allies, the Medicinal Leeches, do here in Europe in the 
water. Even more interesting are the land Planarians.84 They 
breathe, like the leech, through the skin (fig. 53, d, ¢), and dry 
up even more readily ; they move by means of fine, micro- 
scopically small hairs, the cilia or flagella, which are attached 
to the skin, and which by their peculiar motions can carry the 
animal forward when it is surrounded by a sufficient quantity 
of trickling water or of mucilage. On a perfectly dry surface, 
therefore, they cannot creep about for any length of time ; the 
rapidly drying skin would soon yield up all the moisture which 
the cilia on the under side require for their motions. Hence 
these creatures are always found only in very damp spots; on 
vocks, however, as well as on trees, or even on the walls of 
houses. A few small land Planarians, two or three species, 
occur even in Europe, where they have already been found in 
Denmark, Germany, Spain, and France. Planaria terrestris, 
which was discovered at the end of the last century by 


AMPHIBIOUS WORMS. 187 


O. F. Miiller, was observed by me two years ago in the Balearic 

Islands—Minorca—where I found it under stones in a shady 

and very damp spot, far from all stagnant or running water. 
Besides those just named there is still a considerable number 


Fig. 53.—a, Geonemertes palaensis, a land Nemertean of the natural size ; 6, the head 
magnified ; c, the proboscis and spine, with the poison-gland of the same creature ; 
d, Dolichopluna striata (Moseley) ; e, Bipalium sp., both land Planarians from the 
Philippines ; 7, Land-leech from the Philippines. 


of aquatic animals that live normally on land. To the land Ne- 
mertean discovered by me in the Pelew Islands of the Pacific 
Ocean (Geonemertes palaensis, fig. 53, a—c) a second has been 
added by Von Willemoes-Suhm ; they live with land Planarians 
under stones or low-growing plants.85 Many crabs of the 


188 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


family of Orchestide (fig. 54) live exclusively on land, although 
they have the gills proper to all aquatic species. In the summer 
of 1876 I found in Minorca an euormous number of individuals 
of one species *° under large stones in the perfectly dry bed of 
a stream, during the driest season of the year; and in the 
islands of the Indian Archipelago they are often quite as fre- 
quent as the Land-leeches in damp and constantly shady woods. 
Various species of Neritina §* frequently occur on dry land 
far from any water; other species live constantly or during the 
chief part of the year high up on trees in mangrove swamps— 
groups of Veritina dubia and V. ziczae. 

In most of the cases here adduced, the organisation of 
the animal appears, so far as we know, to be entirely that of 
a creature living and breathing in water, or only very slightly 


Fie. 54.— Talis us saltator, 


modified. The Orchestide, Nemertidz, Snails, and Leeches show 
not the smallest difference from their nearest allies living in 
water ; in the land Planarians, however, a creeping surface has 
developed on the under side, which acts physiologically in the 
same way as the foot of the land snails, and which is not found 
in Planarians living in water. But undoubtedly there are, 
among Fishes and Crabs (Brachyura) for instance, many forms 
which constantly, or only occasionally, live on land in damp 
spots, and have undergone a more or less considerable transfor- 
mation, particularly in their organs of respiration. As these 
cases are of more general significance, we will investigate them 
somewhat more in detail. 

III, The accommodation of water-breathers to breathing 
air.—Fish, as is well known, breathe through their gills, which, 
being set at the sides of the head and covered by the operculum, 


AMPHIBIOUS FISHES. 189 


absorb oxygen from a current of water which enters by the 
mouth, bathes the gills, and passes out again behind the oper- 
culum through the gill-opening. Hence fishes would appear to 
be confined exclusively to a life in the water. Nevertheless 
there are a few forms which are able to breathe out of water, 
and which in fact even pass.a great part of their lives out of 
water. Such are the two genera, both belonging to the 
Gobiidee, Pertophthalmus and Boleophthalmus ; these skip along 
close to the water-line on the sea-shore, where they hunt for 
Molluscs (Onchidium) and Insects. In their branchial cavity, 
like all fishes, they have true gills; but these, though not dif- 
fering widely from those of other fishes living constantly in the 
water, are far from filling up the cavity, which is rather large ; 
and this seems to contain not merely water, but air as well. 
In other fishes that occasionally visit land, the branchial cavity 
on each side is prolonged, and penetrates upwards far into the 
head, while its mucous membrane sometimes forms labyrinthine 
folds of highly complicated structure (see fig. 47); hence their sur- 
face is often much more extensive than that of the true gills. 
Formerly the species which possess this accessory labyrinthine 
organ in the gill-cavity were classed in one family of Labyrin- 
thict, for it was thought that their internal affinities were 
clearly denoted by the presence of this organ. Now, on the 
contrary, they are distributed into several different families,88 
since it is considered as proved that their real genealogical 
affinity is indicated by other characters, while the labyrinthine 
organ must have originated independently in certain forms of 
these different families, though with great similarity of struc- 
ture and identical physiological functions. Formerly this func- 
tion was explained by the hypothesis that water could be stored 
in the cavity of the labyrinthine organ, which might be closed, 
and that this water, being rich in oxygen, and unable to 
escape from the gill-cavity, enabled the creature during its 
excursions on land still to breathe in or through water. There 
can be no doubt, since numerous observations exist on this 
point, that they are capable of living for days out of water ; 
many of them make long journeys over land, and some are even 
said to be able, by means of the spines on their scales and gill- 


190 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


covers, to climb up palm-trees—Anabas scandens. I have 
certainly never seen this, though I have often caught Anabas 
scandens in the Philippines. But the hypothesis that their 
labyrinthine organs are merely auxiliary gills destined to en- 
able the fish still to breathe through water when on land, finds 
no confirmation in the observations made by the most esteemed 
naturalists; indeed, it is quite incomprehensible how, in so minute 
an amount of water as could find place within the labyrinthine 
organ, so much oxygen could exist as the creature must con- 
sume even in a few hours. And there can be no possibi- 
lity of a renewal of the water deprived of its oxygen so long as 
the animals live on land. It is, however, at this day, almost 
superfluous to point out the absurdity of this early and often 
disputed assumption by an analysis of the physiological pro- 
cesses; for the direct observations of Dr. Francis Day ®°— 
known by his great work on the fishes of Malabar—have proved 
that the accessory gill-cavities, or labyrinthine organs, as they 
were called, of the Labyrinthici never contain water, but always 
air only. So that these organs must be simply designated as 
organs for respiring air, 7.e, as lungs which have been formed 
by modification of a portion of the water-breathing gill-cavity ; 
the fishes that have them are therefore to be regarded as Amphi- 
bians with quite as much reason as toads and frogs, or even 
better, since they are capable of changing the nature of their 
respiration—of air, that is, or of water—at will and suddenly 
without any interruption ; nay, are actually accustomed so to 
change it. Finally, in some Brazilian fishes—Sudis gigas, Ery- 
thrinus teniatus and brasiliensis—the air-bladder, as Jobert 
has lately discovered, serves directly as lungs subsidiary to the 
gills, since they inhale air through a connecting passage which 
subsists between the throat and the air-bladder. If this air- 
passage (ductus pneumaticus) is ligatured, the fishes die of 
suffocation, since the amount of air obtained through the gills 
does not suffice them for respiration. By these observations it 
is made intelligible how an air-bladder could be transformed 
into a lung. Insufficient absorption through the gills brought 
the fish to swallowing air ; instead of passing out through the 
gill-openings, as in other fresh-water fishes, the air passed into 


AMPHIBIOUS MOLLUSCA. 191 


the intestine or even into the air-duct leading to the air-bladder, 
and thus both organs might become organs of respiration, since, 
fundamentally, every growing or living cell must breathe as soon 
as it comes into contact with a highly oxygenated medium. 
Even among the Invertebrata we know of animals which may, 
in this sense, be designated as true Ampbibians, The opercu- 


Fig. 55.— Gill lungs of Ampullaria. a, Ampullaria insutarum (D’Orb.). 1, long respira~ 
tory siphon ; 2, section in the direction of the arrow b; 4h, the upper lung-cavity; 4, 
branchial cavity wita the right and left gills; the cavities communicate bya passage 
in the centre of the dividing wall. 

lated snail Ampullaria (fig. 55) has a well-developed branchial 

cavity and gills, and above these, and separated from them, it 

has a well-developed lung-cavity, of which the structure is pre- 
cisely similar to that of the lungs of our common land-snails. 

The Ampullaria uses both organs in rapid alternation ; lying 

not far from the surface of the water, it protrudes above it a 

breathing siphon, and inhales air through it; then it closes its 


192 TUE INFLUENCE OF INANIMATE SURROUNDINGS. 


lungs, reopens the siphon, and admits a stream of water through 
it into the branchial cavity.°° Some species of Wert/ina of 
the Philippine and Pelew Islands live constantly on land, and 
apparently go into the water only when they want to lay their 
eggs; other species actually living in the water often make 
long journeys over land, as ] myself have frequently had the 
opportunity of observing in the Pelew Islands. In these species 
the gills are comparatively small, and the roof of the branchial 
cavity is furnished with a dense vascular network of which 
the main branches unite in one large vessel; this is inserted 
in the heart—the auricle—and thus stands in precisely the 
same relation to the lungs as the pulmonary vein of the true 


Fic. 56.—Gecarcinus rusticola, a Land Crab. 


land-snails—elicide. Thus the branchial cavity in this case 
seems to be capable of fulfilling not only its own proper func- 
tions, but also that of alung. We know, moreover,”! that many 
species of crabs—Birgus latro, Grearcinus (fig. 56), Grapsoidea, 
Sesarma, and others—live far from all running or stagnant 
water in damp woods, under stones or decaying trees, and are 
even able to expose themselves to the sun for hours. In most 
of these species true gills are present in the branchial cavity, 
but they fill at most a third or fourth of the space, and the 
cavities contain, besides water, a considerable quantity of air, 
as is shown by their constantly expelling air-bubbles at the 
sides. The supply of air thus driven out can of course be 


LUNGS IN A LAND-CRAB. 193 


replaced only by air, since the animals live in the air, and it 
obviously follows that they generally breathe air with their 
branchial cavities, and only exceptionally water.2? In one 
single case this change of function has induced a modifica- 
tion of structure; this case is that of Birgus latro, according to 
my own observations. The lower portion (see fig. 2, p. 5) of 
the gill-cavity, which contains numerous but small gills, is 
divided from the upper half by a transverse fold which turns 
inwards at the edge of the thorax-plate. The cavity thus 
enclosed is a true lung (see fig. 2), since it never contains any- 
thing but air, and the arrangement of the vessels traversing its 
walls proves that blood poor in oxygen enters it from the 
body, und the vessels leading from it open directly into the 
auricle. The skin on the outer and upper lung-covering bears 
a great number of ramified tufts, which add to the extent of 
the respiratory surface, and contain in their interior an extra- 
‘ ordinarily developed network of vascular spaces, intervening 
between the afferent and efferent pulmonary vessels. These 
spring from two large vessels, proceeding one on each side from 
the anterior half of the body cavity ; each divides close to the 
foremost angle of the lung into four pulmonary vessels, of which 
three ramify over the upper and one over the lower lung-cover- 
ing. These are merged in the before-mentioned network which 
traverses the villi of the lungs. From this again proceed 
several vessels which unite at the angle of the lung to form a 
large trunk, the afferent pulmonary vessel ; this passes at first 
from the front backwards, then bends round, corresponding to 
the curve of the thorax-shield, and passes from behind forwards, 
uniting with the branchial vein coming from the interior, shortly 
before it enters the auricle of the heart. This arrangement of 
the vessels is such as we should expect to find in a true lung; 
the expansion of the respiratory surface which is here afforded 
by the villous structure of the lungs equally corresponds to the 
typical structure of all organs for breathing air; finally, it is 
positively established that the animals pass the greater portion 
of their life on land, and that their lung-cavities contain 
actually air, and never water in any greater quantity than is 
requisite for maintaining the moisture of the respiratory surface. 


194 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


Now, since this lung perfectly corresponds, morphologically, to 
the upper half of the branchial cavity of other crabs, proof is 
furnished that here a portion of the gill-cavity has been trans- 
formed into an organ for breathing air, and has at the same 
time undergone very characteristic modifications of structure. 

The objection which certain morphologists feel to admit- 
_ ting that land crabs, and above all Birgus latro, are animals 
actually breathing air and provided with lungs, appears to be 
caused by their incapacity for understanding that the same 
organ which to-day acts asa lung may be used to-morrow as 
an organ for breathing water by the employment of the gills 
placed in it, or close to it. This objection is all the more to 
be wondered at, since the same zoologists readily admit that 
this same process—the transformation of a gill-cavity into a 
lung—has actually taken place in snails. The lung of Ampul- 
laria spoken of above might be here adduced ; still it might 
perhaps be said that it is not proved that it belongs morpho- 
logically to the branchial cavity, and that it may bea new organ 
occurring only in this genus. But the same objection could 
not be made with regard to the lungs of the Helicinide, 
Cyclostomacee, Siphonariade, and hermaphrodite Pulmonata, 
for all these Molluscs have only one respiratory cavity, which 
breathes air, but which, by reason of its position and its 
relation to the other organs, may be regarded as a gill-cavity 
transformed into a lung, and which is even regarded as such by 
all zoologists. But there are among them certain forms— 
Siphonaria, the aquatic Pulmonata, Awricula—which bear a 
small gill or gill-like organ in this lung, notwithstanding that it 
is filled with air; consequently even those zoologists who dis- 
pute the air-breathing powers of Birgus latro, merely because 
it possesses gills, must regard the above-mentioned molluscs 
as water-breathers also. This, however, they do not do, for 
they cannot deny the fact that these creatures breathe air; 
hence they will be obliged by degrees to accustom themselves 
to the idea that land-crabs breathe air, and to regard the lungs, 
in them as well as in the snails, as gill-cavities which have 
exchanged their normal or primary function for another. 

At the first glance it certainly appears singular that an 


OXYGEN IN AIR AND WATER. 195 


aquatic animal whose organs of respiration are adapted to 
breathe water should be capable of learning to breathe air with 
them. If, however, we enquire somewhat more closely into the 
character of the process, it loses much of its strangeness. In 
both cases the oxygen is absorbed from the surrounding 
medium by a membrane which is kept moist, and to which it 
must be a matter of indifference whether it receives it from 
air or from water. Thus, granting that in both cases the 
osmotic power of the respiratory skin remains the same, as 
the amount of oxygen taken up within a given time naturally 
depends on the proportion of oxygen contained in equal deter- 
mined volumes of the air. or of the water, the respiratory 
surface may be in a position to take up more oxygen from 
the air than from the water in the same unit of time, be- 
cause air hasa larger admixture of freeoxygen. Thus—if there 
is no other hindrance to an alteration in the mode of life—on 
the above hypothesis, an animal, which has hitherto breathed 
in water, will more easily accustom itself to breathe in air 
than an animal living in the air, on the contrary, can accom- 
modate itself to breathing in the water ; for in this latter case 
a deficiency, which must inevitably arise, must first be covered 
by auxiliary organs—by the skin, for instance—while in the 
former the originally small requirements of the water-breathing 
animal will be much more easily supplied by accommodation to 
the more copious respiration of air than by its continuing to 
breathe in water. But whether the creature is, without excep- 
tion, benefited by a change of function, by which a medium 
poor in oxygen is exchanged for one rich in that element, of 
course is not proved; while, on the other hand, it cannot be 
disputed that some such advantage may be connected with it.%? 

The causes which prompt an animal to quit the water and 
to accustom itself to breathe air may be of very various natures ; 
lack of food, need. of shelter, and the pursuit of prey or flight 
from foes most likely play the most important part. It is, 
however, also possible that other causes which are less obvious 
may have led to the same result; thus, for instance, the 
absolute deficiency of air in the water under some circum- 
stances may undoubtedly have exerted this influence. When 


196 THU INFLUENCE OF INANIMATE SURROUNDINGS. 


river cray-fish are kept in stagnant water, they soon die for 
want of oxygen, or else they quitit and prefer to live in the air , 
it is well known that they are packed for carriage in wet moss, 
and not in water. Further back I have already mentioned 
(see p. 172) that fishes are easily drowned if they are prevented 
supplementing the amount of air they derive from the water, 
which is insufficient for their respiration, by swallowing 
air at the surface. Fiitz Miiller has made us acquainted 


Fia. 57.—a, Lupea, a swimming crab that breathes only in water ; b, Ocypoda, a marine 
crab which easily suffocates in water. 


with a few other examples. He showed that the Ocypoda, 
which lives half its time on land and in part breathes air 
(fig. 57, 6), can easily be drowned if it is held in sea-water, 
which yet contains enough oxygen to allow 4 Lupea diacantha 
to recover itself perfectly when it has been almost killed by 
being kept in the air. It follows from this that the osmotic 
power of the respiratory dermal surface is extremely different in 
the two animals, and that in the Ocypoda it is not great enough 
to extract the considerable amount of oxygen necessary to the 


LUNGS OF LYMN@A. 197 


creature from sea-water, which contains but little air, in the 
same time as from the air directly. The Lupea, on the other 
hand, cannot breathe in the air; perhaps for this reason, that 
its gills completely fill the,cavity, and so the ingress of air is 
hindered by the gill-laminz which lie too close together; most 
fishes die quickly in the air in the same way and for the same 
reason, because their gills collapse and the respiratory surface is 
conspicuously diminished. 

It is, however, only an hypothesis when we speak, as above, 
of a transformation of the gill-cavity into a lung; for in none 
of these cases have we ourselves observed the process or 
attempted hitherto to induce it by experiment ; nay, we do not 
even know whether Crustaceans living on land, and breathing 
air—as, for instance, Birgus latro or Gecarcinus—tfill their lungs 
with water and breathe through water, when in the water, or 
breathe exclusively through their gills so long as they are 
under water. From the morphologist’s point of view, however, 
this hypothesis may be regarded as satisfactory ; for, in the first 
place, the structure of a gill-lung is precisely such as we should 
ascribe to an organ for breathing air, and, in the second place, its 
position in the body is such that its derivation from the gill- 
cavity or from some portion of it is immediately apparent. 

Moreover, and finally, there is another instance that has long 
been known of such a transformation of a branchial cavity into 
a lung, but whose full significance has only quite lately been 
duly estimated. The Lymneide, living in fresh water, have 
true lungs; they go from time to time to the surface of the 
water to take air into these lungs, and the oxygen contained in 
it suffices for some time as supplementary to that which they 
absorb through the skin. Now it has long been known that 
the lung-cavity of the young Lymnzz when they escape from 
the egg is full of water, and it apparently acts as a gill-cavity so 
long as the animals do not find their way to the surface of the 
water to inhale air into the gill-cavity and thus transform this 
intoalung. Generally, it is true, this period of water-breathing 
through gills is not of very long duration, perhaps of only a few 
hours ; but Professor Forel, of Lausanne, has made us acquainted 
with some experiments which prove that there are actually 


198 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


Lymneide which breathe water throughout their lives. In the 
course of his investigations of the deep-water fauna of the Lake 
of Geneva, he brought up from a depth of 130 fathoms, 
among other animals, some Lymnege which had in their lungs 
no air, b.t only water, and which lived there in great numbers 
and at various ages and stages of growth. But as soon as the 
creatures brought up by the drag-net were transferred to a small 
vessel with water in it, both old and young soon crawled to the 
surface, opened the orifice of their lungs, and inhaled air into 
the respiratory cavity, which was still filled with water, and 
with which, till within a few moments, they had breathed only 
water all their lives. This incontrovertibly proves that an 
organ of respiration may be able to alter its function, not only 
gradually but quite suddenly, and the apparent contrast between 
air and water breathing consequently loses much of the signifi- 
cance hitherto ascribed to it. On the contrary, every water- 
breathing organ can easily be brought to breathe in the air, if 
only two conditions are fulfilled: First, the maintenance of 
moisture in the respiratory surface by the condensation of the 
water contained in the atmosphere; and, secondly, the preven- 
tion of the collapse of the organ, and the consequent diminu- 
tion of the respiratory surface. 

These observations accidentally made by Forel have given 
rise to a very interesting experimental treatment of the ques- 
tion by Dr. Pauly. He showed that the Lymneide that exist 
in the depths of various lakes, of Geneva, Constance, and Starn- 
berg, where they can never come to the surface to breathe, 
can live by respiration through the skin and by using their 
lungs as gills ; but according to Pauly the chief action is to be 
attributed to the former. He further found that they fre- 
quently contained air in their lung-cavities without coming to 
the surface of the water, and that they obtained it by taking the 
numerous air-bubbles that cling to water-plants or stones into 
the respiratcry orifice. Finally he proved by experiment that 
a Lymnea from deep water, which had for the first time become 
accustomed to breathe air, never returned to water-breathing ; on 
the contrary, it kept its respiratory cavity completely closed, and 
breathed subsequently by the skin alone while under water. 


FUNCTION OF RESPIRATION. 199 


Concluding remarks.— With regard to the selective influence 
of air and of the matters held in it—oxygen, water, carbonic agid, 
&c.—the same general conclusion may evidently again be drawn 
as we arrived at in the former chapters ; it results from the fact 
that different animals or even individuals never react in pre- 
cisely the same way under the influences of the atmosphere. 
Every change in its composition must therefore essentially 
alter the fauna of a country or of a locality by selection, if this 
change is not merely a transitory one—in which case an inju- 
rious or favourable influence may find compensation—but is 
continuous for a lengthened period. A conflict between the 
individuals thus affected need no more take place under the 
selection caused by the conditions we have now been investiga- 
ting than under any we have hitherto discussed. Such a 
struggle can arise from this cause only when, from a super- 
abundance of animals, the quantity of air at their disposal for 
respiration has to be so greatly subdivided, that it fails to be 
equally favourable to all the individuals. But while in all the 
former ‘chapters we recognised not selective effects only, but 
also with more or less success a direct transforming influence 
as exerted by the conditions under discussion, and even could 
sometimes experimentally prove their existence, this has not 
been the case as to air. The proof that a change in the fune- 
tion of respiration is possible may indeed be acquired by ex- 
periment, and a not insignificant number of differences in the 
structure of the respiratory organs concerned may be very 
naturally conceived of as an immediate consequence of such a 
change of function; but in no single case have we as yet suc- 
ceeded in proving that such a change of function as is involved 
in the transformation of a gill-cavity into a lung must neces- 
sarily be accompanied by definite changes in the structure of 
that organ. Still it must not be forgotten that in this respect 
we are not yet past the stage of the most superficial and 
elementary knowledge. 


200 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


CHAPTER VII. 
THE INFLUENCE OF WATER IN MOTION. 


In the previous chapter we saw that the distribution of animals 
on our globe would be essentially modified by a change in the 
proportions or in the chemical composition of air or of water. 
If the air were deprived of the greater part of its oxygen, 
only a very few species of animals could continue to live—only 
those, that is to say, which could endure such a diminution of the 
respirable oxygen contained in the air. If, on the other hand, 
a larger quantity of oxygen could be added to the water than it 
usually contains, it would appear probable that many animals 
fitted for breathing in the air would be thus enabled to live 
in the water if any other cause made such a change of habit 
inevitable ; consequently land animals would become aquatic. 
It is not probable that such a complete change could now 
ever actually take place; but smaller changes in those con- 
ditions of life might occur, in fact actually do occur. We 
know, for instance, that, according to the direction of the wind, 
the air at the surface of the earth is light or heavy, that it is of 
different density in low plains and on heights, and varies very 
greatly in its composition ; it is different in the dwelling-places 
of man and under the shady roof of forest trees, on the open sea 
and in the Sahara or the boreal regions of the eastern and 
western hemispheres ; and the percentage of moisture in the air 
varies with the temperature and the prevailing winds. The 
constituents of water are equally variable ; in lakes with marshy 
shores they are not the same as in running brooks or rivers; 
they are different on a limestone soil and on sandstone; the 
amount of saline matter in solution (sensu strictiort) varies con- 


SECULAR VARIATIONS. 201 


spicuously in the different oceans and inland seas; some water 
is rich in oxygen or carbonic acid, in others these are wanting ; 
in some cases we find large quantities of calcic carbonate, magnesic 
sulphate, and other salts in the water, which is then termed 
hard; in others, as is usually the case with rain-water, these 
salts are almost entirely absent. 

All the differences here briefly enumerated, and many others 
not specified, which affect those conditions of existence which 
depend on air and water, must have more or less in- 
fluence on the forms of animal life; some species will abso- 
lutely die out, others will remain wholly unaffected, while 
others again will become modified in their habits of life as well 
as in the structural relations of their organs (for instance, 
Branchipus and Artemia). Now, if we assume that these 
variations must be perfectly inappreciable to our individual 
perceptions—secular variations, as they are termed—the modi- 
fications which are caused by these secular variations in the 
animal life of any given country must also be inappreciable by 
man ; the apparent constancy of the conditions of life, so far as 
they depend on air and water, will make the fauna appear 
equally constant to our unaided vision. This hypothetical 
constancy does actually exist, if we disregard the variations 
occurring within the space of a day, a month, ora year; the salt 
constituents of the different oceans and inland seas remain 
perfectly identical, as well as the moisture contained in the air 
or in the composition of the atmosphere; we are in no way 
cognisant of any perceptible variations in the conditions of 
existence within the historical epoch of our globe. Hence we 
may without hesitation assume that any alteration caused by 
such variations in the fauna of any locality or in the mode of 
life of any animal and in the structure of its organs can never 
have been perceptible to us. 

Nevertheless these two conditions of existence, air and 
water, are precisely those which are most constantly at work 
on animal types, and which are also the best qualified of any to 
bring them into ever fluctuating conditions of existence, and 
it is their capability for being moved of which nature avails 
herself to effect the constant transfer of animals from one 


10 


202 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


place to another. The passive migrations of animals are 
effected entirely by the winds or currents, and their voluntary 
movements are limited or favoured by them. Finally, the 
strencth and direction of these currents in the air and water 
provide nature with so many instruments for influencing 
different animals in their individual life and growth; among 
those so influenced, above all, are those which we may call 
sedentary, in which a marked effect on the mode and vigour 
of their growth can be traced to the moving medium. 

We will begin our enquiry into these important problems 
by investigating the influences of water i motion. But it 
must once more be pointed out that the influences discussed in 
our former chapters must always be inseparable from those 
proper to the currents, so that the total effect of the water arises 
from the combination of several influences, not one of which 
need ever act in the same direction as any cther; on the con- 
trary, they frequently neutralise each other. If, for instance, 
the larve swimming in a sea were, without exception, drifted 
by the same current simultaneously into an estuary, they would 
apparently be thus enabled to take possession of the new 
territory ; nevertheless all those forms would die out which 
were not at once able to endure the reduced saltness of the 
water in the estuary. A stream of warmer water, as, for instance, 
the Mozambique Channel flowing past the east coast of Africa, 
will have a tendency to convey animals of warm latitudes into 
the colder seas ; but only a few species—those we have designated 
as Eurythermal—will be able to establish themselves in them 
easily. It is true that the difference in the conditions of life 
under migration by means of sea-currents is not always so 
conspicuous as in these extreme cases; but all creatures are 
exposed to changes of less intensity, if, in the larva or in the 
fully grown stage, they are borne from one place to another. 

If we now leave out of the question those influences of the 
constituents and temperature of the water which are inseparable 
from its currents, and direct our attention solely to these, the 
mechanical factor of their momentum is what we have to 
consider as exclusively important. The direction, the rapidity, 
and the strength of the current unite to affect the animals 


FORCE OF WATER. 203 


exposed to them; some they will annihilate, others they will 
transport against their will to other spots, and others again they 
will affect by hindering or promoting their growth. 

The resistance of animals to currents.—The effects of water 
in motion may be displayed in two different ways : in the first 
place, as sudden and irregular blows, as in the beating of 
waves or surf; secondly, equally and uninterruptedly, as in 
currents. Excepting in some few instances, we need not en- 
quire more closely into the effects of sudden shocks, for either 
they at once destroy the creatures exposed to them, or these are 
able to withstand them ; this, however, can but rarely be the 
case under heavy blows.- Currents are far more important. 

In general we estimate the pressure of a current, in seas, 
rivers, or torrents, by its velocity, assuming that the current 
exerts its force perpendicularly to the body resisting it. Such 


Fia. 58.—Mollusca that cling tightly to rocks by the foot. a, Patella, the shell of which 
entirely covers the soft parts, which are pressed down on the rock; 6, Nuvicella, fully 
extended, only the tentac:es projecting beyond the front of the shell. 


cases, however, but rarely occur in nature, and in placcs where 
a current or high waves break perpendicularly to the cliff a 
very small number of animals can live—such, for instance, as 
are sufficiently protected by the strength and form of their shells 
against destruction, or by the sucking power of their foot 
against being torn or washed away, like Patella in the sea 
(fig. 58) or WNavicella® in strong mountain torrents; or 
forms which not only are covered by a hard external shell, but 
whose shell is grown to the rock, such as the Sea-acorns: 
(Balanide). The pressure to which these animals are subjected, 
either perpendicular or lateral, must sometimes be enormous ; 
it would be interesting to ascertain by experiment how 
great it actually is in individual cases, A knowledge, how- 
ever, of the maximum of pressure which can be generally borne 
by the animals above mentioned and others of analogous 


204 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


structure, would not, as it would seem, be of any universal 
interest ; for it must be very difficult, if not actually impossible, 
to make experiments on such creatures. We must remain 
satisfied with the fact that permanently fixed animals, or such 
as can attach themselves temporarily, exhibit, without exception, 
organs of such structure, form, and character as most clearly 
prove the adaptation of the animal to tke development: of its 
powers of resistance. Their fitness to live depends exclusively 
on this power of becoming fixed or of clinging ; and if once they 
are displaced by a too violent lateral shock, death almost always 
ensues. 

This group of sedentary animals, spending their lives fixed 
to one spot, exposed to and resisting the pressure of water in 
motion, may be contrasted with another which, to express 
myself for once in the terms of the Teleologist, so far over- 
come the momentum of the water as to use it to their own 
profit. Swimming animals do not feel the strength of the 
current so long as they swim with it or are carried on by it; it 
is only when they try to swim against it, directly or at an 
angle, that they feel its force, and it is not till then that they 
develope those organs or characters from which they derive the 
requisite power of resistance. It is perfectly immaterial to all 
the very tender sea creatures, containing not more than from two 
to three per cent. of dry matter—such as Siphonophora, Meduse, 
Salpze, and larve of aj] kind—whether they are carried by the 
current at arate of two or of ten miles an hour; nor, indeed, 
can they be conscious of it so long as they are not flung against 
a hard object. 

We will consider these two groups, and the organs which 
distinguish and demarcate them, somewhat more closely. 

The organs and characters which are possessed by migratory 
animals, and allow of their moving and of their taking advan- 
tage of the transporting power of the currents, are extraordinarily 
various; and yet they admit of our easily dividing the animals 
into two great groups. These arc (a) the animals that move 
voluntarily or swim, and that move from place to place of their 
own free will; and (5) those that move involuntarily or float 
and are carried passively by the stream. The former require 


ORGANS FOR FLOATING. 205 


swimming organs, the latter do not. In the former tke specific 
gravity, compared with that of the stratum of water in which — 
they live, must be a little in excess. In the latter it may”: 
be the same as that of the water, or even less. This comparison, 
however, is not, and cannot be, absolute ; for since the strength 
of the current varies with that of the wind, with the fall and 
the mass of water, &c,, most of the actively swimming animals 
even, must often be transported passively, as, for instance, when 
. their strength is insufficient to contend with the current; nay, 
even the strongest swimmers, as the Porpoises, Whales, Sharks, 
&c., must often voluntarily allow themselves to be borne along 
by it. Those organs which serve to enable swimming or floating 
creatures to maintain a position at the surface or at a certain 
level, are known in zoology as hydrostatic organs; to these 
belong, for instance, the swim-bladders of many fishes, the air- 
bladders which keep floating colonies of polyps (Siphonophora) 
at the surface of the sea,®® also those air-vacuoles which are 
sometimes found in the protoplasm of testaceous Rhizopoda 
(Arcella). Even creeping animals, such as the fresh-water snails 
(Lymnea, Physa, Planorbis, &c.), can become floaters, for they 
fill their lungs so full of air that they become specifically 
lighter than the water, and consequently rise to the surface.97 
This enables them to creep by the motion of the cilia or by the 
action of the foot on the stratum of air in contact with the 
surface of the water. It is easy to see from these examples 
that nature employs an infinite variety of means for solving so 
simple a problem. In Arcella it is a temporary bubble, which 
disappears when the creature desires to sink to the bottom; in 
the Siphonophora the organ appears to serve no purpose but that 
of keeping the colony at the surface ; in Fishes, as we have seen 
above, the hydrostatic organ also subserves the purpose of an air 
reservoir, and in some cases even of a lung in the physiological 
sense; in the pulmonate water-snails, again, the lung itself 
can, at the will of the animal, become a hydrostatic organ. 

Far more various and important are the actual swimming 
organs of the true swimmers, by which they are enabled to ex- 
change an unfavourable spot for one more suitable, to escape 
the pursuit of their enemies, or to extend their own hunting- 


206 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


ground. In the first place, in all swimmers, the whole body 
serves as a means of motion ; snakes and eels or similar crea- 
tures with very long bodies swim exclusively or in preference 
by a wriggling motion, and even much shorter animals, as 
bleak, &e., can swim without fins, but then their power of 
directing their movements is greatly impaired. Such organs as 


Fic. 59.—Various animals that swim by means of fins. Above a fish (Dors?); below a 
Cachalot Whale; to the right a Pteropod, Hyalea; left, a Pterupsdous larva 
(Creseis?). 

serve exclusively or chiefly to enable swimming animals to 

move in a determined direction are known by the general term 

of fins. Notwithstanding the widest difference in their structure, 
and though they may have but small morphological correspondence 
in different creatures—as will be understood, without any more 

detailed comparison, from the subjoined illustration (fig. 59)— 

they have, without exception, certain peculiar charact21s, which 


ORGANS FOR SWIMMING. 207 


have a direct reference to their purpose and function. In the first 
place, they have cutting edges and a broad surface, by which 
the animal is enabled to exert pressure on the water in the 
most efficient and natural manner ; in the second place, they 
are, without exception, attached by movable joints in such 
a way as to serve for steering. Such fins occur in the greatest 
variety in both Vertebrate and Invertebrate animals; in whales, 
sea-serpents, and tailed Amphibia, on the tail; in many Fishes, 
as single fins on the back, belly, and tail, and as paired 
fins on the body, where the lower or hinder extremities 
or limbs are modified into fins. In many Birds, both wings and 
legs serve as fins; °° Mammalia and Reptiles alike are often 
fin- or web-footed (crocodiles, turtles, and aquatic mammalia). 
In insects, the legs (in Motonecta) and sometimes special ap- 
pendages of the body (as in the larve of Ephemera) or even 
the wings (Polynema), are used for swimming ; in many Crusta- 
ceans all the legs are true fins, and in several of the Annelida 
each segment of the body bears a pair of fins. The larve of 
Mollusca have them on the head, Cuttle-fish at the hinder end 
of the abdomen; in short, there is hardly any portion of the 
body on which some little lobe or process might not serve as a 
fin. A small and delicate creature, like a Medusa or the larva 
of a Molluse, may find a ciliated dise or margin, or even a few 
scattered cilia, efficient as fins in spite of their fragility ; large 
animals, as whales, sharks, &c., require larger fins provided 
with strong muscles, and supported on an internal skeleton. 
In all cases the serviceableness of these organs depends on their 
being fitted to move the whole mass of the creature in a defi- 
nite direction, with or against currents; if the extent of sur- 
face of the fins, or the strength of their motor muscles, or the 
supporting power of the skeleton, is insufficient for this purpose, 
the individual possessing such inefficient fins must necessarily 
perish. Thousands or millions of such inefficiently equipped 
aquatic animals must be swept away every day and every hour 
by the currents to which they are forced to commit themselves, 
and here again the external conditions of existence select the 
stronger, and eliminate the weaker, individuals, without the 
need of any personal struggle between them. 


208 THR INFLUENCE OF INANIMATE SURROUNDINGS. 


Sedentary animals, not properly swimmers, are either fixed 
to their dwelling-place or only temporarily attached. The way 
in which these animals either yield to or overcome the effects 
of currents has already been indicated ; the hardness and shape 
of the shells, the adhering power of the foot, or of the skeleton— 
as in corals—protect them against the steady lateral pressure of 
currents, but if they lose their hold or are broken off, they 
are soon destroyed. Only certain creatures, as many sponges 
and some polyps (Hydroida), although they are fixed, escape by 
other means the destructive effects of strong currents; their 
extreme tenacity of texture, elasticity, and pliancy, in which 


Fic. 60.--Creeping Mollusca. a, Natiea, which can extend its foot very widely; 6, 
Evycina (?), which creeps by means of its foot, like a univalye, on the skin of a 
Synapta. 

they resemble many water plants, qualify them to live even in 

strongly agitated currents. 

Creeping creatures, which attach themselves only tempo- 
rarily to stones and plants, and can quit them at pleasure, have 
special orgaus of attachment. Thus, in most univalves and 
some bivalves, the part known as the foot serves this purpose ; 
the broad under-surface clings closely to the object they adhere 
to, but at the same time can leave go ofits hold. The force, 
however, with which the foot adheres is not merely not absolute, 
but is even relatively different in individuals of the same 
species at various ages. In the course of my experiments on 
the pond-snail (Lymnaea stagnalis) I found that those just come 


EFFECT OF CURRENTS. 209 


from the egg have so little clinging power in the foot, that the 
feeblest current suffices to sweep them off the leaves on which 
they seek for food, although the clinging surface is just as 
large in proportion to the length of the shell asin the fully 
grown snails, but the older ones can move about freely in a 
current of moderate strength. Dr. Kobelt, in Frankfort, took 
occasion, when discussing my experiments on Lymnea, to show 
that this fact tends to explain the remarkably wide distri- 
bution of this mollusc. It usually lives in ponds and lakes ; 
sometimes, however, it is found in brooks or rivers where the 
current is feeble; but in these, only fully grown specimens are 
found, and they are by no means so numerous as in stagnant 
waters. Hence we may infer that they were carried by floods 
into the stream at these spots, when already half grown, while 
the young ones transported at the same time perished for lack 
of food, in consequence of their inability to cling to leaves in the 
current. Multiplication in a strong current is equally impos- 
sible to the Lymnea, and for the same reason ; since, even if the 
young escaped from the egg, they must shortly perish in conse- 
quence of the weakness of the foot. This explains why Lymnaea 
stagnalis seeks situations where the water is stagnant, for it is 
only in such spots that the conditions of life are suitable to 
every stage of its growth. It would be interesting to institute 
similar experiments with other. species of Molluscs, and I have 
no doubt that the results that might thus be obtained would 
contribute to explain many facts hitherto inexplicable as to the 
distribution of fresh-water snails. 

In some cases an increase of clinging power is accompanied 
by peculiar modifications of structure in the creature itself, which 
may perhaps be even regarded as its direct result. Among the 
fresh-water Univalves of the tropics of the eastern hemisphere 
there is a genus, Vavicella, which is identical with Veritina in 
all the essential characters of its anatomical structure.°® Both 
belong to the same family of univalves, and bear a calcareous 
plate on the hinder part of the foot, known as the operculum 
(see fig. 61), which is commonly of the same shape as the mouth| 
of the shell, so as to close it perfectly when the animal draws in! 
the foot. But, in order to do this, it must necessarily lcave 


210 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


its hold of the object to which it is clinging, and, in point of 
fact, many Neritine attached to stones fall off at the slightest 
touch. There is no doubt that they thus easily escape from 
the pursuit of their enemies, precisely as many beetles living 
on leaves elude the search of the entomologist by dropping off 
them. If such a Neritina were suddenly transferred to a 
rushing mountain stream, it would undoubtedly soon be crushed 
by the rolling of the pebbles, if it preserved its habit of falling 
at an unexpected jar. 

All animals possessing this peculiarity are, therefore, unfitted 
to live in currents of any force ; and the créeping creatures living 
in such a situation must be able either to hide in the narrowest 
cracks and fissures in the rocks, or to bore into the earth or the 
stone itself, or else must have such clinging powers of the foot 
as may enable them to resist the shock and pressure of the water 
by attaching themselves firmly to the rock, and applying the 
margin of the shell so closely to the surface of the stone that it 
is difficult or impossible to remove them. This is in fact the 
case; all the Neritinas that I found clinging to the surface of 
the rock in exposed spots in the mountain streams of the 
Philippines, adhered as closely as possible, as soon as I touched 
them. But in this respect many Navicelle are still better 
qualified ; they often live in the midst of a dashing torrent, 
adhering to the stones by suction, so closely that it is difficult 
to raise them, even with a knife, without injuring the margin of 
the shell. This is effected partly by the sucking power of the 
foot and partly by the form of the shell, which is flattish, 
conical, and oval, and has an operculum which is smaller than 
the marginal circumference of the shell. The relatively small 
size of the mouth is owing to a calcareous plate forming a sort 
of half-deck across the back of the inside of the shell, and which 
may be considered as identical with the inner whorl of other 
Univalve Shells, while the foot is so broad that it quite covers 
the under surface of this half-deck, while its margin corresponds 
exactly with the oval of the mantle, and also corresponds with 
that of the shell. Ifa Navicella is removed from the spot 
to which it is clinging—and this is not difficult to do if it is 
taken by surprise—it can be seen at once that the animal cannot 


ON MOLLUSCA. 211 


bend up its large foot and withdraw it completely into the 
shell like other univalves; even when it is plunged at once 
alive into spirit, the foot remaius rigid and almost straight. 
Thus an operculum placed on the hinder surface of the foot, as 
in Neritina, would be useless, since it cannot serve to close the 
opening and to protect the soft parts, if the foot cannot be 
withdrawn; but the Navicella, as has been said, does net 
need any such protection, since it clings more closely to the 
rock when threatened with danger, and we might expect to 


a 
iN TET 


LTT 


4) 

Ai / 2 

: 

bf 5 

/ zs J 
Ps “ y 
BY Ae QS” 
B ASRS AK ®, Op 

SO 


Fig. 61.—Diagram of section through Weritina and Navicella to show the position of 
the operculum ineach genus. «@, Navicella (ep, operculum), b, operculum of Navi- 
cella ; c, Neritina (op, operculum), d, operculum of Neritina. 


find, on examination, that it had no operculum. In this, how- 
ever, we should be quite mistaken ; in all the Navicelle, without 
exception, there is a true operculum placed, exactly as in all the 
Operculata, on the back or upper side of the foot. It agrees 
too in structure with that of Neritina, but it is much smaller 
than the mouth of the shell (fig. 61), and the little hook 
which serves to attach it to the muscle of the foot, and 


212 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


which in Neritina is very small, is modified in Navicella into 
a large thin plate appearing as a direct prolongation of the 
free portion of the operculum ; finally this lies in a small fold 
between the back of the foot and the horizontal deck inside the 
back of the shell. Thus it is evident that the operculum of 
Navicella comes under the head of rudimentary organs, for it 
can neither revolve through an are of 180°, as it would have to 
do to close the shell, nor could it close the aperture even if it 
were brought into the proper position. Now, as Navicella is in 
every other respect a true Neritina, it may be regarded as one 
which, by long inurement to living in rushing mountain streams, 
has had its shell modified in the way most saited to those con- 
ditions, while the operculum, in consequence of long disuse, has 
become a peculiar degenerate or rudimentary organ.!0¢ 

The foregoing cases show us that sometimes even the 
weakest currents may act as potent means of selection between 
different species or individuals, and they also prove that animals 
nearly allied, or individuals of the same species, may be quali- 
fied at different stages of growth to resist the strongest cur- 
rents or the most violent shocks by some modification in their 
mode of life. The characters thus developed clearly exhibit 
their connection with the creature’s mode of life, since its 
power of resistance evidently depends on them; and in the 
instances where, as in Navicella, rudimentary organs occur, 
their derivation from organs formerly of physiological value 
may be attributed to the indirect action of the currents. But 
it unust not, at the same time, be forgotten that the degenc- 
ration of an organ is, in point of fact, no more explained by 
its disuse than the continued existence of a still serviceable 
organ, as ¢.g. the eye, is explained by the fact of its utility or 
by the evidence of the vast importance of its use. 

Nevertheless, there are a few instances in which the direct 
effects of more or less constant currents of different force on the 
form of certain animals admit of easy proof. 

The-direct mechanical effect of currents on the structure 
and growth of animals.—It is evident that this influence 
cannot be exerted in any considerable degree on any but fixed 
animals, at any rate in such a way as to modify or determine 


STRUCTURE OF SIIELLS. 213 


their growth ; and it is plain that if freely creeping or swimming 
creatures are exposed to it they will probably in most cases be 
destroyed. Sometimes, however, these are capable of escaping 
its effects by their own strength. 

This occurs, for instance, in the case of many Mollusca, of 
which the shells are often much injured by erosion. All 
conchologists are well aware that the shells of fresh-water 
Mollusca in particular are generally more or less eroded; in 
many places it is very difficult to find any number of certain 
species of shells that are not thus worn away, and the process 
commonly begins at the apex of the shell. It can hardly be 
doubted that there are very various causes for this; in many 
cases, which I myself have observed and studied minutely, 


WEL 
d RETTORCELU i) 


Fie. 62.—Oblique section through the shell of a fresh-water mussel, Unio. a, Cuticle, ex- 
ternal and organic; 0, tho Prismatic Layer ncxt to it; c, Nacreous Layer, slightly 
magnified. 


two causes have combined—namely, the boring powers of the 

.filaments of certain Fungi and the constant wear of fresh- 
water currents, by which, in the Philippines, the shells of 
various species of Melania, Navicella, and Veritina are attacked. 
How this occurs can only be understood by briefly studying 
the structure of such shells. 

In the shells of Bivalves as well as in those of Univalves 
three typical layers of structure may be distinguished : the outer 
one consists invariably of purely organic matter, known to 
conchologists as the epidermis; the two inner layers consist 
of calciec carbonate, combined with a very small quantity of 
feebly developed organic matter. The outer layer of these two 
is conimonly designated as the prismatic layer; the inner one 
as the mother-of-pearl or nacreous layer.!°! In most of the 
Mollusca living in fresh water the external organic layer 


214 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


is, relatively speaking, of considerable thickness, very tenacious, 
and perfectly impermeable by water ; and there can be no doubt 
that, merely by its resistance to the action of water, it serves to 
protect the calcareous inner portion of the shell against its 
solvent and destructive effects. Indeed, this is proved by the 
subjoined sketches of one or two shells, of which the calcareous 
layers have been deeply eaten into, while the organic cuticle 
hangs about the shell in thin rags, as it were, above the holes 
or pits (fig. 63). The question now is: How did erosion first 
begin in these cases? For, though it would be perfectly 
intelligible that the face of the calcareous layer, when once 
laid bare, should be easily eaten into by the action of the 


Fra, 63.—Shells of living molluscs partly eroded. u, Melania; 6, Navicella; c, Nevitina, 


carbonic acid in solution in the water, it must have been per- 
fectly protected against this action by the cutic’e so long as it 
remained perfect. 

Microscopic examination of the shells here depicted has 
shown that the substance is penetrated all over, but more 
especially in the immediate circumference of the eroded spots, 
by innumerable perforations caused by a minute boring Fungus; 
these perforations are usually perpendicular to the surface of 
the shell, and might easily be supposed to be a normal pecu- 
liavity of its texture. These boring fungi are also to be found 
in shells which appear to be perfectly sound, being still com- 
pletely enclosed in their brown cuticle, while others are already 
slightly eroded at the apex. This supplies us with the required 
explanation. The boring fungi, of which the spores are con- 


EROSION OF SHELLS. 215 


stantly being carried down towards the shells by the water, 
attach themselves to the most prominent portion, which is at 
the same time the oldest and has the thinnest cuticle; they 
gradually penetrate the shell, and thus the calcareous layers are 
exposed to the action of the water; this, in consequence of the 
carbonic acid it contains, eats into the prismatic layer, and, as 
a minute vortex must be established in each little hole by the 
action of the current, the chemical effect may be enhanced by 
the mechanical action of the stream. By degrees this erosion 
proceeds more rapidly than the destruction of the cuticle by 
the action of the fungus, and thus long strips or rags remain 
free, covering the pits worn by the water. The pits at last 
show to some extent unmistakable traces of the chiselling action 
of minute whirlpools. 

Of course, in time, these will gradually eat through the 
nacreous as well as the prismatic layer, even without the 
assistance of the fungus, and finally the soft portion of the 
animal itself will be laid bare. The creature protects itself 
against these injurious effects simply by secreting fresh layers 
of calcareous matter, and thus the structure of the shell 
is considerably altered. It is certainly difficult to under- 
stand how the creature is abie to secrete a new supply of 
calcareous matter precisely at the spot where the shell grows 
thin; for this does not take place in the first instance only 
when the shell is actually worn through, but without exception, 
on the contrary, at a much earlier stage; as is proved by the fact 
that shells pierced quite through are never, or extremely seldom, 
found. It may perhaps be assumed that the impact or pressure 
of the whirlpool is more perceptible at the thinner portions of the 
shell to the creature within, until at length the local irritation 
it produces excites a more copious secretion of the shell-forming 
fluid by the skin. Thus the same power which is exerted to 
destroy the shell at the same time incites the animal to defend 
itself against its injurious effects. 

Similar results from currents on the animals living exposed 
to them, whether free and creeping or occasionally sedentary, 
could be pointed out in many other cases; thus, for instance, 
the forms of the shells of many univalves seem especially 


216 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


adapted to resist pressure or impact in a certain direction. In 
the total absence of any exact knowledge on this point, we may 
set such cases aside and address ourselves to the investigation of 
those where the mechanical influence of a constant current is, 
by itself, of manifest importance in modifying the growth of 
fixed or sedentary animals. 

In this respect the Coral animals are of predominant im- 
portance, for in al] the specics, large and small, the tendency 
is conspicuously manifest—and in the individual Polyps as well 
as in the whole mass—to exert the vigour and direction of 
their growth to counteract the strength and direction of the 
current or pressure they may be exposed to. On the other hand, 
the question as to the way in which the growth of Corals may be 
affected or modified by external circumstances is of the greatest 
importance with reference to Darwin’s well-known theories as 
to the origin and formation of coral reefs. We should have no 
occasion to give much attention to the phenomena I allude to 
if they either simply availed to confirm Darwin’s views, or, on 
the other hand, in no way affected them; but as they elucidate 
in detail the same views, antagonistic to Darwin’s, which 
I have gradually arrived at after a careful investigation of the 
whole mode of growth of coral reefs in general, I feel called 
upon here to describe them fully. The high authority which 
every opinion expressed by Darwin has, and always must have, 
in my estimation, would of itself justify our giving our best 
attention toa thorough investigation of any question bearing 
upon them; and it seems all the more permissible in this in- 
stance, because the easy application of Darwin's theories of 
coral-reef formation, their extreme simplicity, and partly also 
the great interest which has always been excited in the popular 
mind by the processes of coral growth, have made them almost 
universally known to the geologist as a convenient hypothesis, 
and to the layman as one easily grasped and understood. In 
the following disquisition I shall proceed from special cases, and 
afterwards discuss the more general question. 

A. The influence on growing corals of a constant current 
produced by other animals.—So long ago as the year 1837 
Stimpson described a small crab, under the name of Hapalo- 


GALLS ON CORALS. 217 


carcinus marsupials, which had been discovered in the Pacific 
Ocean by Dana, in the course of his great voyage under the 
command of Wilkes. Irrespective of other peculiarities, this 
was distinguished from all other crabs by a remarkable pouch 
in which the female carries the young, formed by a prolonga- 
tion of the lateral plates of the abdomen. Subsequently Heller 
described another species of crab from the Red Sea, under the 
name of Cryptochirus coralliodytes, of which the female has 
egg-pouches similar to those of the other genus, but the form of 
the body is otherwise quite different. While the general form 
of Hapalocarcinus is lenticular, Cryptochirus has a thorax of 
perfectly cylindrical shape, with a head terminating obliquely, 
so that it strikingly resembles several of the cylindrical wood- 
boring beetles. The singular mode of life of these crabs was, 
however, unknown to both these naturalists. 


Fig. 64.—The crabs forming galls on corals. a, Cryptochirus (male) ; b, Coralliodyles 
(female) ; c, Hapalecarcinus marsupialis (female). 

As I was able to study both species alive in the Philippine 
Islands, I will here give my observations in detail. 

For both of them an association with living Corals is indis- 
pensable, and the influence of the Corals on the Crabs is as 
‘direct and important as that of the Crabs on the Corals. Hapalo- 
carcinus '°2 has hitherto been detected only in pieces of branch- 
ing coral; I have found it on Sideropora digitata and palmata, 
and on species of Seriatopora; Verrill found it on Pocillopora 
ceespitosa in the Sandwich Islands, and D. Graeffe discovered it 
on two species of Seriatopora. On all these corals the crabs 
produce a peculiar excrescence on the twigs (so to speak) of a 
branch ; these growths, which are sometimes very broad and 
massive, and sometimes very slender, grow opposite each other 
in such a way that the crab settled between them is perfectly 


218 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


surrounded, and thus enclosed, in the gall which gradually 
forms. 

It is not difficult to infer what the whole process must be 
from the different stages observable on one single mass of coral. 
A diseased excrescence is first produced by the young crab 
establishing itself between two branches, and the twig thus 
originating takes various forms according to the character of 
the species of coral. This is very conspicuous in the different 
specimens lying before me. In the Seriatopora, both the twigs 
are leaf-shaped and beset with more or less numerous offshoots 
terminating in sharp spines ; in the more solid Pocillopora the 


Fic. 65.—Sideropora hystrir, with galls inhabited by Hapalocarcinus marsupialis. aaa, 
young galls still half open; 6, an older one, closed, in which a close inspection may 
detect two opposite fissures, 

twigs also have spines, but they are more massive ; finally, in 

Sideropora, spines ave wholly absent, and the two twigs between 

which the crab lives are altogether more massive. In the first 

instance the two leaf-like twigs are of course far apart, so that 
the crab could easily get in and out; but as it does not do this 
it is soon so surrounded by the growing together of the twigs, 
that it must remain a prisoner. The creature requires a con- 
stant and rapid renewal of the water in the gall in which it 
lives, for the purpose of respiration; at first the water finds 

a free passage on all sides, but when the two twigs have 

bent over towards each other, the space through which it can 

find entrance and exit must grow narrower and narrower. 

Moreover, from the structure exhibited by galls broken off from 


CRABS FORMING SUCH GALLS. 219 


the coral, it may be concluded with certainty that the crab 
moves about very little in the cavity, for otherwise we should not 
find the very distinct scars which are evidently produced by 
continual scratching in one spot. Since, in all the crabs of this 
group, the current of water for breathing enters the body close to 
the mouth, and passes out again at the hinder margin of the 
branchial cavity, the stream passing through the gall must 
always flow in one and the same direction. The results are 
easily recognisable in the half or wholly 
closed gall. The two excrescences on the 
coral grow together quickest in those spots 
which are least exposed to the current 
through the gall; there also they first come 
into contact, till at length only two fissures, 
more or less wide, are left, which plainly , 
show, by their position opposite to each wt 
other, that it is through them that the cur- ™,66.~-An open gall of 


2 . Seriatopora hystrix, 
rent for respiration passes ; one fissure serves «the normal develop- 


ment of the coral: 6, 
for the influx, the other for the exit, of the the gall with a cavity 
2 , —here Jaid open—in 
water. These two slits remain open so long — which a crab was en- 
as the crab is alive; no living crab is ever ics 
found in a clesed gall, and they are for the most part perfectly 
empty. 

It is impossible not to conclude from this state of things 
that the fissures are kept open by the force of the current 
flowing through them ; but still this can only occur when the 
force of the current is exactly commensurate with the strength 
working in antagonism to it, which is exerted by the growing 
polyps. These are constantly tending, as is shown by the 
different stages of the gall, to reduce the space between the two 
sides of it; at first this may be quite easy, but as the force of 
the current is gradually increased by the diminution of the 
fissure, at last a state of equilibrium must be reached in which 
the forces neutralise each other. Thus, though in the first in- 
stance the coral was able to continue its growth unhindered, 
after the manner of its species, it ere long found an obstacle, 
which it was unable to contend with, in the current produced 
by the crab. Hence we are justified in supposing that similar 


220 = THE INFLUENCE OF INANIMATE SURROUNDINGS, 


hindrances, which might be opposed to the growth, in any 
spot, of much larger masses of coral, would check or modify 
their growth in the same way. 

I may for the present postpone the application of the prin- 
ciple thus arrived at; but it will repay our trouble if we direct 
our attention to some other phenomena observable in these 
same galls. The walls of the leaf-shaped excrescences that 


Fic. 67.—Sideropora palmata, with a gall which is hardly visible from outside, but 
shows a distinct fissure dividing the two halves of the closed gall. 


form them bear polyps not only on the outer surface, but on the 
inner surface as well; this is proved by the fact that both are 
closely covered by little depressions, which, from their structure, 
can only have been formed by polyps. Now, as the polyps 
situated in the cavity were just as much exposed to the effects 
of the current as those on the margin of the fissure, they must 
show the traces of this influence, if indeed any such influence has 
been exercised during the growth of the gall. This is, in fact, 


CRABS FORMING PITS. 221 


quite unmistakably the case. Not one of the cups is normal 
in structure ; the depression, which in the external polyps is 
very deep, is here no more than a shallow pit, and the septa (or 
party walls) of the cup are very slightly developed. Hence it 
follows, with some degree of certainty, that the polyps on the 
inner surface were not able wholly to overcome the resistance 
of the current passing over them. This direct action of the 
stream is unmistakable in many of the cups, where the polyps 
were exposed to the greatest force of the current produced by 
the crab ; for they are placed obliquely on the fissure and directed 
outwards, as they must have grown, supposing them unable to 
grow against the stream. 

We see from this that the current caused by the crab is 
sufficient not merely to force the diseased excrescence on the 
coral to take a particular direction, but also to check the growth 
of the individual polyps quite as considerably, and to divert 
them from their normal growth. 

The influence of the respiratory current of crabs of the 
genus Cryptochirus, which live only in the more massive forms 
of coral, appears to be exerted in quite a different way. I 
found them in the Philippine Archipelago in cavities in 
Gontastrea Bournont, in an undetermined true Astrea, which 
was unfortunately lest, also in an undescribed Z'rachyphyllia ; 
finally I received a new form through A. Agassiz from the 
West Indian seas, which may perhaps form a distinct genus, 
though it is very nearly allied to the first. It also lives ina 
Trachyphyllia.!°3 The reader will see that they all belong to the 
massive corals, and, in correspondence with this circumstance, 
the cavities in which these crabs live are totally unlike those 
in which Hapalocarcinus is found. Here there are no galls, 
but merely cylindrical or funnel-sbaped hollows, which are 
never closed during the lifetime of the crab, so that it cer- 
tainly would be able to quit its position. Nevertheless, it as 
certainly does not do.so; but the species I observed living 
thrust the forepart of their bodies very far out of their pecu- 
liar ‘ cave-dwellings,’ so that only their pouches, ¢.e, the hind 
part of the body, remained within. The cavity itself exhibits 
some remarkable peculiarities. The bottom of it, on which the 


222 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


pouch rests when the creature has completely withdrawn itself 
into it, displays the radial septa of a polyp-cup one above 
another. They there are perfectly distinct, while the side 
walls of the cylindrical cavity are so completely lined with a 
thin calcareous crust that nothing can be seen of the perpen- 
dicular septa of the polyp-cup. From this it is evident that 
the young crab, or the larva of it, takes up its abode in the 
centre of a cup, and so kills the polyp inhabiting it. A speci- 
men now lying before me, with an incomplete cave-dwelling, 
shows that the crab grows at first at the same rate as the 


Fic, 68.—@eniastreea Bournont, M. Edw., with a funnel, at the bottom of which a crab 
(Cryptochirus coratliodyles, Hiller) is sitting, only the head being visible. 


surrounding polyps; for the margin of the crab’s hole, which 
is perfectly cylindrical, is on exactly the same level as the 
neighbouring cups, and its breadth too is exactly the same. 
The cavity is six millimétres long, and the length of the crab 
found in it exactly corresponds. In another example, however, 
the length of the pit is twenty millimétres, while that of the 
crab belonging to it is not more than seven millimétres, at any 
rate in the dried state. This proves that the crab ceases to 
grow much sooner than the coral; and this conclusion is strik- 
ingly confirmed by the fact that the margin of the cylindrical 
pit is not on the same level as that of the surrounding polyp- 


THE PROCESS OF GROWTH. 223 


cups, but much deeper in. From the margin of the crab’s 
dwelling, properly so called, there is a funnel that widens to 
the top, and of which the margin, as is shown in the cut (fig. 
68), is gradually merged in the upper prominences of the coral. 
The crab living in the funnel thus formed was carefully 
observed by me during a long period of its life, and I was 
enabled to see that it protruded itself far enough out of its hole 
to be able to reach with its outstretched fore-claws almost 
to the highest portion of the funnel. 

The whole conditions here described !%4 allow of no other ex- 
planation than the following: At first the crab and the coral 
grow at an equal rate ; for, if the coral grew more rapidly than 
the crab, an inverted funnel or hollow cone would be formed 
over the crab, while, if the crab grew the faster, the margin 
of its cave-dwelling, so long as it was small, could not be exactly 
on a level with the margin of the contiguous polyp-cups. But 
when the crab has reached its full length, about seven milli- 
métres, the polyps outgrow its funnel-shaped dwelling, and 
would no doubt soon wholly overgrow it, if it were not that 
they find a certain resistance in the current set up by the crab 
for breathing and in the movements of the creature ; and this 
resistance is sufficient to compel the growth of the coral in a 
particular and determined direction. The two powers in 
opposition thus reach an equilibrium, and it is their reciprocal 
action which gives the funnel its characteristic form. 

Here too, as in the former instance, the individual polyps 
plainly show the effects of the current. While in general the 
cups are perpendicular to the surface of the coral, in most of 
those which grow within the funnel this is not the case; they 
have an oblique direction upwards, and are most oblique where 
the strength of the current is greatest, te. at the narrow 
bottom part of the funnel. 

We see thus, in these two examples, that tne same force— 
namely, the respiratory current caused by a crab—affects the in- 
dividual polyps in the same way, forcing them to grow obliquely ; 
but at the same time it also produces very ditterent effects, 
resulting from the different law of growth of the two forms 
of coral-stock. Thus galls-are produced only on branched 


224 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


species; on the more massive ones, either funnel-shaped or 
cylindrical pitsare formed. In the course of my travels I have 
made numerous other observations as to the similar effects of 
currents on individual coral animals and on whole blocks of coral. 
It will suffice here to select two particularly instructive cases. 

One of these bears upon the growth of certain massive 
species of coral, among which we may especially consider the 
species of Porites, so common in reefs, and a few of the 
Astreide. The smallest, and so the youngest, colonies had, as 
a rule, a regular convex surface; but only, of course, when they 
were healthy and not eaten away by other animals or by 
plants. In them the polyps situated at the summit were as 
well developed as those at the margin of the mass, and they 
were never left dry even by the lowest ebb-tide. Larger blocks 
of the same species, which seemed to be more often exposed to 
the effects of the air, were flat ; the polyps at the summit looked 
feeble, and many of them were dead ; indeed, small stones and 
sand were often lying on the centre of the surface. In still 
larger masses, of which the upper surface was commonly laid 
bare by even ordinary ebb-tides, nearly all the polyps at the top 
were dead, and often entirely covered with sand, Vullipora, and 
other Alge. The summit of the largest stocks, finally, was 
concave, with a raised border, from a few lines to an inch 
higher than the central portion. These old stocks exhibited 
some important peculiarities. It is well known that even in 
corals of a massive and solid type there are often slight furrows 
between the separate cups, and in many stocks of Porites of 
moderate size these are enlarged to trenches of various breadth. 
In the largest, again, they have become narrow but often very 
deep channels which traverse the concave surface and even the 
raised margin in various directions. 

We can, as it seems to me, without any forcing, avail our- 
selves of the conditions here described to construct a theory of 
the process of growth of a knoll of Porites. So long as 
the young colonies are completely under water even at the 
lowest tides, the separate polyps grow out in every direction, 
giving the coral an equal convex surface; a section through 
it gives an outline like that shown in the subjoined wood- 


GROWTH OF CORAL KNOLLS. 225 


cut (see fig. 69, a). If the growth were to continue equal on 
all parts of the surface, the polyps at the summit would reach 
the normal low-water mark sooner than those at the sides ; 
thus, too, they would be the first exposed to the action of the 


eel = — 


= : ss 
=. — = 


=—s 


= = 


Fie. 69.—Diagram of the growth of the colonies of Porites. a, first stage, in which the 
summit of the coral touches the surface of the sea; 6, the second, in which it grows 
only in circumference; ¢, the third, during which the surface, intact at b, has 
died away and been hollowed out. 


air and rain at low ebbs, and so die the sooner. The polyps 
at the sides continue .to grow, tending not merely to raise the 
mags of the coral, but also to extend it horizontally ; and thus 
the upper and rapidly dying surface spreads laterally so much 
that sand, stones, plants, and various forms of animal life find 
room upon it. At first this level remains at very much the 
height to which sucha block may normally grow (6); but when 
it has reached a considerable size —from 6 to 8 feet in diameter or 
even more—the centre plateau is large enough to afford room for 
the collection of a considerable body of water, with sand, plants, 
and animals. In consequence of this the upper surface must 
perish, and then is easily hollowed out by the waves washing 
over it (c). If moreover, in the rainy season, any considerable 
amount of water falls into this basin at low tides—and that this 
is possible cannot be disputed—the fresh water collected in it 
will soon overflow the margin for lack of room. Now the whole 
coral is very porous, and is always traversed already, as we 
have seen, by more or less deep furrows. The water natu. 
rally seeks these ready-formed channels and widens them, 
working through the margin in favourable spots where the 
formation is softest, and thus slowly but surely eating channels 
through the raised ring round the coral-block. In the cuta 
diagram of these processes of growth is given. 


11 


226 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


Any other explanation appears to me impossible. We 
might be tempted to account for the elevation of the marginal 
ring above the central plateau by assuming that it had grown 
more rapidly than the centre; but if this were the true explana- 
tion the wall would be visible even in knolls of moderaie size, 
and this is never the case. Or it might perhaps be said that it is 
well known that all corals grow most vigorously at the margin 
of the block, and that the concav.ty of the upper surface may 
be easily explained by assuming a subsidence; but the first 
statement would be simply untrue, and the second perfectly 
absurd, for at any rate it is impossible to see why such a 
subsidence should have taken place in the case of the largest 
masses and not in the medium-sized or small ones. In point of 
fact, I see no other explanation which agrees so perfectly with 
the observed facts as that which I have given. 

As the last point under this head, we must now consider the 
way in which the whole reef is affected where it is acted on by 
currents of different force and flowing in varying directions. 
I will illustrate this by a highly significant instance which I 
myself carefully observed. 

To the south-west of Mindanao, and exactly opposite the 
famous old Spanish colony of Zamboanga, is the little island of 
Basilan. I visited it in 1859, when by far the larger part of it 
was still occupied by hostile Mohammedans ; the Spaniards were 
restricted to a village at the northern end of the island, which 
lay opposite to the still smaller island called Malaunavi, 
separated from Basilan by a narrow channel. This little strait, 
which, though very narrow, is of some length, runs from north- 
east to south-west; to the east it opens with a wide mouth 
into the straits of Zamboanga, while to the west it is barred by 
a very small island lying between the two others. To this form 
of the channel and to the particular divergence of the main 
current in the straits between Basilan and Zamboanga we 
must ascribe the fact that the current between these islands 
flows always in one direction, and never changes with the turn 
of the tides ; at least this was the case during the two months 
I spent there, and it is so throughout the year according to the 
information given me by Don Claudio Montero, the chief of the 


EFFECT OF CONSTANT CURRENTS. 227 


Commission of Hydrography at that time. The effect of the 
ebb and flow of the tide is only visible in the increased or 
diminished force of the current flowing through the channel. 
Now I willingly admit that possibly a return flow of the 
current may sometimes take place; but this certainly is not of 
frequent occurrence, and there can be no doubt that a strong 
stream passes through the channel for long pericds in the same 
direction. And itis this fact which, above all others, enables us 
to understand the peculiar structure of the reefs which fringe the 
islands, and consequently the channel, on both sides. Reefs 
proper, with a raised margin, do not exist here; the water in 
the channel is always still, and, as even the waves raised by 
storms on the main do not affect it, it offers to such small craft 
as can navigate it a secure shelter in spite of its small extent. 
But the current is often very strong, running sometimes at from 
4 to 5 or even 6 knots an hour. 

Both banks of the channel are formed of coral, those of the 
shore of Malaunavi on the north side being the most developed ; 
they consist of the usual reef-forming species, Astrea, Porites, 
Madrepores, &c. Now these, like all the species that form large 
blocks, have a tendency to extend in all directions; but here 
they are interfered with by the strong current impinging on 
them at an angle, and flowing, as has been said, during the 
greater part of the year in the same direction. If it were weaker 
than the growing power of the coral, its resistance would be 
easily overcome; but it is, on the contrary, strong enough to 
force it to grow perfectly perpendicularly. Thus the reef on 
the Malaunavi side is not more than a few paces wide, with an 
abrupt perpendicular fall, though the depth is certainly incon- 
siderable. 

The reef which fringes the little island lying in the western 
outlet of the channel is quite different in its structure. The 
same species are present as in the channel, but they here grow 
quite differently in different spots. Where the current is met 
by the island, of course it parts; thus, at the eastern end of the 
island, which is that turned inwards towards the channel, a 
triangular space is found, where the water is full of feeble 
currents flowing in various directions. Both branches cf the 


228 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


divided current first strike the island further in, and still at an 
angle. In this comparatively calm triangle of water, the 
Madrepores, Astrea, Porites, and other corals do not grow 
vertically upwards, but on the contrary in all directions, and 
usually in isolated blocks; and though the branched species 
generally grow more in height than the more massive forms, 
even in them it is impossible to overlook a tendency to increase 
as much in breadth as in height. Hence, on the eastern point 
of the island there is no perpendicular termination to the reef; 
it sinks quite gradually to the bottom of the channel. But where 
the two currents produced by the division of the main stream 
impinge upon the island, we at once find the perpendicular 
cliffs again, and the upper surface of the reef is as narrow as in 
the channel on the shore of Malaunavi. At the opposite end 
of the island—the western end, that is to say—there is a second 
calm triangle formed by the meeting of the two streams, and 
there the reef again assumes the structure which has been 
minutely described in speaking of the eastern end. Every- 
where in the Philippines, and likewise in the Pelew Islands 
in the Pacific, I have observed the same phenomena; wher- 
ever there was an eddy or a calm spot formed between the 
current and an island, the corals grew in irregular masses, and, 
more or legs, in all directions ; and wherever the stream ran 
parallel with the shore there was an abrupt and perpendicular 
fall of the reef. 

It is not difficult to find an explanation of these phenomena. 
Tt is the same as was given above in the case of individual 
coral knolls; where the sea impinges on coral reefs, with feeble, 
irregular, and variable currents, flowing in various: directions, 
the reefs, like separate blocks, will be enabled to extend in 
every direction, since they will not in the course of their growth 
have to overcome a constant resistance in one direction. But 
wherever strong and unopposed currents flow constantly in the 
same direction parallel to the cvast, impinging on the reef, the 
corals forming it must be forced, individually and collectively, 
to grow perpendicularly—assuming, of course, that their power 
of growth is insufficient to overcome the resistance of the stream. 
Between the perpendicular growth thus induced and the perfectly 


ON CORAL ISLANDS. 229 


horizontal mode of growth not checked in any way, we find 
every grade of transition. The precipitous fall is more or less 
oblique as the stream impinging on the reef is more or less 
strong. !% 

It is my conviction, derived from my own long-continued 
study of coral reefs, that the connection here indicated between 
the strength and direction of the stream on the one hand and the 
vigour of growth in the corals and in the reefs they form on the 
other, is one of the principal causes that have given the reefs 
their frequently very remarkable forms. This view is in Cirect 
contradiction to Darwin’s theory, which now finds universal 
acceptance, as to the formation of coral reefs. The reception 
which that has met with, as well as the high respect and vene- 
ration which I am most ready to pay to every opinion uttered 
by Darwin, compels me to devote the ensuing chapter to a de- 
tailed description of the reefs of the Pelew Islands in the Pacific, 
which during ten months I thoroughly investigated. Every 
variety of reef occurs there within a limited space, and a 
close study of their structure will show that Darwin’s theory 
is not sufficient to explain them all, but, on the contrary, that 
in every instance hitherto investigated, whether on a large or 
small scale, the same effects on their growth, as produced by 
strong and constant currents, may be detected. 


230 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


CHAPTER VITT. 


THE INFLUENCE OF WATER IN MOTION—(continwed). 
The Formation of the Coral Reefs of the Pelew or Palaos 
Islands in the Pacific Ocean. 


Ir is always an unsatisfactory task, and often an unpleasant 
one, to feel forced to contravene a view which is universally 
accepted as a true one, and which is supposed to be evidently 
sufficient to explain all the observed phenomena as completely 
as is on the whole possible. Nevertheless I cannot here escape 
this necessity; for it is precisely because I delight to pay to 
such men as Darwin and Dana the high respect that is due to 
them, that I find it impossible to be silent on those points 
where I dissent from their views. The constantly repeated state- 
ment that Darwin’s theory of coral reefs amply suffices to ex- 
plain to geologists the origin of fossil reefs would, it is true, 
‘scarcely provoke me to a discussion ; but I feel that I owe it to 
Darwin himself, here to state once more the views I hold, 
founded as they are on a long series of observations. For it 
seems to me that in the second edition of his universally known 
work on coral reefs * he has fallen into error as to some obser- 
vations of mine, inasmuch as he has attributed to me some 
opinions which at that time certainly I had only very lately 
held. But itis also due to myself that I should give a more 
detailed account of them in this place, because Dana, in the 
second edition of his book on corals which has lately appeared, 
has not even alluded to my views and observations, although 


* On the Structure and Distribution of Coral Reefs. Smith, Elder, 
& Co. 


FORMATION OF REEFS. 231 


Darwin himself recognises the force of my objections; though 
he certainly attempts to set them aside by means of an assump- 
tion of which the fallacy is amply proved by the very obser- 
vations I have published. 

The latest labours of the American naturalist prove that 
his views with regard to the origin of reefs differ very consider- 
ably from Darwin’s, and in a very essential point. Darwin, as 
is well known, assumes that wherever fringing reefs occur, a 
period of elevation or of repose now prevails, while atolls and 
barrier-reefs can be formed only in regions of recent subsidence. 
As an essential argument for the correctness of this view he 
adduces the fact, prominently brought out in a map of his con- 
structing, that active volcanoes occur only in those regions 
which, from the structure of their reefs, must also be regions of 
elevation. 

Dana agrees with Darwin in so far that he also assumes 
that atolls or barrier-reefs can only be formed in regions of sub- 
sidence; but fringing reefs, according to him, indicate not merely 
no upheaval in recent times, but, on the contrary, a more con- 
siderable subsidence than is pointed to by other reef formations. 
For instance, he says expressly : ‘I still hold that, while barrier- 
reefs are proofs of subsidence, small or fringing reefs are in 
themselves no certain evidence of a stationary level, and are 
often evidence of subsidence, even a greater subsidence than is 
implied by barrier reefs. I have already stated that one cause 
limiting the distribution of reefs is bold shores; a wall of rock 
of even a hundred and fifty feet producing a complete exclu- 
sion. . . . Such bold shores are evidence of subsidence ; and 
as only very small reefs, if any, could find footing about such a 
shore, the narrow reef would be another consequence of the 
subsidence, and no evidence of a stationary cundition.’ * 

Now, although Darwin admits that, under certain circum- 
stances, narrow (7.¢. fringing) reefs might be formed on such 
steep and precipitous shores, he adheres to his former opinion 
that this does not occur, as a rule, and that, in most cases, reefs 
of this class prove upheaval in the most recent periods, or even 


* Am. Journ. Sci. Ser. IU, vol. viii. p. 316. 


232 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


a process of elevation still going on. He does so, indeed, with 
the full understanding that his theories would be undermined 
if he were to accept Dana's views ; for it is clear that no theory 
of upheaval or subsidence, strictly speaking, could then come 
under discussion. According to Dana’s views we may assume 
that, in regions of general subsidence, local upheavals may 
occur ; and, vice versd, that local subsidence may occur in regions 
of general upheaval. From this, however, it foilows that the 
structure of the reef itself can give no certain evidence what- 
ever, as the basis of an assertion that this or that region is at 
a given moment undergoing upheaval or subsidence. Thus 
to settle this question we must avail ourselves of other argu- 
ments than those used by Darwin to establish his theory ; 
Dana in fact seeks for such, and his investigations led him to 
the conclusion that the whole of the Pacific Ocean is a region of 
subsidence, while Darwin recognises in it certain distinct 
regions of subsidence and others of upheaval. Dana further 
assumes that the West Indian Ocean is at present sinking, 
while Darwin, on the contrary, regards it as rising, since the 
reefs occurring in it belong almost exclusively to the group 
which, according to his theory, ought to prove a condition of 
upheaval. 

Now it might perhaps be objected that, with regard to the 
formation of atolls and barrier-reefs, both writers perfectly 
agree, and that the possibility granted by Darwin of the occur- 
rence of fringing reefs on steep and subsiding coasts certainly 
affords no argument against the view that atolls and barrier- 
reefs can under any circumstances be formed only during subsi- 
dence. This must of course be admitted; but I must never- 
theless maintain my opinion that Dana’s and Darwin’s theories 
do contradict each other, and that if, as Dana says, all kinds of 
reefs may originate during subsidence, the structure of the reef 
itself is of no importance in investigating the question as te 
whether in any given spot subsidence or upheaval is taking 
place or has taken place. 

Hence it seems to me allowable to ignore Dana’s views 
when the matter in point—as in this place—is to establish in 
what way the form of the reef has been determined or altered 


DARWIN’S VIEWS. 233 


by the combined action of internal and external causes. For 
those forces on which Dana relies for his argument in no way 
depend on the particular nature of the corals or of the reefs 
formed by them. Hence we have to deal exclusively with the 
original unaltered view of Darwin. 

There are two modes which may be adopted in contravening 
or criticising a generally accepted theory. In the first place, 
its general fundamental basis may be attacked, or, in the 
second, its value or worthlessness may be tested in a special in- 
stance. I shall here adopt this latter method, and am prompted 
to do so by several reasons. First of all, in this way only is it 
possible to set in the clearest light the intimate connection 
between the main subject of this section and the form assumed 
by reefs, that is to say the direct influence of a constant current 
on the growth of the reef. But I do so in the second place, in 
order to oppose the idea that in this particular instance it is 
difficult or even impossible to test and criticise a general theory 
by individual examples. Darwin himself, it is true, says that 
it would be exceptionally difficult to draw any conclusion as to 
any particular small group of islands or separate atoll or 
barrier-reef, even when the depth of the sea outside the reef, 
and the angle at which the enclosed land slopes, are well 
known. But ought this difficulty—more imaginary than real— 
to withhold us from making the attempt? I think not. It is 
usually assumed, and with justice, that the theory which must 
always lie at the bottom of an hypothesis appears to be soundest 
when it can be successfully applied in explaining particular 
cases of difficulty. But this is granting, on the other hand, that 
we do not simply set aside such difficulties as facts or observa- 
ticn seem to oppose to the theory, because it has already been 
proved by cumulative evidence. On the contrary, we should 
rather require that each new difficulty that arises, if only be- 
cause it is a difficulty, should be applied to test it. The fol- 
lowing attempt to refer the reefs of the Pelew Islands in the 
Pacific—which I myself have thoroughly studied—to their origi- 
nating causes will plainly show that certain facts, easily ex~ 
plicable according to my theories, present insurmountable difii- 
culties to Darwin’s hypothesis. 


234 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


I. The general form and structure of the Palaos Islands 
and Reefs.—The Palaos Islands—the Pelew Islands of English 
maps—form a small chain lying almost exactly NNE. and 
SSW., and of which the greatest length, from the northern- 
most island, Kriangle,* to the southernmost, Ngaur (or Angaur), 
is about 80 geographical miles. The nearest islands are 
Sonsorol, about 120 miles to the south-west, and Yap, belong- 
ing to the West Caroline Isles, at a distance of fully 150 miles to 
the north-east. 

The structure of these islands is very peculiar. The most 
northerly group is formed of five small, low islets lying on 
the eastern side of a true atoll; the largest of them, Kriangle 
proper, has given its name to the whole little cluster. At about 
forty miles to the north-west is the bank or reef of Aruangle, 
which is uninhabited, but which, from the description of the 
natives of Kviangle, seems to be a true atoll. Due south of 
Kriangle is the bank of Kossol, which is open to the south west, 
and may be described as a horse-shoe atoll. From this begins an 
extensive barrier-veef which encloses the largest island of the 
group, Balelthuap. Its northern end is very narrow, in many 
places scarcely haf a mile wide; then it suddenly widens, 
so that the island towards the middle is about ten miles ¢ or 
rather more across; its length is about twenty-five miles. 
The southern portion of the group is composed of innumerable 
islets, the greater part of them being included in the same reef 
that hems in Babelthuap ; but this, as it runs south, alters more 
and more in its structure. The most southerly island, Pelelew, 
marks the end of the reef, though further south still is the island of 
Negaur—the Angaur of the maps—which is divided from the 
others by a very deep channel, and is now surrounded by no 
reef at all. It is high land, and the surf beats directly on the 
foot of its raised coral cliffs, which have ceased to grow and 
which are as white as chalk, A glance at the subjoined map 
suffices to show that from north to south there is a gradual 


Written as Kiangle or Kyangle on many maps. 
+ Geographical miles, 60 to a degree, 


MAP OF THE PELEW ISLANDS. 


235 

ters 10! of 134530 Jo j aK) 

a 
of cy rust ra 
bf | Krvrn 

\ Yj Aoss 
/ & Me 
SO on reess _—o 5! 


73413 


all 
Map J.—General sketch-map of the Pelew Islands in the Pacific ; the deviations from 
Coello’s and Friedrichsen’s maps are from my own observations and measurements. 


236 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


passage from atolls to barrier-reefs, and then to true fringing 
reefs, till, at the southernmost end of the group, all reefs have 
disappeared. 

The fact that all the varieties of reef are met with in combina- 
tion in a district which, in accordance with Darwin’s theory, 
we were accustomed to regard as a district of subsidence, and 
in which, consequently, only atolls and barrier-reefs ought to 
occur, is fiankly admitted, in the second edition of Darwin’s 
well-known work, to be a very grave difficulty. We might 
evade this difficulty, as Darwin does—that is, by assuming 
that even islands which are in process of subsidence, and which 
therefore ought properly to be enclosed in true barrier-reefs, 
might sometimes form true fringing reefs, particularly if the fall 
of the coast were so steep that even during a period of slow sub- 
sidence the reef remained close to the shore, and thus preserved 
the form of a fringing reef. This assumption, however, stands 
in sharp contradiction to the facts long since observed and 
announced by myself, and into which I must presently go some- 
what more closely. We may also endeavour to overcome this 
difficulty in another way—for instance, by supposing that within 
the Pelew Group itself there exists an axis of motion independent 
of the general change of level of the bottom of the Pacific Ocean. 
This point might be sought perhaps somewhat to the north of 
Pelelew, or in that island itself, to the north of which the 
whole group would be constantly sinking more and more 
rapidly, while to the south it was rising in proportion. This 
would, in fact, apparently account for the circumstance that 
Angaur has hardly any reefs, that Pelelew exhibits fringing 
reefs, and plainly though feebly developed barrier-reefs. It 
would also explain why the reefs to the north of this fulerum 
become deeper and deeper in the deep sea, till at Jast, at the 
extreme north, only atolls occur, or reefs of the atoll character. 
Now I purposely avoid laying any stress on the fact that 
such an assumption is in the highest degree improbable, as 
that there should be such a fulcrum or point of rest localised in 
so small and isolated a district as these islands constitute in 
the Pacific Ocean, with upheaval proceeding to the south of it, 
and subsidence to the north. But, even granting this as pos: 


THE PELEW ISLANDS. 237 


sible, I believe I have discovered so many proofs of its inac- 
curacy, notwithstanding its theoretical possibility, in the struc- 
tural conditions which I have observed in those reefs, that the 
task of disproving it will not be a hard one. In order to do 
this thoroughly, it will be necessary to investigate the structure 
of the islands and of the reefs enclosing them. 

II. The atoll of Kriangle.—This is a true atoll of almost 
oval form, with its ]agoon wholly enclosed by the outer reef 
and the islands upon it; there is no channel leading into it 
from the sea. In order to get into i*, the reef must be crossed 
at high water at the deepest spot, which is to the south; but 
even then this lowest spot is so high that it requires some skill 
to cross it without accident. The lagoon is from three to four 
fathoms deep, in some places quite five. The bottom, which 
is perfectly visible through the transparent water, is covered 
with sand, and on its level surface lie scattered blocks of coral 
a few feet high; here and there plants are growing on them. 

The four islands, only one of which is permanently in- 
habited, lie on the east side and the south-east end; the most 
northerly, which, however, does not mark the northernmost 
point of the group, is alone called Kriangle; the others, count- 
ing from north to south, are called Nariungas, Nasingis, and 
Korack ; they are all low, composed of sand, fragments of 
corals, and large masses of a peculiar stone which consists 
almost exclusively of the innumerable shells of a recent 
Foraminifer, the well-known Tinoporus baculatus. This crea- 
ture is now living in extraordinary quantities on the exterior of 
the reef, and in smaller numbers within the lagoon. The islands 
are quite flat; the difference in level in their inner and outer 
edges is not more than a foot or two at most. The rocks of 
Tinoporus, on the inner side of the islands, lie so high that it is 
only at high tide that the water touches their base, and they 
slope slightly inwards towards the lagoon. 

The reef which encloses all the islands on the east is but nar- 
row ; the outer circumference, as indicated by the breakers at low 
water, is at most at five or six hundred feet from the shore. It 
is here quite dead, and exhibits very striking details of structure. 
It consists of large, almost horizontal banks, covered in spots 


238 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


with fragments of shells and corals; these strata are intersected 
by others lying at from 1 to 1:5 foot lower, which are quite 
free from detritus, and are formed of blocks of coral so perfectly 
baked together that in many places they appear to form a 
homogeneous and compact coralline limestone, Such strata of 
metamorphosed coralline limestone also occur, but much more 
rarely, on the summits or inner sides of the islands further to 
the south. The greater part of this eastern reef is covered by 
the sea only at high tide; then its surface lies at from 1 to 2 
feet below water. Above this the raised margin of the islands 
stands up from 5 to 6 feet. Its outer slope is steep, and the 
summit is crowned by blocks of coral of no conspicuous size, 
which have evidently been carried up during violent storms, 
The eastern slope of the reef to the sea is not steep, as can 
be seen by the colour of the water ; in the south-west monsoons 
good anchorage is found here at some distance from the reef. 

The reef to the west is devoid of islands, and its structure 
differs essentially from that of the eastern reef. While this is 
laid quite dry at every ebh tide, the former cannot be crossed 
dry-foot excepting at spring-tide ebbs. It follows from this 
that the western reef must lie from 4 to 5 feet lower than the 
eastern one; for in so small an island, bathed all round by the 
same ocean, it is impossible to suppose that this difference can 
be caused solely by the water being dammed up on the 
eastern side. 

This western reef rises with a not very steep slope from 
the lagoon; it is moderately broad, almost level, and at 
first only covered by coralline sand very equally distributed ; 
towards the north and south points of the atoll it rises 
more than on the west. Nearer to the outer margin ofthe 
reef occur living but isolated blocks of coral, at first in 
small masses and few in number, but the further we go west- 
ward the larger and more frequent they become, till at length 
all sand has disappeared and the blocks of coral are no longer 
isolated, but a solid, connected mass. Here we rarely meet 
with dead blocks among the living coral. Of course the coral 
grows most vigorously outside the external reef. The slope of 
this western reef, unlike the eastern one, is very abrupt; ata 


RAISED CORAL BLOCKS. 239 


distance of from 200 to 300 feet from the cuter edge, the colour 
of the water is as dark as in the channel of Kossol, which is 
from 50 to 60 fathoms deep. But the most striking peculiarity 
of the western reef is the accumulation of immense blocks of 
coral on the south-western point (see Map II.). These are, with- 
out exception, dead, and their summits are never covered even 
at the highest tides, unless perhaps during storms. The largest 
of these blocks lie on the south-west point, where they not un- 
frequently measure ten feet or more in diameter; towards the 
north and east they gradually grow smaller and less numerous, 
and disappear altogether amid the sand and living coral before 
the first is'and is reached on the eastern side; ou the western 
side, going northwards, they extend about ha'fway. The 
subjoined map makes this sufficiently clear. 

The position of these blocks, and their height above the 
highest water line, it seems to me can only be regarded as 
proof of a recent upheaval. It is true indeed, with regard to 
similarly situated blocks on other reefs, that they have always 
been said to have been piled up by violent storms; but even 
Wilkes remarked, as it appears to me with perfect truth, that 
such enormous stones as are sometimes found on the margin of 
the outer reefs could not possibly be tossed up so high by 
waves. For the argument so often used, that in violent tem- 
pests even large ships have been carried over the edge of a reef, 
proves nothing ; since a ship will always float if it has enough 
water under its keel, while a stone always sinks. The blow 
of a wave may certainly be strong enough to roll such a block 
a few paces forward on the reef; but even if the force were 
sufficient to rend blocks of ten feet or more in diameter from the 
living reef, it certainly would be incapable of raising them over 
the edge of the reef. In the case now in point the position of 
the blocks is still more adverse to such an assumption; for on 
Kriangle they do not lie where the most violent attack of the 
waves takes place during a storm, but precisely in those spots 
most sheltered from it. By far the greater number of storms 
in these islands come from the east; and even though, during 
the short period of the western monsoon, storms may sometimes 
sweep up from the west, it is not clear why in this case such 


240 THE INFLUENCE OF INANIMATE SURROUNDINGS, 


ENGLISH YA 


aan jen ‘ 
1000) Boo 250 OYARDS. 


The Zig-zag fino numbored at Intorrals shows the places & depth af the soundings taken by me 


Map I1.—The Atoll of Kriangl» from my own measurements and soundings. 


AN ARTIFICIAL CHANNEL. 241 


blocks should not have been flung up along the whole extent 
of the western reef. They do, in fact, lie in a spot which may 
undoubtedly be regarded as the least exposed of the whole 
reef. 

The argument here put forward, and which seems to prove a 
recent upheaval, can be supplemented by others from which it 
derives weight. The little island of Nariungas, before spoken 
of, includes a small lagoon quite divided from the larger one 
(see Map II.) ; its mean depth is from 2 to 6 feet, and it is not en- 
closed by trees on its eastern side, though in every other direc- 
tion it is hidden by shrubs. This lagoon formerly communicated 
directly with the sea by a narrow canal which cut through the 
eastern reef in a perfectly straight line. The natives of 
Kriangle told me that this channel had been cut by the crew 
of a Spanish vessel, and a few of the oldest remembered having 
seen the ship there. The accuracy of this story was confirmed 
to me by Captain Woodin, of the ‘Lady Leigh,’ who had called 
there thirty years ago, and within from five to ten years after 
that Spanish ship, which had been despatched with others from 
Manila to these islands for the regular trade in trepang. The 
channel cut through the reef, and of which the banks are still 
distinctly recognisable, was too wide for an ordinary boat-canal, 
and itis probable that it served to admit the ship itself into 
the small lagoon of Nariungas. The ships which at that time 
traded with the islands were quite small—schooners of from 50 
to 100 tons at most—and such a canal must have been quite 
large enough for them. But, in its present condition, the reef 
is raised far too much above ordinary flood-tide to allow of our 
supposing that the depth of the channel is the same at present as 
it was then; it must, on the contrary, have been very much 
deeper if the canal was to be of any practical use. Even now 
it would be quite superfluous to cut such a channel for boats, 
since it could be available for them only at the highest tides, 
when the passage across the reef itself is practicable. The 
present high position of the canal can thus be accounted for 
only by supposing that it has been raised together with the reef, 
and there is further evidence for this in the fact that the lagoon 
is now much too shallow to admit a schooner; and, above a!l, 


242 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


the canal as it now exists could never have served for it to pass, 
as in the deepest spots the bottom is only a few feet below high- 
water mark. Thus everything points to a very recent upheaval, 
which however can certainly not have been merely local, but 
must have acted on the whole group of islets which are now 
known under the name of the largest, Kriangle. 

This may be admitted without giving up the opinion that 
the whole atoll was formed during subsidence ; for in order to 
maintain this view it is merely necessary to assume that the 
elevation thus proved only began ata very recent period, per- 
haps at the very time when the Spanish ship cut the channel 
in the eastern reef. In opposition to this only one obj ction 
can be raised, but itis one of considerable importince. The 
great difference above pointed out between the external slope 
of the eastern and western reefs can hardly be reconciled with 
the hypothesis of an equable subsidence throughout the atoll ; 
for, if such had occurred, the slope would be equally steep on 
both sides. In point of fact it is steep only on the west, and 
on the east very gentle, although this is the windward side. 
This difficulty, again, might be removed by an arbitrary as- 
sumption that the eastern side of the atoll might have re- 
mained stationary while only the western side was sinking. 
This hypothesis also would be easy to upset; but I will post- 
pone the discussion of it to a more favourable opportunity. 

III. The bank of Kossol—The small reef known by this 
name lies to the south of Kriangle ; its shape is a well-defined 
horseshoe ; the channel that divides them is, according to the 
maps, from 50 to 60 fathoms deep, and, so far as I have been 
able to detect, entirely free from corals. The channel between 
Kossol and the northern point of the large island of Babel- 
thuap is, on the contrary, very shallow, at least in comparison 
with the southern one; on the maps, it is true, a considerable 
depth is marked, but I must positively contradict the accuracy 
of these indications ; the water in it is every where of a pale blue 
colour, while in the channel between Kossol and Kriangle it is 
quite dark, almost blue-black. And while in this channel coral- 
blocks are nowhere to be found standing up from the bottom, 
they occur in considerable numbers in the channel south of 


THE BANK OF KOSSOL. 243 


Kossol, rising to within from two to five or six fathoms below 
the surface of the water. 

The bank itself is open to the south and south-west, but 
entirely closed everywhere else. Its outer margin is highest on 
the east side; on the west there are enormous blocks of coral- 
line limestone exactly resembling those that lie on the south- 
west side of Kriangle. At low ebbs the reef is laid quite dry, 
but its western portion is conspicuously lower than the western 
reef of Kriangle, since, while I was able to cross the former at 
five in the afternoon, I had to wait till eight o’clock, even at 
the lowest portion of the Kriangle reef, before I could get 
across. The general outline of the reef is oval; it encloses a 
lagoon, very differently formed from that of Kriangle; for it is 
almost entirely filled with masses of living corals which vary 
remarkably in size and form. Towards the south the lagoon 
gradually deepens, and the biocks of coral increase in size and in 
number. Towards the other sides the isolated blocks within 
the lagoon grow together more and more, till at last they form 
amass; at the same time, they are so much raised that they 
help to form, on the reef itself, a spot which is laid dry at every 
ebb-tide. This inner surface of the reef is not level and sandy, 
as in Kriangle, but quite rough with knolls of living and dead 
coral, cut through on all sides by small channels radiating from 
the centre. We followed up one of these channels, but it only 
led as far as the outer border of the reef, and we had to wait 
there a tolerably long time before the tide rose enough to allow 
us to cross. The outer slope to the west was very abrupt; ata 
distance of from 150 to 200 feet the water was almost black, 
and much darker than in the channel between Kossol and 
Babelthuap. The eastern declivity, on the other hand, was far 
less steep, as is the case in Kriangle; for the water at some 
distance from the reef was still quite light blue, and the corals 
living at the bottom were clearly distinguishable here and there. 

Thus, as will have been seen, the structure of the reef of 
Kossol, and that of the reef of Kriangle, are equally adverse to 
the theory of their origin by subsidence. With regard to this, 
one point above all must be brought forward. The reefs are 
beyond doubt connected, as is proved by the soundings between 


244 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


Kriangle and Kossol. Now, if the two channels had been 
formed by subsidence, as is required by Darwin’s theory, it 
would be quite incomprehensible why, in the channel south of 
Kossol only, corals should have grown in blocks of various 
sizes, from the bottom nearly up to the surface, while in the 
northern channel they are totally absent. For though reef 
corals certainly do not seem usually to build up from a depth 
of sixty fathoms, since, on the hypothesis of subsidence, the 
southern channel must at some time have been high and dry, 
even the bottom of the north channel must have lain high 
enough to allow of the establishment of reef-corals. But then 
it is not clear why they should not have continued to grow up 
to the present time, and hence, on the theory of subsidence, we 
might expect to find exactly the opposite of what in fact has 
happened. But all these difficulties vanish as soon as we sup- 
pose that both these channels, and of course the reef also, have 
been formed during a period of upheaval. In that case the 
Kvriangle channel, which was originally the deeper, would not 
yet be raised high enough to allow of the establishment on any 
large scale of reef corals, while the bottom of the channel be- 
tween Kossol and Babelthuap much sooner attained the re- 
quisite level; and it is in accordance with this idea that we 
find in this channel numerous blocks of various sizes. To this 
point I must return in a later section. 

IV. The northern reef of Babelthuap.—This is a true 
barrier-reef running round the island of Babelthuap, which 
rises to a tolerably conspicuous height, and is almost entirely 
composed of quite recent eruptive rock, The northern portion 
of the island is extremely narrow, and beyond the northern 
point lie three or four islets in the channel within the reef. 
According to the maps hitherto published this would be in- 
correct ; even in the latest by Friedrichsen, founded on data 
supplied by a naturalist named Kubary, its breadth at the line 
passing through Aibukit (see Map I.) is made at least three 
times as great as in mine, constructed on triangular measure- 
ments. I found that at this point the width from reef to reef— 
from east to west—was at most 5 to 6 geographical miles, while 
Friedrichsen gives it as about 20. The island enclosed by the 


REEF OF BABELTHUAP. 245 


reef is naturally much narrower; starting from Roll, it takes 
at most twenty minutes to “go across the ridge of the island to 
the east coast. The narrowest part is a little farther to the 
south; according to my measurements the island here is at 
most 3,700 feet * wide. In this place, too, Friedrichsen gives the 
island a breadth of some miles. Close below Aibukit to the 
south, the island suddenly widens, without, however, anywhere 
reaching the considerable width of fourteen to fifteen miles at- 
tributed to it by Fredrichsen ; I am convinced that even at the 
widest part it is at most from seven to eight miles across. 

The reef which runs round the whole of the narrow island 
exhibits the following remarkable peculiarities of structure. 
At the northern point and on the west it is at a considerable 
distance from the land; at the latitude of my house (7° 38’ N.) 
it was from four to five miles from a small hill on which I had 
set up my theodolite. Between the inner reefs lying close to 
the shore and the exterior reef is a channel of from five to six 
thousand feet in average width, and from thirty to forty-five 
fathoms deep. This communicates with the ocean by three 
channels, which certainly do not lie opposite to large rivers 
or even brooks. The one marked on Friedrichsen’s map as 
‘ Kavasak passage’ is very narrow, and certainly not navigable 
for ships. The second, called by Friedrichsen ‘ Woodin’s 
passage,’ is placed by him much too far south ; its position is 
more accurately given on the accompanying map. I passed 
through this canal with Captain Woodin himself in the ‘Lady 
Leigh’ in 1861. The third channel is the widest; it is almost 
due west from the highest point of Babelthuap, which is de- 
signated in Friedrichsen’s map as ‘Royoss Aremolongui.’ The 
inner lagoon channel runs almost parallel to the coast ; it is not 
very wide, and the depth varies between thirty and forty-five 
fathoms, and into it debouch, almost all at a right angle, a 
number of channels which intersect the surface of the shore reef 
in all directions ; one of the last led us as much as 1,200 paces 
inland at high water, and we there anchored close to the per- 
pendicular bank of living coral which formed the shore of the 
canal. 


* German feet, about 3,810 English feet. 


246 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


Here, to the west and north, the reef, so far as it belongs 
to the island of Babelthuap, may be regarded as a quite 
characteristically formed barrier reef. According to the pre- 
vailing theory—which, however, I am contending against as 
to its general validity—-we ought to infer that a subsidence has 
lately taken place in this island, since it is in this way only 
that a barrier reef, as it is said, can originate. I of course 
cannot admit this proposition as correct, since I dispute the 
whole argument on which it is founded. But, quite irrespec- 
tive of this, the eastern reef at the northern end of Babelthuap 
displays certain peculiarities of structure which directly contra- 
dict the theory of subsidence. 

For instance, while the western reef stands off from the 
shore, so that, a true channel is formed, navigable for sea-going 
vessels, this is by no means the case on the north-eastern side 
of the island. By means of the theodolite, I accurately mea- 
sured its exact width almost exactly opposite to the village of 
Aibukit; here the outer reef was distant not more than 1,200 
feet in a north-westerly direction—at right angles, that is, to the 
shore. So far as I could see with the telescope of my instru- 
ment, both to north and south, the distance between the shore 
and reef, as calculated from triangulation, was nowhere much 
more, and it is not till about the parallel of Athernal that 
it seems to become greater on the eastern coast, according 
to Friedrichsen’s map. But as the distances and heights of 
hills are, on the whole, very incorrectly given by Friedrichsen, T 
see no urgent reason for giving unconditional credence to his 
map in this particular; however, be that as it may, it is cer- 
tain, by my own measurements, that in the northern part of 
the island the outer reef is not more than from four to five hun- 
dred paces from the shore. Between them, moreover, there is 
no reef-channel. The outer reef is no doubt a little raised 
above the general level of the reef, but not enough to form a 
channel which ships can navigate ; at high tide it is possible to 
cross it in boats, and in many places it might then be desig- 
nated as a boat channel. But at low ebbs it is easy to see that 
no such channel, properly speaking, exists; in many places it 
would be possible to cross the whole reef on foot, almost dry- 


SLOPE OF THE SHORE. 247 


shod. The surface is, however, traversed by numerous channels 
running in all directions, without any arrangement, and fre- 
quently ending in a deep hole. Nor are there here, as on the 
western side, true passages ; for though the outer reef is cut 
through in a few places, these channels nowhere lead to a 
navigable deep water canal. Hence the natives, when going 
out to sea, never follow these rifts in the reef, but steer across 
in a straight line for the outer edge of the reef, where they 
seek a spot low enough to allow them at high tide to float 
across the raised wall of the reef by skilfully avai'ing them- 
selves of the high surf dashing over it. 

The structure of the reef, as we see, is here essentially dis- 
similar to what we ought to expect on the hypothesis of subsi- 
dence. However, this deviation from the normal conditions 
may be explained in the way actually attempted by Darwin— 
by the assumption that on the more precipitous east coast the 
reef necessarily comes nearer to the shore, and that consequently 
so deep a channel would not be formed as has been the case on 
the western side. A very successful passage across the eastern 
reef out into the open sea, however, provides me with an argu- 
ment against this hypothesis, of which I have already made 
use in a former small communication on this subject, which has, 
however, remained unnoticed by both Darwinand Dana. It was 
on the occasion of my passage to Kriangle. After crossing the reef 
early in the day, at about nine in the morning, I occupicd 
myself for several hours till the afternoon on the outer edge 
of the eastern reef, being favoured by most beautiful weather. 
My investigations yielded a result which at that time I thought 
very unsatisfactory ; I saw plainly that the reef certainly does 
not fali abruptly into the sea, as it ought according to theory, 
but that its slope, on the contrary, is quite gradual. I could 
push some thousand paces straight away from the reef seawards 
without losing sight of the bottom ; the separate blocks of coral 
lying there were plainly distinguishable in their various forms. 
The sea was almost still, but in a great ocean it is never free 
from that slow swaying motion known as a ‘swell.’ This 
exhibited perfectly the phenomena observable on all shelving 
coasts ; i.e. the upward wave rises more and more strongly as 


248 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


it nears the land, but very equably and hardly perceptibly to 
the eye, till it breaks at last on the wall of the outer reef with 
aroar; but as this wall does not rise abruptly from the deep 
purple sea, as on the western reef, a phenomenon here becomes 
visible which may frequently be observed on sloping coasts; a 
second line of breakers succeeds the outer line, nearer in, and 
often even a third. This phenomenon is very familiar to the 
natives; to escape the danger of their boats filling at the 
second or third line of breakers, after crossing the first row of 
breakers, they shove the boat with long poles as quickly as 
they can over the outer level of the reef, soas to pass as rapidly 
as possible the two dangerous lines of surf lying beyond. On 
the western reef, on the other hand, there is never more than 
a single broad belt of breakers. 

These facts alone suffice to prove that the outward slope of 
the eastern reef is quite gradual. I investigated the matter 
very carefully, and with the express view of forming my own 
judgment on the assertion I had so often read that, on the 
weather side of a reef, the fall was always very abrupt, But 
my own investigations were certainly not favourable to this 
statement ; on the contrary I saw, as I have said, that even at 
some thousand paces from the shore the species of corals were 
still easily distinguishable, and at a distance even of from two to 
three sea-miles from the outer reef the water was still much 
lighter in colour than in the channel between Kriangle and 
Kossol, where, according to the soundings of navigators, 
it is about sixty fathoms deep. This exactly agrees with those 
observations as to Kossol and Kriangle which I mentioned 
before, without, however, adding much to thoir significance. 
But I may now assert with the utmost decisiveness that every- 
where in the northern part of this group of islands the eastern 
slope outside the barrier reef is particularly gentle, while that on 
the west is so precipitous that, at a few hundred paces outside 
the reef, the bottom is quite invisible. 

The facts here adduced are wholly irreconcilable with 
Darwin’s theory of subsidence. Before entering on that ques- 
tion more in detail it will he well to make an equally exact survey 
of the reefs lying to the south. 


PELELEW. 249 


V. The southern reef of the Pelew Islands—The most 
southerly point of Babelthuap is connected with a perfectly 
irregular system of islands of various sizes, and of the most dis- 
similar form and structure, which are separated by channels, 
some very narrow and others of considerable width. They are 
most numerous close to the main island, and more and more 
scattered as we proceed farther to the south. Quite to the south 
the single island of Pelelew is enclosed by the lower end of the 
great barrier-reef. The character of this reef, which surrounds 
the elevated islands, alters conspicuously as we pass from north 
to south, but this change is not gradual. As far as the latitude 
of Coroere (Corror on Friedrichsen’s and some English maps), the 
channels between the outer reef and the islands become on the 
whole somewhat shallower, but the difference is not great; nay, 
in many spots, as, for instance, in the eastern passage into the 
harbour of Coroere, they attain at least as great a depth (accord- 
ing to Friedrichsen, whose map may in this matter be relied 
upon) as in the western channel at the latitude of Aibukit, and 
in this median region of the islands the reef, both in the east 
and west, lies at a considerable distance from the land it en- 
closes. 

Farther to the south, however, the condition of things alters 
considerably. From about the latitude of Urudzapel (Urucktapel 
of some maps) the two sides of the reef rapidly close in towards 
the islands, till Pelelew exhibits a barrier-reef with a shallow 
boat channel on the north-west side only, while the south and 
the whole east side are surrounded by a true fringing reef close 
to the shore. 

Even the barricr-reef to the north-west of Pelelew is scarcely 
to be called a barrier-reef; its outer edge is at about 600 
paces from the shore; the surface of the reef, like that of the 
eastern reef of Babelthuap, is only navigable by boats, and 
at high water ; a true channel is wholly wanting, and the reef is 
merely intersected by a great number of smaller canals of various 
widths and depths, as is the case on the eastern reef of the 
northern island. Finally, its outer margin is not much 
yaised. However, this portion of the reef of Pelelew may cer- 
tainly be designated as a barrier-reef, though with a certain 


12 


250 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


straining of the term; but there can be no doubt whatever that 
to the southwards it very gradually passes into a fringing reef 
as characteristic as any to be found in the Philippine Archi- 
pelago ; and those islands, according to Darwin, may with the 
greatest certainty be classed among those which, being sur- 
rounded by fringing reefs, ought to indicate a region of recent 
upheaval. 

The space included within this southern reef exhibits a 
few significant peculiarities. No true deep channels occur 
here, and where they do occur, about the border region near 
Coroere, they soon disappear as we proceed southwards. The 
western and eastern reef, enclosing the numerous small 
islands, surrounds, on the contrary, an almost horizontal level 
which from west to east may be at least ten nautical miles 
across ; and, extending from the southernmost point of Pelelew 
almost to Coroere, it is about twenty-two miles long. This enor- 
mous and, as bas been said, almost flat surface is traversed in 
every direction by numerous channels intersecting it at right 
angles. The average depth of this pool itself may be a few 
fathoms, but on its northern side it suddenly falls to the depth 
of the channels there, namely, from fifteen to twenty fathoms. 
On the east side the reef becomes at last so decidedly a fring- 
ing reef that natives are always forced to gain the open sea 
if they wish to visit the villages lying on the east coast. 
Towards the north again, in the vicinity of Malacca (see Map), 
this fringing reef becomes a barrier reef. Finally, it must 
be observed that here, to the south, the eastern and western 
reefs show the same differences as I have already described 
minutely in speaking of Kriangle; those to the west generally 
seem to lie deeper than those on the eastern side, and they are 
strewn with numerous large blocks of dead coral, which are only 
very seldom covered by water at the highest flood tides ; those 
on the eastern side lie, on the whole, at a higher level, are formed 
almost entirely of dead coral, and large blocks of dead coral are 
never found on their exterior edge. 

VI. The prolongation of the Pelews to the north and 
south.—It cannot, I think, be disputed that the reefs and islands I 
have been describing belong to each other; but it is very pro- 


A SUBMARINE RIDGE. 251 


bable that the submarine mountain ridge on which the Pelews 
stand extends much farther to the north and south. The island 
of Ngaur (Angaur of Friedrichsen’s, and other maps) lies 
almost exactly south of Pelelew, and is divided from it hy a 
very deep channel about five miles wide. Still farther south- 
west of Ngaur a small shoal is marked on the same map, which 
is certainly formed of corals, and which has not more than ten 
fathoms of water over it. Unfortunately I was not able to 
visit the island of Ngaur myself; the natives of Pelelew, among 
whom I lived for nearly three months, persistently refused to 
take me there, because, as they declared, it was possible to land 
at one point only, and even there it was always very dangerous. 
They asserted that the reef everywhere clung to the shore, so that 
it was impossible to land excepting in a quite calm sea, I had 
already formed an idea that true reefs did not occur on the 
coast of Ngaur when I had passed between that island and 
Pelelew on the way hither, and this conjecture was confirmed by 
the statements of Herr Kubary, who, more fortunate than J, 
was able to visit Ngaur, and by his account (communicated to 
the Journal of the Godeffroy Museum) this island is in fact 
devoid of a reef. 

The submarine ridge on which, to the north of Babel- 
thuap, Kossol and Kriangle stand, is indicated in older maps 
as reaching much further to the north. On this there is 
a series of soundings extending five nautical miles to the west 
and north of Kossol, and within these limits, indicated by 
a dotted line, there is, according to Friedrichsen, a small 
shoal only shown by the words ‘heavy breakers’ (starke 
Brandung). Thus there is here a reef of about the same 
height as that of Kossol, but quite divided from it by a 
channel five miles wide. Directly north-west of Kriangle there 
lies the well-known reef of Aruangle, at a distance of twelve 
miles, yet within the line of breakers marked on the maps. 
Aruangle seems to be a true atoll; it is always said to be one, 
and, as I believe, correctly, At any rate the description given 
me by the natives of the island, which was formerly inhabited, 
entirely agreed with this hypothesis. An attempt I made to 
induce the people of Kriangle to visit Aruangle failed entirely, 


252 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


and my escort from Aibukit was too unfamiliar with the route, 
which is said to be not without danger, for me to undertake it 
without any other guide. 

I believe that we may unhesitatingly include these last- 
mentioned reefs and islands with the Pelew group proper, as 
belonging to the same system of elevations rising from the 
bottom of the ocean—a system extending about eighty-five 
miles from north to south, and measuring ten to fifteen miles at 
its greatest width. I intentionally say a system of el:vations, for 
it seems to me quite impossible to suppose that this group of 
islands and reefs can have been formed by a subsidence, as 
Darwin’s theory requires us to assume. To make this quite 
clear, we will endeavour to account by the theory of subsidence 
for the observed phenomena that I have described. 

VIT. The theory of subsidence as a means of explaining 
the origin of the Pelew Islands.—If we suppose that the sub- 
sidence has been equal everywhere throughout the group, as 
must be allowed according to Darwin, it is, in the first place, 
difficult to see why in the north only isolated atolls, in the 
middle barrier-reefs, and in the south only fringing reefs, have 
been formed ; and why, farther south still at Ngaur, all reef 
structure should have almost ceased. According to the pre- 
dominant views it is allowable to regard the depth of the reef 
channel of Aibukit as a standard of measurement for the sub- 
sidence that has taken place. This would thus amount at a 
maximum to about fifty fathoms. Now if we imagine the 
islands in the north to have been raised to a height exactly 
corresponding to the actual amount of the assumed subsidence, 
the bottom of the channel between Babelthuap and Kossol, as 
well as that between this atoll and Kriangle, must have been 
in the highest degree favourable to the establishment of a growth 
of corals. But this lias not been the case; on the contrary, 
the channel to the north of Kossol is entirely free from them, 
and that to the south more or less so, If the form of the reef 
were indeed due to subsidence only, the present fifty-fathom 
line of soundings must coincide with the outline, at any rate 
of the main features of the reef now existing; the facts of the 
case are precisely the contrary. Hence we must in the first 


ACTION OF CURRENTS ON THE SHORE. 253 


place conclude that subsidence alone has not here sufficed to 
produce the forms of the northern reef of this archipelago ; 
since, for instance, it must have caused the isolated blocks of 
coral in the southern channel of Kossol to form high reefs, just 
as much and in the same way as has happened in Kriangle or 
on Kossol itself. 

Thus we are obliged, under all the circumstances, to assume 
the co-operation of some other force besides subsidence when en- 
deavouring to explain the peculiar formation of the northern 
reef, but still, without wholly excluding the effects of subsidence. 
That force, as I believe, can only be sought in the action of the 
constant currents of the sea ; for no other influence which might 
check the growth of coral—such as sand, ooze in the water, 
strong streams of fresh water, &c.—can be adduced in explanation 
of the conditions and behaviour of this reef, nor have they ever 
come into play in these spots. But the currents which during 
rising tides here run strongly to the east, and at ebbing tides to 
the west, might very well have acted on the original furrows 
between Kriangle, Kossol, and Babelthuap, so as to widen 
them to channels as these first sank below the surface of the sea, 
and so have opened the way to the influx of the tidal currents. 
Thus, combined with the currents, subsidence might have pro- 
duced the form exhibited by the northern reef. However, it 
has always seemed to me to be a very weak point in the 
theory of subsidence, that it is evidently insufficient to explain 
some particular cases, and requires to be supplemented by some 
auxiliary force standing in no direct causal connection with the 
theory itself; and I am convinced that many similar instances 
will be found as investigators begin to take the trouble to study 
separate reefs more exactly than has hitherto been done. 

A second difficulty occurs as follows. I have already stated 
that the triangle lying between Pelelew, Urudzapel, and the 
Urulong Channel, constituting nearly a fifth of the super- 
ficies of the whole group, is almost level, and lies, in section, at 
from two to four fathoms under water at high tide. This level 
bottom consists of solid limestone which, rising gradually, con- 
stitutes the numerous islands, composed of the same stone, 
which are scattered about the suiface. It is cut through by 


254 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


numbers of narrow canals with perpendicular sides, usually 
from two to three fathoms deep or even more, and evidently 
cut into the stone by the action of currents; they grow deeper 
and broader to the east and north-east, and there ultimately fall 
into a wide basin, extending as far as the south point of Babel- 
thuap, and between fifteen and twenty-five fathoms deep. The 
rise of this limestone bottom is just as gradual towards the 
outer reef—where it appears as its internal declivity—as its 
slope where it forms the rocky islands ; still it nowhere consti- 
tutes any portion of the present living reef. These conditions 
are very difficult to explain by the hypothesis of recent sub- 
sidence. If such a process had in fact and exclusively effected 
the submergence of the limestone plateau, this must have ac- 
quired its peculiar structure before this recent subsidence was 
initiated, which is in itself highly improbable. The whole 
plateau, on the contrary, with its channels, produces the im- 
pression that it has been recently formed during a period of 
rest or of slow elevation. But the limestone plateau affords 
even better arguments against the assumption that subsidence 
was going on during the formation of these reefs. Wherever 
the limestone islands of the southern region are exposed to the 
” denuding action of the rising tides, a wall like a hollow cornice 
rises abruptly from the base of the island which slopes up 
gradually from the submarine level, and its summit, which 
often overhangs to a dangerous extent, has a growth of shrubs 
or trees. This hollowed-out base, which would show a highly 
concave section, is from six to ten feet high; at the highest 
flood-tides the water rushes into it with great force, and its 
face exhibits many smaller holes and fissures, often eaten into the 
solid rock for some feet. In many spots the overnanging sum- 
mit threatens to fall, and that such catastrophes are certainly 
not rare, may be seen hy the piled-up fragments which in many 
places lie -half in the water at the foot of the islands. This 
seems to me to prove convincingly that a subsidence has not 
taken place, for it is not clear how, during subsidence, such a 
form of the internal limestone plateau and its transition into 
the islands rising from it could have originated. 

It might, however, be conceded that the phenomena here 


SUBSIDENCE AND UPHEAVAL. 255 


described make the theory of a recent subsidence untenable as 
regards the southern portion of the group, without our being 
obliged to deny the possibility of such action for the whole 
group. If, for instance, it is assumed that the subsidence has 
not taken place equally, but that by a sort of tilt the northern 
portion has subsided most, the middle but very little, and the 
south not at all, all the observed phenomena might be accounted 
for, or at least apparently explained. Nay, more, the pre- 
sence of true fringing reefs to the extreme south of Pelelew, and 
thetotal absence of all visible reefs round Ngaur, would, from the 
point of view of the Darwinian theory, indicate a past process of 
elevation of the region south of the axis, if it were assumed that 
this point of equilibrium and perfect rest had been somewhere 
rear the northern point of Pelelew. 

Now I shall lay no great stress on the internal improba- 
bility of the hypothesis that such an unequal movement should 
actually have occurred in so small an area, which, besides, rises 
from the depths of the ocean perfectly isolated from other 
groups of islands. Indeed, the difficulties here raised would by 
no means be removed in this way; they would rather be 
essentially enhanced, from the fact that south of Ngaur a 
submarine reef is found very near to that island, and its 
being divided from it can be explained only by supposing that 
some other forces have been at work besides subsidence only. 
Thus even if it were admitted, in spite of the facts which are 
directly opposed to it, that the impossibility of such a subsidence 
is not conclusively proved by them, we should still be com- 
pelled to acknowledge that the influence of the constant cur- 
rents must be, in all cases, an essential factor in the history of 
the origin and formation of these islands. For, to insist once 
more very positively on this point, during subsidence the coast- 
line of the sinking island would necessarily be preserved, as 
Darwin even will admit ; but this, as is proved by my data, has 
not been the case, and hence the subsidence theory, by itself, 
ceases to be applicable. This, naturally, narrows its whole 
general value; for if it is impossible in a special case like the 
present to explain the circumstances by the only force which 
that theory admits to be efficient and determining, and if, in- 


256 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


stead of it, we are compelled to have recourse to various other 
forces to aid us toa solution, the problem of showing that a 
subsidence must nevertheless have taken place belongs more 
than ever to the prevailing theory. But in the case here under 
discussion such an attempt is met by an insuperable difficulty ; 
this was briefly alluded to above, and must now be discussed 
somewhat more fully. 

The submarine mountain district which serves as the foun- 
dation of the whole group is extremely narrow; at the widest 
part the western reef is at most from twelve to fifteen miles 
from the eastern one. Nevertheless, these reefs are extraor- 
dinarily dissimilar in structure ; even in the small and indepen- 
dent atoll of Kriang'e this difference is conspicuous. The 
eastern reef is everywhere much nearer to the shore than the 
western. A navigable channel between it and the included 
islands occurs only at about the middle of Babelthuap, where 
Altngot Passage (see Map I.) has been formed, according to 
Friedrichsen’s map, by an outer reef lying in front of the 
island reef proper. The eastern reef, so far as I have seen it, 
nowhere exhibits a line of dead coral-blocks above the highest 
tide mark. And yet this is the weather side, on which only, it 
is said, such blocks ever occur. The western reef, on the con- 
trary, is invariably characterised by them. Moreover, while 
the western cliff is throughout a true barrier reef, almost down 
to the southern point of Pelelew, the eastern reef can hardly be 
regarded as a barrier reef even at the north of Babelthuap ; and 
southwards for about the lower third of the group, it assumes 
the character of a true fringing reef. 

This difference, and the incontrovertible fact that fringing 
reefs predominate on the eastern coast, cannot by any means 
be reconciled with Darwin’s theory unless we suppose, as 
Darwin has done and as Dana evidently fain would do, that 
such a reef formation might take place even duving subsidence. 
But, according to Darwin’s own admission, this involves the 
assumption that the seaward declivity of the outer reef is ex- 
tremely steep. This hypothetical steepness, however, occurs in 
the Pelew Islands only on the west side, while, in absolute con- 
tradiction, the eastern declivity is remarkably gradual. From 


TIE SLOPE OF TIIE SEA-BOTTOM. 257 


the tops of trees in Kriangle I could clearly discern that there, 
too, the light hue of the water to the eastward denoted a 
shelving shore, whi'e on the west the deep blue sea comes 
close up to the outer edge of the reef. Anchorage can be found 
outside the reef on the eastern coast, but not on the west, and 
the same is true of Kossol and the northern portion of Babel- 
thuap. We could venture on the west side close up to the 
reef in our schooner; in cruising we were not forced to tack 
till within a few hundred paces of the shore. This would 
be quite impossible on the eastern side, where for the whole 
distance from Roll to Kossol I could still clearly see the corals 
at from one to two geographical miles from the shore. Again, 
it is now very difficult to enter the harbour of Coroere, formerly 
a very good one, on account of the numerous shoals and the 
shelving upwards of the sea bottom, and this I was assured of 
by Captain Woodin at a time when I still believed implicitly in 
the absolute correctness of the subsidence theory. Finally, on the 
western side of Pelelew, where the reef is not a perfectly 
characterised barrier-reef, the ‘Lady Leigh’ approached within 
a hundred paces of the reef, while on the east coast a wide 
belt of light blue water extends far out to sea. 

Here we find a state of things precisely the reverse of what 
Darwin in the second edition of his book has adduced as an 
argument against my views. Where, according to him, there 
should be an extremely abrupt declivity, it is not to be found, 
and where it does in fact occur, as on the west coast, no fringing 
reef is formed. 

A glance at an ideal but correct section of the Pelews in 
various spots will here throw light on what I have advanced, 
and at the same time conclusively show that in this particular 
instance the principles of the subsidence theory do not suffice to 
explain the peculiar formation of the reefs. And even if, like 
Darwin and Dana, we set aside this difficulty, so many, as 
we have seen, still remain that their theory must call in 
the aid of other forces—namely, currents—to enable us to 
understand the structure and origin of these reefs. Hence, in- 
evitably, the question arises whether perhaps the auxiliary cause. 
which is indispensable to the subsidence theory, may not have 


258 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


been the only efficient cause ; and, moreover, the further enquiry 
whether this auxiliary cause, in combination with a slow up- 
heaval, might not have been perfectly competent to give ii-e to 
every form of reef, simultaneously and side by side on the same 
area. Many celestial phenomena can be explained, and long 
were explained, by assuming that the sun moved round the 
earth ; the consideration that it was insufficient to explain otber 
facts allowed us to perceive its complete absurdity. But it is 
not always the case that one explanation perfectly excludes the 
other. We know that in most plants chlorophyll is formed only 
under the influence of light; an exception is found in many 


3 nautical males. 


I) Pp, 
YOY) 
UY YY, 


grauticul mile 
Fic, 70.—u, section through Kriangle; 6, section through Babelthuap near Aibukit; 
¢, section through Pelclew. 

Conifer, in which the same leaf-green is elaborated even in the 
dark. Hence we must conclude that other causes besides those 
that act on broad-leaved plants are capalle of producing the same 
results. We know, generally, that nature has in many cases 
made use of different means to produce results which to our 
eyes seem identical. Thus, to return to the matter in hand, 
we still have to investigate the question whether the same 
results might not have been produced by the auxiliary cause, 
assuming an upheaval, as hy the hypothetical subsidence ; since, 


EVIDENCES OF UPIMEAVAL. 259 


even under the assumption of the subsidence, they still could 
find only a forced explanation by the help of some other agency. 

To guide us in this enquiry, it will be well to collect and 
collate the facts which may directly prove a recent upheaval 
in the Pelew Archipelago. 

Evidences of recent upheaval in the Pelew Islands.—I 
have already pointed out that the huge blocks of coral lying on 
the outer margin of the western reef, in my opinion, can only be 
regarded as evidence of a recent upheaval. Since, however, this 
may be disputed on familiar, though not perhaps very strong, 
grounds, I will not attribute to it any great importance. 

The following stronger grounds, on the other hand, can 
hardly be doubted. Most of the islands are high ; to the south 
they rise to 200 or even 300 feet (at most), while on Babel- 
thuap there are hills said to be 2,000 feet high. Their structure 
sufficiently proves that they owe their origin to a volcanic 
upheaval in quite recent times. 

Between the islands of the north and south a marked con- 
trast is visible ; while the former are almost exclusively volcanic, 
by far the greater number of the latter are formed of upheaved 
and partly metamorphic coralline limestone. This contrast is 
so sharply defined that even the natives have distinctive names 
to express the difference ; the islands formed of coralline lime- 
stone they call ‘ Kokeal,’ the volcanic islands ‘ Royoss.’ 

A recent work by Dr. Wiechmann treats of the geological 
structure of the northern islands. This geologist !°° has come to 
the conclusion, which is confirmed by Herr Giimbel (inspector 
of mines), who examined my collection of minerals, that the 
eruptive rock is augitic Andesite. Wiechmann also came to 
the conclusion—without having been to the islands, and simply 
from studying the minerals collected there by Dr. Kubary—that 
the eruption must have been submarine—a view I had long 
since taken, from a study of the islands themselves. He also 
approximately determined the period of the eruption ; he is of 
opinion that the upheaval must have taken place during the 
latest period of the tertiary epoch. A remarkable feature, not 
mentioned by Wiechmann, is the distribution of the various 
volcanic rocksin Babelthuap. The solid eruptive rock, Andesite, 


260 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


according to my observations, occurs exclusively, or nearly so, 
on the east side of the island, while, on the west, the lower hills 
consist of red tufa and strata of rolled pebbles; these are but 
rarely traversed by the Andesite core which forms isolated high 
hills. Thus, for instance, the summit of that known as ‘ Royoss 
Armilimui’ (see Map. I), which I did not ascend, from Kubary’s 
information appears to consist of Andesite, and the black 
colour of the peak confirms this. These differences in the 
geology of the country are marked by a corresponding variety 
in the landscape. While on the western side the slope of the 
land is generally gentle, and small islets formed of tufa lie 
scattered on the surface of the inner reef, the eastern side is 
everywhere much more precipitous. There, where the black 
Andesite rock rises to any considerable height, we frequently 
find a quite perpendicular precipice. This is apt to hang over 
at the top, and its base, hollowed away by the surf, generally 
slopes to a continuous bottom of the same rock; but we no- 
where meet with separate islets, such as are to be seen on the 
western side. There the strata of tufa are almost horizontal, 
with a slight dip to the west; according to Friedrichsen’s map 
these also occur in the island of Aruangle, since, from Kubary’s 
account, the natives declare that it consists of ‘ Royoss.’ Cer- 
tainly the description I had of it in Kriangle does not agree 
with this. However, be this as it may, so much at least is cer- 
tain, that at a former period the tuta of the west extended 
much further than it now does, since a few disrupted islets of 
tufa lie on the surface of the inner reef at one or two nautical 
miles from the main island. 

At the southern end of Babelthuap the eruptive rocks are 
combined with the limestone rocks called ‘ Kokeal.’ Still there 
are but a few spots where they lie directly one on the other. I 
myself, in fact, have never seen them in such juxtaposition ; but 
Wiechmann states, on the strength of Kubary’s observations, that 
solid limestone lies immediately upon the black Andesite at the 
south-east end of Babelthuap, a spot I have not visited. Simi- 
lar examples occur, according to this observer, on two small 
islands lying to the south of Coroere. But, irrespective of these 
localities, the islands composed of volcanic rock and of coralline 


TRACES OF YOLCANIC ACTION. 261 


limestone mutually exclude each other. In the border region, 
about the middle of the group, they are mingled without order ; 
thus, for instance, the island of Coroere, consisting entirely of 
tufa, is close to the limestone islands which lie by the southern 
point of Babelthuap ; then again we find a limestone island, and 
then the wholly volcanic island of Malacca, and between these 
larger islands there are numerous smaller ones, some of coral- 
line limestone and some of tufa. 

Yet farther to the south the Andesite and volcanic tufa 
wholly disappear, and all the islands south of the latitude of 
Urulong (see Map I.) consist exclusively of upheaved coralline 
limestone, partly very highly metamorphosed. These are, with- 
out exception, true raised reefs, as is seen from their general 
form, the equal level of their summits, and the fossils found in 
their strata; their structure and their connection with the still 
living reef prove, too, that they are of quite recent origin. It 
will be well worth the trouble to study the arguments for this 
last statement somewhat more closely. 

The height of the cliffs of the various ‘ Kokeal’ islands’ 
differs greatly ; the highest are from two hundred and fifty to three 
hundred feet above the sea, the lowest often scarcely ten f eet 
above the water. Even in the same island, as in Pelelew, this 
difference occurs. The western cliffs of this island rise to 
about 250 feet with a perfectly horizontal top ; the eastern cliffs 
at Ardelollec, on the contrary, are at most eighty feet above the 
sea ; their top, too, is almost horizontal. Quite at the south of 
the island, again, we find cliffs which stand barely five to ten 
feet above the sand thrown up by the waves. In general the 
northern islands of the Kokeal are the loftiest ; but even there 
they certainly never reach the height ascribed to them by 
Friedvichsen of from fifteen hundred to two thousand feet. 

The geognostic structure of the Kokeal islands is also very 
various. Sometimes the cliffs are composed of very dense 
limestone in which hardly a trace of fossils can be found, or in 
which the coralline structure has been preserved ; in the latter 
case the rock is sometimes hard, as if infiltrated by a dense 
almost crystalline limestone; or it may be chalky, as white as 
snow, and easily friable. The cliffs of Ngaur and Pelelew, 


262 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


for instance, consist of such a snow-white rock, and their 
gleaming perpendicular walls form a landmark from afar for 
the sailor, The masses of fossils found in them, both corals and 
shells, are for the most part merely impressions preserved in the 
limestone. This is difficult to detect at the first glance ; for the 
corals especially, or rather their impressions, lie so wonderfully 
closely that they seem to form a dense mass of compressed 
corals. 

The geographical distribution of these different kinds 
of coralline limestone also offers some striking peculiarities. 
All the Kokeal islands which I saw, and which lay in the 
vicinity of volcanic recks—as, for instance, those in the neigh- 
bourhood of Coroere—consist of dense limestone which is often 
semi-crystalline ; when fossils occur, they are firmly imbedded in 
the rock and preserved uninjured. These islands also yield 
exclusively the large pebbles of arragonite which are used by the 
natives of the island of Yap lying a hundred miles to the 
northward—as a kind of money in great request. The more 
remote the islands are from the centre of volcanic action 
presumably situated in the middle of Babelthuap, the less 
prevalent is the limestone, dense or crystalline, till in the 
south it finally quite disappears. 

The fossils contained in these rocks show, in conjunction 
with other peculiarities in the structure of the upheaved reefs, 
that these reefs belong to quite a recent period, and that, in 
fact, we must regard them only as the beginning of the reefs 
now in course of construction in the neighbouring seas. On the 
island of Noerkessul, lying on the eastern reef of Pelelew, 
which is only about twenty to twenty-five feet high, I found 
with true Astreide, imbedded in the rock, a tooth of the 
Indian crocodile, which still is found there, though it is not 
frequent. On the little island of Calacoligoll, which is almost 
on the outer ridge of the western reef of Pelelew (that is to say 
not more than 120 feet from it), I found a large block, at least 
five feet in length, in which the imbedded corals stood upright, 
and among them were shells of Pholas and numerous tubes of 
Vermetus gigas, which still lives and is very common in the 
sea close by. The centre of Pelelew is from twenty to twenty- 


TERRACED STRUCTURE OF A REEF. 263 


five feet above high-water mark ; the chalk-like cliffs rise from 
the sea perpendicularly, or at any rate very steeply, and the 
inner base shows clear traces of the effects of surf at a former 
period. Here constantly are found heaps of torn-up rocks and 
fossils ; but those fossils which were found on the inner side 
of the cliffs were quite different from those of the outer side— 
numerous Fungiz, which are very near to living species, or 
may be identical, many Pectens, with enormous masses of two 
or three species of J/ycedium and Agaricia, belonging to the 
most delicate forms of the genera. These two corals form the 
greater part of the rock of which the cliffs consist. 

But besides these reasons, already indicated by Wiechmann, 
for assuming a recent upheaval, there are other factors which 
confirm this evidence. The eastern reef of Pelelew has all the 
character of a fringing reef; the outer edge, which is scarcely 
raised, and which in many places lies at most at 100 feet from 
the shore, gradually passes into a manifold series of raised cliffs. 
Those of the first series are mostly only six to ten feet 
above the strand, which slopes down to the reef; the base is 
much hollowed out by the waves, and passes without interrup- 
tion into the surface of the dead reef; it also consists entirely 
of Astrea and Meandrina, partly metamorphosed. The surface 
of the dead reef next to the strand is almost horizontal, but it 
drops by little shelves hardly more than a foot high, till the 
lowest of the terraces thus formed is covered at high tide by 
from two to three feet of water; then, gently shelving down, 
the dead portion of the reef merges in the living, of which, as I 
have said, the outer edge is but littleraised. All these terraces 
are quite smooth, and without any trace of large coral blocks 
thrown up by the waves. From these indications we may safely 
infer that elevation has taken place within a very recent 
period ; for otherwise it is difficult to see how the still living 
reef could be a direct continuation of the raised dead coral. 
Quite identical phenomena are displayed; as I have said, on the 
east side of Kriangle. 

The facts here adduced suffice, as it seems to me, to prove 
that, in the first place, a quite recent upheaval must have 
ceourred ; and, secondly, that that period of upheaval must have 


264 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


passed into the present condition of very slow elevation or 
absolute rest without any conspicuous break. 

An attempt to explain the structure of the reefs of the 
Pelew Islands—I have said that the theory of subsidence is 
insufficient to explain the sections given on p. 258, since, according 
to that theory, on the steep west coast there ought to be a fringing 
reef, and on the shelving east coast a barrier reef. Exactly the 
contrary is the case. The occurrence of shallows without reefs 
close to atolls, as at Kossol, and of high reefless islands, as 
Ngaur, the high blocks on the west side and outer edge of 
all the western reefs, the extensive, almost horizontal, sub- 
marine level to the north of Pelelew, the uninterrupted connec- 
tion of the eastern reefs of Pelelew and Kriangle with the 
dead raised coralline cliffs—ail these facts are arguments, hardly 
to be refuted, against a recent subsidence. And if all of these 
should be explained away by arbitrary assumptions of which 
the baselessness could only be proved by fresh investigations 
carried out on the spot, we still should be obliged to accept the 
degrading action of the movement of the sea, and, above all, 
that of constant currents, as causes co-operating with the 
supposed subsidence. I, of course, readily admit that these 
must have had their effect, but I positively dispute that the 
recognition of these effects proves the necessity of a subsi- 
dence. On the contrary, I believe that those apparently 
secondary causes would be far more likely to be effective during 
a period of elevation than when combined with subsidence, and 
that all the conditions I have described which argue against 
a subsidence under the other hypothesis may be perfectly ex- 
plained by easy and independent assumptions. This, in the first 
instance, applies of course only to the structure of the Pelew 
reef, and it must remain for further investigations to determine 
how far similar conditions may exist or not in other coral 
islands ; since the proof that here, in the Pelew Islands, subsi- 
dence cannot have been the special cause which has determined 
the form of the reefs is, self-evidently, no proof that in other 
groups subsidence may not have been combined with the 
upward growth of the reef in the form impressed on it by other 
causes, 


CONSPICUOUS EFFECTS OF CURRENTS. 265 


In the seventh chapter we have already seen that not 
individual corals only but whole reefs are influenced in the most 
decided manner by two forces, ¢.e. the strength and the direc- 
tion of constant currents impinging on them. Their favourable 
development depends, no doubt, on other things, as the warmth 
of the water, its chemical composition, the accidental mingling 
of species, &c. ; but all these influences, in my opinion, sink into 
the background in comparison with currents; for though they 
may impede or even destroy the existence of the polyps, they 
never, so far as I can see, force them to develope in any par- 
ticular direction. Now this, as I have amply explained in 
Chapter VIT., is in an eminent degree the effect of constant 
currents. Moreover in this investigation it is not our business 
to determine whether corals can thrive, or even grow at all, in 
this or that particular spot, but exclusively to decide whether 
the forms of particular reefs, under circumstances of unhindered 
growth, can be explained by known causes. 

‘We must remember that wherever constant and deep 
currents impinge on a coast at an angle, the reef will inevi- 
tably grow upwards perpendicularly if the force of the current 
is sufficient ; and, on the other hand, that many species of coral 
have a tendency to grow equally in every direction, as far as 
circumstances allow, so long as shallow currents flow horizon- 
tally over them. 

The high seas, the open ocean, exhibit both these modes of 
motion of water in the most conspicuous degree. Late researches 
have shown us that strong currents often flow at great depths ; 
these constant currents maintain the same direction the whole 
year through, though their force may vary ; even those arising 
from the ebb and flow of the tide vary in strength under the 
influence of the prevailing winds, but never, or rarely, in 
direction. Besides these currents flowing at great depths there 
are, in the second place, quite superticial ones, which are some- 
times variable, particularly if they are affected by the prevalent 
winds, or sometimes very constant, as in the case of the drift 
in great seas. 

The Pelew Islands lie, as is well known, within the region 
of the north equatorial current flowing from east to west in the 


266 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


Pacific Ocean. This current, in conjunction with those caused 
by the ebb and flow of the tide, impinges perpendicularly on the 
broad side of the group ; hence on their eastern side there is a 
triangle of comparatively still water, since the main current 
must part before the insuperable barrier; and, within it, only 
the more superficial currents can produce any effect. These, 
however, here in the Pelew Islands flow almost constantly from 
east to west ; on the eastern reef a long line of breakers is always 
visible even during the short period of the south-west monsoon, 
and even at the highest tides it is always dangerous to cross the 
reef here. On the western reef, on the contrary, at high tide 
and in a calm sea, the water over the exterior edge of the reef 
is so perfectly still that it may be paddled across in a boat with- 
out any danger. The currents which run past the islands to 
the north and south, or between the separate islands, whether 
as tidal currents or as part of the great north equatorial current, 
on the west side, turn at an angle to the north or south. In 
correspondence with these facts we see that in a rough sea a 
wave falling on the outer reef propagates itself in the direction 
indicated, while analogous waves on the east break simultane- 
ously on almost the whole length of the shore. In connection 
with this, indeed, there is another fact which surprised me very 
much the first time 1 observed it. It is usually supposed that 
while the tide is rising, the water that flows in the lagoons or 
lagoon channels is thrown into them over the outer margin of 
the reef. This is certainly not the case in the Pelews; almost 
all the water flows into the natural channels as readily as it 
flows out of them. This is proved by the fact that during the 
rising tide the current produced on the surface of the reef 
does not flow into the channel from the outer reef, as would be 
expected if that hypothesis were correct, but on the contrary 
from the channel towards the reef. During my first expedition 
on the western reef, my life was in some danger from this ci- 
cumstance, then unknown to me, for I had gone so far from the 
boat that I had great difticulty in getting Lack to it again. 

Let us now endeavour to explain the observations I have 
communicated on the assumption that both these classes of 
currents were active agents during a period of upheaval in 


EFFECTS OF OCEAN WAVES. 267 


these islands. I hope thus to succeed in showing that in this 
particular instance every difficulty vanishes which can be raised 
by the hypothesis of subsidence. 

In Kriangle the boat-canal cut through the eastern reef, 
and the large coral blocks lying at the south-west point are 
easily explained by upheaval, as also the structure of the 
eastern reef. On the western side the current attacks the reef 
at an angle; hence it must grow upwards perpendicularly. 
Thus, by degrees, the corals elevated above the level of the 
living reef must die out ; they remain standing, however, and 
are slowly destroyed by wind and weather, the softer parts first 
and the harder portions later, and these naturally endure the 
longest where they are least and most rarely exposed to waves 
and storms. This in Kriangle is the case precisely in the 
spot where the highest blocks lie on the outer margin of the 
reef. On the eastern side, on the contrary, the ocean drift, 
combined with the constant current to the westward, falls 
perpendicularly upon the reef; the billows are still further 
increased on this side by the gentle slope of the sea-bottom, so 
that a strong, and above all an unremitting, wearing-down 
process is exercised on the reef. Here, on the weather side, it 
is said that the largest thrown-up blocks ought to be found; 
but this is not the case, and it is easily explained. If we 
suppose that such blocks were actually thrown up for once, 
they must soon have been destroyed by the incessant action 
of the waves beating directly on the reef; and the same is the 
case naturally with all corals which have been lifted above the 
highest storm tides during a slow elevation of the outer margin 
of the reef. 

One objection only can be raised. The lagoon, that is to 
say, which is enclosed by the reefs, might with apparent justice 
be adduced as evidence against an upheaval. And this would 
in fact prove a great obstacle to my views if it were necessary 
to assume a very rapid elevation of the whole archipelago. But 
as the case is precisely the reverse we must assume a very slow 
upheaval, and it is easy to offer an explanation of the origin 
of a lagoon, in spite of a slow rising of the bottom. Reflect for 
a moment on the instance, discussed in the previous chapter, of 


268 THE INFLUENCE OF INANIMATE SURROUNDINGS, 


an old colony of Porites. Its surface, in the first instance quite 
level, will be gradually hollowed out by various co-operating 
influences, and so at last a raised margin, only cut through by a 
few channels, will surround a central hollow. <A precisely 
similar result may be produced in a reef undergoing slow up- 
heaval. Suppose, for example, that Kriangle had a tolerably 
level surface, like the shallows to the south of Ngaur, or like 
Kossol, before it was raised to the average level of the sea: from 
the first moment when it was exposed to aerial influences a 
process of destruction must have begun on the surface of the 
reef, perfectly analogous to that which takes effect on large 
isolated coral blocks. Animals and plants first establish them- 
selves on the surface of the reef, which is certainly highly 
favourable to them; they excavate and penetrate the solid 
limestone of the coral in every direction while the rain falling 
on the face of it kills the polyps themselves; the rain and the 
sea-water flung over the margin of the reef must remain there 
if they can find no outlet. Jn the first instance the water on the 
summit of such a rising reef may often remain standing, but 
channels must soon be worn through the constantly growing 
and rising margin, or submarine drainage may easily arise, since 
boring animals and plants are able gradually to destroy even 
the hardest limestone. That there are channels of this kind 
in Kriangle may be inferred from the fact that at low ebb- 
tides the water in the lagoon stands at no higher a level 
than it does outside the atoll; but this -could not occur if the 
water thrown over into the lagoon at high tide did not easily 
find an outlet through the reef itself, for surface channels, 
which might serve the same purpose, are wholly wanting. The 
extreme porousness of the soil of Kriangle below the sea-level is 
also proved by another fact. The inhabitants have sunk deep 
wells, and a very large tank, wbout ten feet deep, for bathing pur- 
poses; the water in these is usually fresh, as they fill during 
the rainy season. But when, after persistent drought in the 
dry season, the level of the water sinks considerably in the 
basin, the bottom of it is frequently brackish, and that without 
storms having arisen to cause the sea-water outside to wash over 
the low island and into the tank or the wells. Thisshows that 


SUBMARINE DRAINAGE. 269 


sea-water can percolate through the walls of the atoll-reef into 
these reservoirs, and consequently proves that the svil must be 
pierced by larger or smaller channels. The raised limestone 
islands of Kokeal are similarly porous. One very small island 
close to Coroere has quite the structure of a true uplifted atoll. 
A high wall of metamorphic coralline limestone surrounds a 
deep lagoon on all sides, and no channel leads from this to the 
sea at the present day. There is a little ridge, which may for- 
merly have been a channel, to be crossed in order to reach the 
lagoon. Nevertheless the same fish are found in the lagoon as 
outside, the water is equally salt, and its level rises and sinks 
regularly with the tidal flow and ebb. The following circum- 
stance ig even more conclusive. Onone of my excursions round 
Pelelew, before going into the harbour of Nasiass from the sea, I 
saw out at sea a wide current of yellowish water, almost fresh, 
flowing from the land; its edge was pretty sharply defined 
against the sea water. It was exactly low water. The natives 
informed me that this current was always there, though some- 
what more feeble at high tide, and strongest during the rainy 
season. Now in the whole of Pelelew there is not one stream 
from which this current could proceed. All the surface water 
percolates at once through the excessively porous soil, and it is 
only during heavy storms of rain that drainage streams form 
in the gullies, and these totally disappear again in a few hours. 
This water then runs away through the soil down to a certain 
level. But, instead of reappearing at the same level on the sea- 
shore, its flow is submarine for a considerable distance from 
land—a proof that it must have found its way through very deep- 
lying channels. 

If we duly consider all these conditions, it becomes clear 
that there is no serious difficulty in the way of the hypothesis 
that the lagoon extant in Kriangle should have been the result 
of the action of currents on the porous soil during a period of 
slow upheaval. 

With regard to the reef of Babelthuap, there are other objec- 
tions in the way of the subsidence theory ; by far the most grave is 
the gradual seaward slope of the eastern outer reef. But this 
and all other difficulties vanish with the supposition that the reefs 


270 THE INFLUENCE OF INANIMATE SURROUNDINGS, 


were formed during a period of elevation, if we duly realise in 
our mind the effects that must have ensued from the combined 
action of upheaval and of more or less constant currents on the 
form of the growing reef. To the eastward the impact of the 
current on the island is perpendicular to the face of the coast ; 
it therefore beats upon the slowly rising sea-bottom, and, being 
tolerably strong in itself, it prevents the perpendicular growth 
of the reef corals. The result is that the reef itself is driven 
very close to the coast. Moreover, between it and the foot of 
the land no deep channel can be formed by currents, for here 
the very hard black rock of the Andesite cliff prevents any such 
rapid grinding down as is easily effected in the coralline limestone. 
On the west coast it is otherwise. Here the currents rarely 
impinge directly on the face of the reef, but only at an angle. 
I have already shown that wherever strong currents sweep 
past a reef at an angle these are forced to grow perpendicularly or 
nearly perpendicularly upwards; this is everywhere the case on 
the western shore. The fact that the outer reef here is separated 
from the islands by a channel above forty fathoms deep and 
many miles wide, finds an easy and unforced explanation on the 
assumption of an upheaval. It is certain that the enclosed 
island of Babelthuap was formerly much broader than it now 
is, as is proved by the existence of the little island in front of 
Roll (see Map), which is now far from the land on the surface 
of the inner reef. Now, if we suppose—as we must even on the 
theory of a subsidence—that the island, which here consists 
almost entirely of tufas, originally extended nearly to the 
western outer reef, only narrow fringing reefs could have ori- 
ginally been formed on the western side; but these must from 
the first bave grown perpendicularly, because they were impinged 
on by tangential currents ; and since the tufa on this side could 
offer but a feeble resistance to the action of surface-water and 
rain, as well as of the surf beating on it, a small channel might 
soon be formed between the reef proper and the coast. This 
channel would presently grow wider, in proportion as the enclosed 
island, consisting of soft stone, was gradually eaten away, and 
during s!ow upheaval it would continue to grow deeper, in pro- 
portion as the old porous portions of the reef and the rock in 


THICKNESS OF THE REEF. 271 


which it was forming were more and more worn down by the 
combined action of boring animals and plants, and of the currents 
produced by the tidesand by rain. Every stream has, as is well 
known, a natural tendency to deepen its bed; thus, while the 
surface currents setting directly against the reef on the eastern 
side prevented a perpendicular growth of the reef, and the hard 
rock atthe base of the coral rendered the formation of a channel 
between the reef and the land by denudation impossible, such 
a process could easily be effected on the western coast by the 
destruction of the enclosed island, and thus the reef, forced into 
perpendicular growth by the tangential current, was soon at 
some distance from the coast. 

In a similar way all the other structural features of the 
Pelew reefs can easily be explained by the hypothesis of a slow 
upheaval during their formation. It appears superfluous to 
discuss them here in detail, since it would involve the repetition 
of the same arguments as have been applied to explain the two 
principal points. The presence of deep channels in spots where 
an easily disintegrated tufa has served as the foundation for 
the coral built up on it, the broad and almost horizontal sub- 
marine plateau to the south of Coroere, with the narrow channels 
that intersect it, the large blocks of dead coral everywhere lying 
on the raised outer ridge of the western reef and their total 
absence on the eastern reefs, the gradual transition of the raised 
cliffs of Pulelew into the still living portion of the reef, the 
drying up of the channel cut by man in the eastern reef of 
Kriangle, and the absence of a channel on the eastern side of 
Babelthuap—these and many other facts are easily comprehen- 
sible if we suppose that the most important causes which deter- 
mine the form of the reefs and the growth of the corals are 
those that I have brought into prominence, and that they are 
precisely such as would most easily prove effective during a 
period of slow upheaval. 

In conclusion there are still one or two objections that must 
be discussed, as they are certain to be brought forward against 
my view. First, there is the enormous thickness attributed, 
according to Darwin’s theory, to the reef rising abruptly from 
the sea-bottom ; and secondly, the small depth in which only, as: 


272 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


it is said, reef-building corals can exist. Nay, it even may pos- 
sibly be said that my explanation as regards the Pelew reefs 
deserves no further attention, for the very reason that it ignores 
these fundamental facts, which lie at the root of the subsidence 
theory. I admit that I have, up to this moment, left them out 
of the question, but I have done so on purpose; and I must 
expressly contend that neither of these hypotheses deserves to 
be introduced into the discussion, fur neither of them has been 
established as a fact by investigation, but is merely inferred 
from ill-founded observations. 

With reference to the first point—the great thickness of the 
reef—I must confess that the method of estimating it as set 
forth by Darwin and Dana does not appear to me in any way 
to establish the conclusions arrived at. Both calculate by the 
same method, but with very different results; they agree in 
estimating the thickness of the reef on the arbitrary hypothesis 
that the foundation on which the submarine base of the reef 
rests, must have the same inclination as is visible to observation in 
the islands enclosed by the reef. Dana indeed assumed a some- 
what less steep incline than Darwin ; but it is quite possible, if 
not probable, that even Dana allows a too great incline for the 
submarine fall of the coast ; for we know that even in regular 
cones the base usually exhibits a more gradual slope than the 
peak, and according to the subsidence theory it can only be the 
peaks of the mountains that rise above the surface of the sea; 
hence the base, which is covered by the reef, must probably have 
a much less steep incline than can here come under our direct 
observation. Thus, under all circumstances, the calculated 
thickness of the reef remains hypothetical, since it is founded on 
an assumption which is unproved by observation. Hence, so 
long as it is not demonstrated by borings that the reef is in fact 
as thick at its outer edge as has been estimated from the angle 
of inclination of the land surrounded by it, this estimated thick- 
ness cannot be regarded as an available argument for any further 
conclusions. 

If these assertions as to the depth of the reef were actu- 
ally correct, it would inevitably follow—from the fact, which 
hardly admits of dispute, that the reef-forming species of coral 


SUBMARINE PLATEAU. 273 


can live only in water of moderate depth, combined with the 
assumption that the whole vast thickness of the reef was formed 
by such shallow-water species—that such a reef could only bave 
been formed during a period of subsidence. Of course I freely 
admit that the reef-building corals occur only down to a depth 
of twenty fathoms at most. It is consequently impossible that, 
during a period of upheaval, they should form a reef of more than 
120 feet in perpendicular thickness. But we now know 
that numerous animals live at much greater depths than had 
hitherto been supposed. The great ‘ Challenger’ expedition and 
other similar voyages have furnished proof that a very rich and 
peculiar fauna is universally distributed throughout the ocean, 
and we know, moreover, from Carpenter’s investigations in the 
Mediterranean, that merely local causes have there prevented 
the development of an equally rich fauna. We may therefore 
infer that the constructive activity of the greater number of 
marine animals might very well supply materials for the forma- 
tion of a solid submarine soil, even at much greater depths 
than those at which the true reef-forming corals are found. 
This probability is raised to certainty by the observations of 
American naturalists in the West Indian seas. Pourtalés 
there discovered a plateau of solid rock many miles in ex- 
tent, lying at an average depth of 150 fathoms, and consisting 
of a compact conglomerate of fragments of shells and corals 
in a calcareous mud with transported rolled pebbles. This 
has been called the Pourtalés Plateau, after its discoverer. 
If this plateau were to be slowly elevated, at a rate somewhat 
less rapid than that required for the deposition of the new layers 
of solid rock, by degrees a sufficiently solid base would be raised 
to the level requisite for the existence of reef-building corals, and 
these might begin their work on the foundation prepared for 
them by other animals. 

From all this I infer the process of reef-formation to be 
somewhat as follows. In the first instance, during the rais- 
ing of the sea-bottom, only the foundation for the coral building 
would be formed, and the inclination of the plateau thus formed 
would certainly be very inconsiderable. Here and there chan- 
nels would already be formed by the action of deep-sea currents, 


13 


274 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


As tne plateau rose to the region of reef-building corals, these 
channels would either become perpendicularly deeper or super- 
ficially wider, according to the force and direction of the 
currents acting upon them. Its tendency to grow upwards, 
combined with the general upheaval, would bring the highest 
part of the coral-stocks to the surface and so expose them to the 
influence of the tides as well as that of periodical or incidental 
rainfalls. The process will now be repeated which J have 
already described as ascertained by observation on separate coral- 
blocks ; if channels have already been formed by submarine 
currents in the reef which has come to the surface, these will 
become the most natural and convenient conduits for the water 
flung up on to the reef ; the shallows lying between the outer reef 
and the land, though of small extent at first, will increase, in the 
first instance by the outward growth of the reef, and then, in a 
much greater degree, by the destruction of the enclosed land ; 
as, at the same time, the amount of water thrown over on to 
this inner space must increase in proportion to the increased 
surface, the channels must, in spite of continued upheaval, eat 
away the rock to a greater depth, and more or less quickly 
according to the nature of the rock itself. 

Now, if we draw the obvious conclusions from the two an- 
tagonistic views above mentioned, all the arguments derived from 
observation again range themselves on my side. If it were true 
that thick reefs could be formed only during subsidence and ex- 
clusively by its action, we should be justified in expecting to find 
the same species of coral throughout the whole depth of the 
mass, and only the base, with a maximum thickness of about 
120 feet, could under the circumstances exhibit a certain variety 
in the materials composing it. To my knowledge, however, no 
observations exist which afford grounds for this conclusion. 
Granting, on the contrary, that the reef was formed during 
a period of elevation, the inference naturally follows that 
the composition of such a raised reef must be heterogeneous, 
since in the first instance only deep-sea creatures can have 
established themselves on it; the reef-corals, properly so called, 
would in such a case form only a thin layer above the deep-sea 
corals, And this actually is the case, according to my personal 


MIXTURE OF FORMS IN DEEP SEAS. 275 


observations, in the raised coral-cliffs of Pelelew. Their base, 
which is everywhere visible, consists entirely of a few species of 
Lophoseris mixed with other deep-sea forms; the species of 
Lophoseris belong indeed to the most delicate forms, and the 
genus itself is one of the most fragile of the inhabitants of the 
deep sea. The higher and highest portions of the Pelelew cliffs 
contain on the contrary only Astreide, Madrepores, Porites, 
and similar natives of small depths, And in direct proportion 
as this structure of the elevated Pelew reef furnishes an argu- 
ment for its origin during a period of slow upheaval it affords a 
no less strong one against the hypothesis that it could have 
been formed during a period of subsidence. 

Thus we have reached the end of our enquiry. It has 
shown us that the subsidence theory !° is insufficient to explain 
the structure of the Pelew reef, and also that all difficulties dis- 
appear if we assume that the really efficient influences which 
have determined the growth of the corals in certain directions 
operated during a period of slow upheaval. And finally, it has 
offered a last, grand example of those direct effects of water in 
motion on the mode of growth of animals, of which, in a former 
section, I have mentioned other less conspicuous instances. 


276 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


CHAPTER Ix. 


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


Aut the parasites that live within other animals are without 
exception adapted to more or less extensive migrations if they 
are to maintain their existence as a species in the struggle for 
existence. If, for instance, a Trichina were incapable of living 
in the stomach of any other animal than man, it would have a 
bad chance of living any longer than the man in whom it had 
accidentally originated ; for since its progeny imbed themselves 
in the muscles of the individual in whose stomach the parent 
producesthem, any transmission to another individual, and con- 
sequently its continuance as a species, could only be secured by 
cannibalism. If an ordinary fluke (Distoma) were obliged to 
complete the whole cycle of its development in one and the 
same individual, it would necessarily perish with the death of 
its host. Thus migration is one—and in fact the most impor- 
tant—of the conditions of life for all Entoparasites. 

On the other hand we know that this rule, indispensable 
to parasites, is not directly applicable to all other animals. 
Many, on the contrary, continue to exist within very limited 
and narrow regions ; most of the land mollusca of small islands, 
for instance, have a very narrow range, and it would appear as 
though their continuance as distinct species for very long periods 
were secured by the maintenance of the balance between themselves 
and the external conditions of life in the locality they inhabit. 
Here migration, as a condition of existence, seems to be abso- 
lutely excluded. But between this and the former extreme 
there is every conceivable degree of transition, and we are there- 


PASSIVE MIGRATION. QF 


fore justified in supposing that either active or passive migra- 
tion is one of the chief general conditions for the existence of a 
species as such. For instance, all sedentary animals, as Corals, 
are just as dependent on migration as parasites; if we suppose 
that one single species of coral suddenly lost the capacity of re- 
production by eggs and swimming larve, and only preserved 
the power of reproduction by buds, it would, no doubt, be able 
to extend itself, like « tree, in the spot it lived on, but it must 
nevertheless presently die out if the external: conditions of its 
existence were considerably changed—as, for instance, by sub- 
marine volcanic action. Consequently the only means by 
which such a species can maintain itself as such, is the power 
possessed by its larve of migrating and transporting the species 
beyond the original limits of its dwelling-place, so that changes 
which might lead to its annihilation in its place of origin may 
not exterminate it, or at any rate not everywhere at once. The 
same is the case for the greater number of animals; the tendency 
to migrate is an important—perhaps even the most important— 
means employed by Nature to preserve a newly originated 
species from destruction. 

This migration, which would seem to be indispensable to 
the great majority of animals, may be either passive or active. 
To the latter category belong the migrations of birds, of certain 
mammals, of many fishes, insects, &c., while many animals 
living in the sea are, on the contrary, passively migratory, as 
Salpee, Meduse, Pyrosome, &c, These are borne involuntarily 
by currents, while the former determine the direction of their 
wanderings by their own choice. In all cases of passive migra- 
tion it is self-evident that the existence and distribution of the 
species will depend on the strength and direction of currents; 
but even in the journeys made by those creatures whose 
migrations are voluntary, the same influence is often more or 
less recognisable. 

The effects of currents in water or in the air may be of two 
different kinds. In the first place they may serve as means for 
the distribution of the species; secondly as Jimits hindering their 
extension. We shall now consider the eflects of currents in 
each of these directions; but at the same time it must never be 


278 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


forgotten that the results thus produced may easily be altogether 
concealed, or even completely nullified and reversed, by the 
simultaneous operation of other conditions of existence which 
are inseparably united with this, the only factor we have imme- 
diately to consider. Granting that a species, be it what it may, 
were borne by a marinecurrent to an uninhabited island, ity exist- 
ence would depend not merely on its safe arrival there, but also 
on the favourable or unfavourable circumstances prevailing in 
the island. It is beyond a doubt that very many larve of marine 
creatures are carried by currents to the shore or into the mouths 
of rivers; but most of them perish because they do not there 
find suitable conditions of existence. The warm Mozambique 
current carries many warm-water animals out of the Indian 
Ocean into latitudes in which they cannot continue to live 
because they are unable to accommodate themselves to the low 
temperature prevailing there. In such researches we are obliged, 
and indeed required, to separate the different influences to which 
animals-are exposed ; still we must not forget that such a 
separation never occurs in nature, but that, on the contrary, 
different -and often antagonistic forces are frequently quite in- 
separable. 

1. Currents and Winds as a means of the diffusion of 
species.—It is a well-known fact that regular winds, as well as 
storms, are able to carry many flying creatures to enormous dis- 
tances from their homes. Insects of all kinds are often caught 
hundreds of miles from the nearest land, out on the high 
seas; North American birds not unfrequently come across 
the Atlantic Ocean to Scotland ; it is well-known, too, that the 
birds of many small islands are identical with, or very nearly 
related to, those of the nearest continent from which the winds 
blow that usually sweep over those islands. Ocean currents act 
in the same manner ; all free-swimming or even diifting animals 
and larve are borne along by them, and the edge of the currents 
often marks a very sharply defined line of limitation. between two 
quite dissimilar faunas. TI shall never forget the impression 
made on my mind, when I was sailing round the Cape of Good 
Hope, by a sudden change in the fauna of the ocean surface from 
this very cause. West of the meridian of Cape Town the 


DISTRIBUTION BY CURRENTS. 279 


animal world was remarkably poor in species, and more pav- 
ticularly was it devoid of all the larger forms, as Medusz, 
swimming Polyps, Ascidians, and such like; quite suddenly the 
ocean teemed with abundant life as we entered on the warm 
Mozambique current to the east of the Cape. The whole sea 
was literally covered with animals of every kind, and for more 
than two days we sailed without interruption through fields of 
gigantic Pyrosomata (P. giganteum, see fig. 71), which swam so 
close together that at night the whole ocean, out to the farthest 
horizon, shone with their blue gleam as if in broad moonlight. 
Now we might easily be led to suppose that the winds must 
act exclusively on flying or land animals, and currents, on the 
other hand, exclusively on creatures living in the water. But 


Fic. 71.—Pyrosoma gigas, 


the investigation of certain special cases will prove, on the con- 
trary, that these influences are often combined, so that land 
animals or air-breathers become dependent on currents in water, 
and also aquatic animals on the winds. It is nevertheless my 
intention to consider the action and effects of winds and cur- 
rents separately. 

(a) Currents as a means of distribution—By far the 
greater number of invertebrate animals freely swimming or 
floating in water are incapable of offering any resistance to the 
current, and are therefore carried along in the direction which 
the current itself takes. All the larve of Sponges, Polyps, 
Annelida, Tunicata, Echinodermata, and very many Mollusca 
as well as fully-grown Radiolariz, the floating Tunicata, Ptero- 
poda, Heteropoda, many Annelida, and the Meduse, though 
many of these are provided with special swimming organs, are 


280 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


perfectly incapable. of swimming against the feeblest stream. 
The only invertebrate animals which are able to overcome 
perhaps the strongest currents are the Cuttle fishes. 

The well-known wealth of forms in the Mediterranean and 
in the Red Sea owes its origin, certainly in great part, to the 
action of the constant marine currents. Both these seas are 
connected with the ocean only by narrow straits through 
which a superficial current incessantly flows in. The strength 
of these currents may vary with the time of year and the direc- 
tion of the prevailing winds, but their direction is invariable the 
whole year through. Hence all the animals drifting on or just 
below the surface, when once they have been carried in through 
these narrow straits, cannot easily get back to the open ocean, 
and so all the forms that never sink below a certain inconsider- 
able depth must remain in the inland sea, and only those few 
species or individuals which reach the deeper return current 
and do not leave it can be in a position to be borne back by 
it to the ocean. Consequently both these seas, by reason of the 
inflowing surface currents, are a sort of trap; everything can 
get in, but nothing can get out again; thus it is inevitable— 
and it is actually the case—that a vast accumulation of species 
as well as of individuals occurs in these seas, wherever the 
other necessary conditions for the existence of the individual 
forms exist. 

Another result which may be directly referred to marine 
currents —in combination, of course, with other influences—is the 
wide distribution of such marine animals as have free-swimming 
larvee. If, for instance, we compare the marine mollusea of the 
Red Sea with those of the Philippines, their extensive resem- 
blanece strikes us at once; nay, even those of the Western 
Pacific closely agree with them both in several respects. In 
strong contrast with this state of things is the fact that the land 
mollusca of Eastern Africa, the Moluccas, the Philippines, and 
western islands of the Pacific, exhibit scarcely a single genus 
which is common to them all, and the species—irrespective of 
a few unimportant forms probably introduced by man—are 
totally different in all these provinces. 

This resemblance between the fauna of extensive marine 


FACILITATAS INTER-BREEDING. 281 


districts proves at once that the constant currents acting in 
these regions have prevented the separation of the varieties that 
probably oceur, into distinct species; for we know from Dar- 
win, that a new species can never be in a position to maintain 
itself unmixed, if the individuals which have been modified by~ 
some influence are not prevented from breeding in again with 
the parent stock. But the currents which keep the Indian 
Ocean and Red Sea in communication with the Pacific Ocean, 
being constant, can just as well transport the parent forms as 
the varieties, and consequently a free crossing between all the 
forms derived from one stock is not only facilitated but rendered 


Fig, 72,—Onchidium tonganum, natural size. 


inevitable. The Crustacea and Fishes of the Red Sea correspond 
very exactly with those of the Indian Ocean and Philippine Sea; 
more than half the genera and a large number of the species 
are actually identical in these two very widely remote regions. 
Even species which have accommodated themselves to a quite 
peculiar mode of life are sometimes dispersed throughout these 
regions; thus, for instance, the C'ryptochirus of the Red Sea 
appears to be in no respect specifically different from that of the 
Philippines, and the Hapalocarcinus of the Pacific, deseribed by 
Stimpson, is found, it would seem identical in species, at the 
Isle of Bourbon not far from Africa. Onchidium v2rrucula- 


282 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


tum, a land molluse living ou the seashore, and having a very 
peculiar mode of life and anatomical structure, is found absolutely 
identical in the Red Sea, the Philippines, and North Australia, 
and on the Chinese and Japanese coasts. Another species of the 
same genus, Onchidiwm tonganum (see fig. 72), is found alike in 
the Tonga Islands and the Mauritius. Different species of the 
genera Aurtcula and Scarabus, belonging to the Pulmonata, of 
which the larve have an operculum and probably float in the 
sea, have an equally wide distribution. All these, and many 
other species which cannot be mentioned here, are easily trans- 
ported by currents, either in the larva state or fully grown, and 
the general similarity I have indicated of the marine fauna, 
from the Red Sea as far as the western half of the Pacific, must 
be the result of a diffusion of species effected by constant 
marine currents. 

The influence of such currents is even more conspicuous in 
the distribution of many land-animals, although at first sight it 
might seem paradoxical to say that the distribution of animals 
living on land can be affected by currents in the sea. The evi- 
dence, however, is easily produced. If we reflect, to begin with, 
that all Land mollusca, the great majority of Insects, and many 
Reptiles and Birds, being vegetable-feeders, are directly depen- 
dent for their existence on certain species of plants, it is evident 
that their presence in particular spots, as well as their geogra- 
phical distribution, depends in the most direct manner on the 
action of currents, since these it is which principally determine 
the distribution of the plants on which they feed. Granting 
that a monophagous insect were by any means transported to 
an island where it did not find the only plant adapted for its 
suppert and to which that plant never could be brought even by 
currents, that species must inevitably perish. But there is yet 
another way in which currents may affect the distribution of 
animals, besides this indirect mode by means of the plants they 
feed on. Many Insects are easily transported as larvee, in and on 
floated trees and wood; smaJ] Mammals, no doubt, are also car- 
ried in this way from one place to another ; the same is the case 
with many Reptiles and Amphibia, and it is more than probable 
that land-snails can trav 1 only in this way, and in no other, 


TRANSITION OF SPECIES. 283 


from one island to another. We may therefore expect to 
find on islands in the neighbourhood of a continent a terres- 
trial fauna closely resembling that of the continent, if the marine 
currents are such as to favour transport from the mainland to 
the island. Examples abound: the Galapagos have a South 
American fauna, the Sandwich Islands a North American one. 
Moreover, the terrestrial fauna of an island may be expected to 
be very dissimilar from that of the nearest mainland when it 
does not lie within the influence of the currents flowing from it. 
Of this too there are plenty of examples; the best known is the 
case of the Canary Islands; their land mollusca have a pro- 
nounced European character, although by their geographical 
position they belong rather to Africa than to Europe. Attempts 
have been made to explain this fact by an assumption that there 
was formerly a connection between these islands and Europe 
through Spain ; an assumption which might certainly find much 
to support it, if animals so large as to be incapable of being 
transported by winds or currents, had been found in these 
islands, either living or fossil. This, however, is not the case, and 
it appears to me that the force and direction of the currents 
sweeping past these islands amply suffice to explain the Euro- 
pean character of the land mollusca and insects of the Canaries. 

Supposing that a long chain of islands had connected twolands 
lying far apart and differing widely in their fauna, it might be 
expected, and with great probability, that the fauna of this group 
could have retained no special homogeneous character. For 
the vicinity of the two terminal countries, and the currents pro- 
bably existing, might easily have caused on the islands a mixture 
of the two dissimilar faunas. This is, in fact, sometimes the case. 
The Philippines lie very nearly north and south ; the northern 
islands are connected with China by the Bashees and Formosa, 
while the southernmost island, Mindanao, is connected by 
Celebes and some smaller islands with the Moluccas, and the 
south-western island, Palawan, hangs on to Born:o by Balabac. 
Wallace certainly includes the Philippines with China; the 
Malayan peninsula, Sumatra, Java, and Borneo, with some 
smaller islands, constitute his ‘Malayan province,’ which he 
contrasts strongly with that of New Guinea and Australia, to 


284 THE INVLUENCE OF INANIMATE SURROUNDINGS. 


which he adds the Moluccas and Celebes. But quite irrespective 
of the question thus raised, which I shall discuss presently, 
the different portions of this Malayan province exhibit great 
and extraordinary differences. A greater contrast can hardly 
be conceived of, than that, for instance, between the fauna of 
Hong Kong, Amoy, or even Siam, on one side, and Borneo, 
Java, and Sumatra, on the other. And this difference is 
repeated in a very striking manner in the Philippines, where the 
northern district displays an unmistakable harmony with the 
true Chinese fauna, while the southern islands show a marked 
resemblance partly to Borneo, partly to Celebes and Gilolo, and 
parily to the western islands of the Australian region. 


Fic. 73,—Shells of Molluscs from the Philippines. a, Cochlostyla stabilis, Sow. ; 6, Chloreea 
n. sp. 3 ¢, Chlorcea benguetensis, 8.; @, Cochlostyla magtanensis, 8. 

As these remarkable facts are probably not universally 
known, I will here give rather fuller details as to the more im- 
portant of them. 

The most prominent feature of the fauna of the Philippines is 
beyond a doubt its terrestrial mollusca.!°’ Setting aside the 
minuter* forms for the present, the following five genera are 
those which give this fauna its peculiar character: Cochlostyla, 
(see fig. 73,a@ and d), Obbina, Chlorea, Helicarion, and Rhysota. 
They are here extremely rich in species, while on the adjacent 
islands, not belonging to the Philippine group, only a few quite 


THE PHILIPPINUYS AN INSTANCE. 285 


distinct species occur which can be included in these genera. 
Thus three species found in Borneo belong to the genus Cochlo- 
styla (antigua, sulcocincta, and axunthostoma). Chlorea (see 
fig. 73, 6 c) is entirely confined to the Philippines, and is not 
found even in Mindanao; Helicarion also isa purely Philippine 
genus, and its nearest allies even do not occur in India, as might 
be expected from Wallace, but in the Australian region; of the 
genus Obbina four species belong to the Australian province 
and one to Borneo; J?hysota is confined to the Philippines. 
Associated with these peculiar forms are other genera which 
are not characteristic of these islands, and which have a very 
remarkable geographical distribution. Closely related to Chlo- 
reea—in anatomical structure, though not in the appearance of 


Fic. 74.—Amphidromus maculiferus, Sow. 


the shell—is the widely diffused group of Helix similaris, which 
is very common throughout the China region; three or four 
species occur in the north of Luzon, and only one extends as far 
south as Bohol ; none at all are found in Mindanao. The only 
Philippine species of the genus Clausilia inhabits the island of 
Camiguin to the north of Luzon; it belongs to a Chinese divi- 
sion of this widely distributed and highly variable family. 
Chiorea itself is most extensively developed in the north of 
Luzon. Thus these three genera give to the fauna of the north- 
ern Philippines a sprinkling, as I may say, of the Chinese. 

At the extreme south, on the other hand, there is a certain 
harmony with the fauna of the Moluccas and the Malayan 
peninsula, For instance, the genus Amphidromus, which is so 
characteristic of the Malayan province (see fig. 74), sends only 


286 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


two species to Mindanao, of which one (A. maculiferus) extends 
as far as Bohol and the south coast of Leyte, while the other (A. 
chJoris) is found only on the south-west point of Mindanao. 
Three species of the genus Xesta, which is at least equally cha- 
racteristic of the Malayan islands and even of India itself, 
as the two species of Amphidromus above mentioned, have the 
same distribution ; two of them have hitherto only been found 
in Mindanao (.Y. Antonii and nobilis), the third (X. Cuming?) 
lives in Mindanao, Camiguin de Mindanao, and Bohol; and since, 
moreover, the number of typical Philippine species is very 
inconsiderable in Mindanao compared with the others, the 
remarkable encroachment of Indian forms on the south Philip- 
pine province is all the more conspicuous. 

Hence, having regard to these differences, the Philippines 
may be divided into three regions: I. The northern, which ex- 
hibits in some degree the Chinese character. IJ. The south- 
ern, which has Malayan or Australian affinities. ITI. The 
Median, which may be called typically Philippine, since it 
has searecly any admixture of foreign elements. This result 
is confirmed when we come to consider the other animals in the 
islands. The stag of the Philippines has, so fur as I know, 
hitherto been found only in the north of the archipelago, and 
its nearest allies are in China. Galeopithecus and Tarsius, 
both highly characteristic of the Indian Islands, only occur 
in the southern Philippines, and never in Luzon ; an apparently 
new species of the Malayan genus Cladobates is found only 
in Mindanao. One single species of Barbus, a fish, I found 
only in Mindanao; this genus is represented in the Malayan 
islands by very numerous species; and the singular Platyptera 
aspro, of the family of Gobioide, in the same way occurs 
only in the south of the Philippines. Among reptiles, Ambly- 
cephalus boa, Dipsas dendrophila, Tropidophorus Grayt, and 
Ptychozoon homalocephalum, all living exclusively in Min- 
danao, belong to the purely Indian fauna. The same mixture 
of autochthonous species with Indian species in the south, and 
Chinese in the north, is displayed by the butterflies, as my brother, 
George Semper, informs me. from studying my collection of 
insects. 


TRANSPORTATION OF LAND-SNAILS. 287 


This mixture of the true Philippine fauna with the two 
foreign elements at the opposite ends of this chain of islands can, 
as it seems to me, be satisfactorily accounted for only by the 
assumption that the marine currents, which in those seas are 
dependent on the monsoons—or at any rate are greatly influenced 
by them—have been the essential means of transporting animal 
forms.!°9 For animals from China could by their aid reach only 
the most northern islands, while those from the Indian and 
Australian islands must first reach Palawan and Mindanao. 
This, as I have shown, is precisely the case; and the almost 
entire absence of such forms, which would easily betray their 
foreign origin, from the middle region of the group confirms the 
idea that in the north immigration has taken place from the 
west and in the south from still farther south. 

The general conclusion to be drawn from this—i.e., that 
the constant currents combined with the changes of the mon- 
soons have been the principal agents in the peculiar distribution 
of many of the land mollusca of the Philippines—is still further 
strengthened by the following considerations. If land-snails in 
general can be conveyed from one island to another at all, it 
certainly can only be by currents, and presumably by the inter- 
vention of trees on which they are borne, and their eggs may be 
transported in the same manner. But it is self-evident that in 
this way a selection will be effected between the different forms, 
according as they are qualified to endure an ocean voyage or not. 
Large species and such as live in the highest branches of trees 
and lay their eggs there, like all the species of Cochlostyla, 
are obviously far more difficult to transport than small species 
which can creep into the rifts in trees or between the roots ; the 
species of a group which, like Helix stmilaris, live on the ground 
among stones and earth, will be almost as well protected on the 
sea-voyage as the operculated snails which have the mouth of the 
shell closed by a lid or shield, which protects the soft part of the 
creature almost perfectly from contact with the salt water. 
Hence we may expect to find in islands a greater multitude of 
different species in those genera which are most easily trans- 
portable. We know that the constant introduction of the parent 
forms in any numbers into a new colony will prevent the 


288 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


formation of a new species in that spot. But the difficulty 
of transport across the sea must, on the contrary, probably 
prevent the frequent importation of new individuals of the 
parent form, and consequently the formation of a new species 
will be facilitated by the impediment thus offered to free cross- 
ing with the original form. 

These inferences from the view that constant currents con- 
stitute an important auxiliary in the diffusion of land mollusca 
are, as every conchologist knows, in perfect accordance with the 
facts. Most of the small species and of the operculated species 
have a much wider range than the large inoperculated forms. 
While the typical Philippine genera Cocshlostyla, Rhysota, 
Helicarion, Chlorwa, and Obbina, which principally dwell in 
trees, are found only, or almost only, in the Philippines, the 
small genera, as Subulina, Trochomorpha, and Ennea, among the 
Helicide, and the Operculata C'yclophorus, Alyceus, Helicina, 


Pe 


€ ot > 


Pic. 75.—Trochomorpha eq. 


and Diplomnatina, have a very wide distribution, and at the 
same time a remarkable uniformity of species. Thus Ennea 
bicolor ranges from India to the Pacific Ocean, where I myself 
found it in the Pelew Islands; the same species of Helicina, 
Pupina, and Leptopoma occur in almost all the islands of 
the Philippine Archipelago, while, notwithstanding the great 
number of species in the genus Cochlostyla, no two identical 
forms are to be found in Mindanao and Luzon. The species of 
the genus Zrochomorpha are extremely similar in appearance, 
whether they come from India, the Moluccas, the Philippines, 
or the islands of the Pacific; nay, several species of this genus 
are distributed throughout this vast region, almost without any 
variation in their shells. 

In the closest connection with these facts is the theory, 
which, under the name of the Migration theory—or, as it isnow 
called, the Separation theory—has heen propounded by its origi- 
nator Moritz Wagner in opposition to Darwin's theory of selec- 


DISPERSAL OF INFUSORIA. 289 


tion. Its principal propositions are as follows: The struggle for 
existence and the selection it gives rise to cannot by itself lead 
to the formation of new species; this can only take place when, 
in the first place, one or a few specimens of a species are intro- 
duced at a variable stage into a new home; when, secondly, 
these by their removal to a distance are prevented from cross- 
breeding with typical individuals of the parent stock ; and when, 
thirdly, a selection is effected by the external conditions of 
existence among the varieties arising in the new colony.!!° 

Setting aside, for the moment, the question as to whether 
this theory is really opposed to Darwin’s or no, we will in the 
first place inquire whether in its strict application it is actually 
capable of explaining all the facts occurring in nature. And I 
must at once confess that in this respect I can by no means 
admit Wagner to be right. 

Many kinds of Infusoria are distributed over the whole 
globe in sharply defined species. Now it is quite certain that 
among these creatures no separation by removal exists to pre- 
vent the free interbreeding of any variety with the parent 
form. They are most easily transportable, either living in 
water, or when dry, as dust by the wind, and indeed we see 
that in no other animal group are there species so cosmopolitan 
and so universally distributed as among the Infusoria; never- 
theless the characters of the different species remain very 
constant. Between the tropics there is no sharply demarcated 
breeding period for the greater number of marine animals, so 
that fully grown individuals, young ones, and eggs, can at all 
times be found side by side in every stage of development. 
Moreover the fertilisation of the eggs is effected while they 
are freely swimming in the ocean in the case of almost all 
Echinodermata, all Coelenterata, many worms, most bivalves, 
and many Tunicata, Brachiopoda, and Bryozoa ; and in the few 
viviparous forms of these groups no union of the sexes takes 
place. On the contrary, the fertilisation of the ova is left to 
chance ; the spermatozoa or male element being expelled into 
the sea and conveyed by currents which bring them into contact 
with the eggs or with the female parent organism. In these 
and algo in all those marine species which have free-swimming 


290 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


larvee, absolute separation of all new varieties from the parent 
species is thus rendered impossible. Nevertheless, these forms 
have specific peculiarities as distinctly marked as those of 
insects, vertebrata, and land mollusca, the only animals which 
Wagner takes into consideration in his investigations. Accord- 
ing to his theory, on the contrary, all such species, whose free 
crossing with the parent form is not prevented by separation, 
should remain very variable, and should not be distinguishable 
into any great number of well-defined species, But, this not 
being the case with the creatures above mentioned, it follows 
that separation by distance cannot be, as Wagner asserts, the 
one exclusive cause of the origin of new species. 

It is admitted, of course, that Wagner in his argument 
recognises the influence of the external conditions of existence 
and duly allows for them; but since these occasionally act as a 
selective power—as in the case of many lower marine animals— 
without the co-operation of a contemporaneous separation of the 
varieties from the parent form, this last circumstance can never 
be the sole cause of the process of forming a species, though it 
may sometimes bear a principal part init. The first question 
is thus answered. 

The second question is: Is it indeed the fact that Wagner's 
separation theory differs so totally from Darwin’s theory of 
selection that each completely excludes the other, as Wagner 
seems to think? It does not appear so to me. Both assume 
that different species are more or less variable ; both assert that 
free crossing with the parent form must be prevented if a new 
species with constant characters is to be developed ; they agree 
in believing that a selection also must be effected among those 
variable forms in order to induce the constancy of specific 
characters and to increase the useful ones by accumulation. I 
can detect only two trifling differences in their respective views. 
Wagner appears to think that physical separation or removal, 
which certainly is a very frequent result of migration, is 
the means exclusively employed by nature to prevent free 
crossing, while Darwin says that this result may often be 
effected by numerous other and very dissimilar causes, as for 
instance by differences in the size of the male and female indi- 


WAGNER’S THEORY. 291 


viduals, by antipathy and sympathy, by incompatible differences 
of structural character, &c. Actual separation, which is often 
but not exclusively the result of migration, may no doubt 
sometimes prove a stronger means of preventing free crossing 
than such physical or structural peculiarities, but it cannot 
possibly be disputed that these, in many cases, certainly suffice to 
effect the same result as, in other cases, is brought about by 
Wagner’s favourite means, local separation. Hence, Wagner’s 
theory lays fur too much stress on migration as a factor, so far 
as regards the indispensable prevention of crossing, and alto- 
gether ignores others which, under some circumstances, are of 
quite equal efficiency. Consequently nrigration must be put in 
the same category with all the other causes which, according to 
Darwin, may interfere to prevent cross-breeding; and so 
Wagener’s theory forms in fact a subsidiary to Darwin’s. 

The second apparent difference between their views seems to 
lie in the method by which the selection between the different 
varieties takes place. Darwin says that it is ‘the struggle for 
existence,’ while Wagner vehemently quarrels with this expres- 
sion and regards the influence of external surroundings as the sole 
efficient means—the influence, that is, of the conditions of exis- 
tence. At the first glance this might appear to be a funda- 
mental difference ; but the difference in the expressions used is 
altogether superficial and may have arisen merely from a mis- 
understanding of the word used by Darwin. Wagner ex 
pressly says, in his latest work, in terms that cannot be misun- 
derstood, that he is of opinion that the words ‘Struggle for 
Existence’ * are used by Darwin to denote exclasively that direct 
combat between two individuals of the same species in their 
efforts to possess themselves of the same prey or of the same 
female.!!!_ This, however, seems to me a quite erroneous inter- 
pretation of Darwin’s expression. For although Darwin him- 
self frequently explains that in his opinion the personal strug- 
gle between two individuals of the same species exerts a far 
greater selective power than the surrounding conditions can 
effect with all their sudden changes, he by no means ignores 


* ‘Not very happily rendered into German,’ says Dr. Semper him- 
self, ‘by the words, Kampf wms Dascin. 


292 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


these influences, and in various places expressly states that they 
may sometimes have had precisely the same results!!2 as 
Natural Selection in its most limited acceptation. In short, if 
I rightly understand Darwin, he applies this expression, not 
exclusively to the struggle or combat between two individuals, 
but conceives of it rather as the sum total of all the efforts 
which a newly constituted species must make to succeed in con- 
quering all the hindrances to its development, and at the same 
time to avail itself to the utmost of every favourable cireum- 
stance that offers. It must certainly be conceded that Darwin 
generally applies the words ‘ Natural Selection’ to those cases 
only of the most direct competition between two animals, of the 
same or of closely allied species. This indeed is the obvious 
inference from the fact that he considers it necessary to contrast 
Natural and Sexual Selection, although the sole difference 
between them properly consists in this: that in the former the 
struggle is for a dead object, in the latter for a living one, 2.¢. 
the female. It may still further be conceded, as indeed Darwin 
himself has admitted, that in the first instance he somewhat 
undervalued the selective influence exerted by the surrounding 
and external conditions of life; but to assert that he wholly 
ignored them is far from the truth. On the contrary, these 
influences constitute an essential part of his theory, though 
Darwin himself assigns them but a small and undoubtedly too 
limited part in it. Still Wagner’s separation theory is not 
thereby opposed to Darwin’s, but, on the contrary, an integral 
part of it; and it is an indisputable fact that the various propo- 
sitions which constitute the ‘separation theory’ had long before, 
if ina different form, been announced in the chapter on the 
geographical distribution of animals in Darwin’s work on the 
Origin of Species. 

But while I must thus, in the most positive manner, dispute 
the idea that Wagner’s theory is in any way essentially opposed 
to those of Darwin, I may on the other hand admit, once more, 
that migration and the separation frequently occasioned by it, 
as well as by currents, may exert a very decisive influence on 
the formation of species. Such an influence is recognisable in 
the fact that such land-snails as are difficult to transport by 


FAUNA OF REMOTE PROVINCES. 293 


currents exhibit a great wealth of different species even in 
adjacent islands, while those species which are easily trans- 
ported have a much wider range, combined with a greater 
constancy of character, than the species of the former group. 

It might perhaps be inferred from the foregoing remarks 
that it is my view that in every case when faunas widely sepa- 
rated by distance exhibit a certain resemblance or affinity of 
species, one and the same influence, i.e. the action of constant 
marine currents, must be regarded as the cause of that correla- 
tion. But I think I hardly need guard against such a mis- 
apprehension ; a brief account of the most interesting cases of 
this kind will suffice for my purpose. 


‘ 


1 


Fig. 76.—Temnocephula chilensis, Blanchard, which lives, absolutely identical in species 
(no difference being perceptible in either the external or anatomical character), in 
Chili, Java, and the Philippines. Parasitical on fresh-water crabs bclonging to quite 
different genera. 

Giinther has shown that the tortoises of the Mauritius ''3 
are very nearly related to those of the Galapagos, which, lying 
near South America, are almost the antipodes of the island in 
the Indian Ocean. The characteristic species of Bulimus (a 
land mollusc) in South America have their nearest allies, not in 
North America nor in the West Indies, but in New Caledonia 
and the Feejee Islands, as I can attest from my own minute 
investigation of such animals. The extinct birds of Madagascar 
show a near relationship to those of New Zealand ; many fresh- 
water fish of New Zealand are identical or very nearly allied 
with those of Chili; Temnocephala chilensis, a small parasite 


294 THE INFLUENCE OF INANIMATE SURROUNDINGS, 


(see fig. 76) on the legs of a fresh-water crab in Chili, occurs 
identical in species in the Philippines and in Java, but on per- 
fectly different crabs. It would be easy to multiply instances, 
but these will suffice, I believe, to show that any attempt to 
explain them by the action of constant marine currents must 
altogether fail. Other causes must here have combined to 
produce so striking a resemblance between the faunas of 
islands lying so far apart ; but it would be difficult to discover 
them in every case. Wallace has justly observed in his great 
work that such cases ought, under the circumstances, to be 
regarded as a proof of the justice of the hypothesis that those 
types which have occasioned the similarity of remote faunas 
must have had a very long historical duration, persisting very 
likely throughout many geological epochs. Also it must not 
be forgotten that the convergence or parallelism of different 
species may sometimes have led to the formation of two simi- 
lar faunas in very remote places in modern times. This, 
however, is not the proper place for a discussion of this inter- 
esting point, and I must refer the reader who is particularly 
interested in it to the brief remarks he will find in the 
Appendix.'!4 

(b) The wind as a means of dispersal—tIt is evident that 
the distribution of all flying creatures, 7.e. the selection of forms 
among them, must in «a great degree depend on the direction 
and strength of atmospheric currents, whether these be regular 
winds or irregular storms; but it is a matter of very great 
difficulty to determine what share each mode of atmospheric 
motion may have, or how they may co-operate. 

Instances of animals being carried by wind-storms far 
beyond the limits of their native province, or even beyond seas, 
are universally known, and it must here suffice to refer the 
reader to the chapter on the Means of Dispersal in Darwin’s 
work, in which a great number of independent examples are 
given. Still we are justified in inquiring whether indeed such 
an accidental transportation of solitary individuals to countries 
where they are merely interlopers, can have often led to the 
acclimatisation of a species in a new country. For it must 
not be forgotten that, independently of the difficulty they will 


CONSPICUOUS EFFECTS OF WINDS. 295 


experience in maintaining themselves under the new conditions 
of existence, their ultimate establishment there must generally 
depend on two individuals of the same species and of different 
sexes being simultaneously, or almost simultaneously, carried to 
the new country. On the other hand, the strength of regular 
winds, such as monsoons, trade winds, &e., would not seem to 
be so great as that flying creatures, such as birds and insects, 
could be carried away by them against their will, nor, indeed, 
with such rapidity as would be requisite to enable them to 
reach some distant destination before they had perished of 
hunger. There are, however, a few cases in which such effects 
of wind are apparently so obvious that they cannot be over- 
looked or disputed. 

he most conspicuous and often-discussed example is afforded 
by the fauna of the islands lying near to, or at no great distance 
from Africa. It has long been known that, as regards their ter- 
restrial fauna, the Azores, Madeira, and the Canary Isles belong 
to Europe, and that even their birds and insects are for the 
most part only specifically distinct from those of Europe. Nay, 
Dohrn has lately shown that even the Cape de Verde Islands, 
which are divided from Africa only by narrow straits, belong, as 
to the greater number of their animals, to the European region, 
although a small admixture of species from the Ethiopian 
region can be pointed out. Wollaston, Murray, and others 
have attempted to explain this remarkable circumstance in the 
following manner. They assume that all these islands were 
formerly connected with Europe by the mythical Atlantis; an 
explanation which escapes all possibility of discussion and 
merely appeals to the greater or less credulity of different in- 
quirers. The explanation offered by Wallace in his latest work 
is founded, on the contrary, on forces of which we can ac- 
curately estimate the efficiency, and it seems to me that his 
views are amply supported by their extreme probability. He 
points out that in this region of the Atlantic, steady winds and 
storms alike blow in the direction which would be required to 
allow of such atmospheric currents having transported European 
animals to these islands. As an indirect argument for the 
correctness of this view he adduces the total absence of all land 


296 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


Mammalia and Reptiles from the Cape de Verde islands, which 
would be incomprehensible if they had formerly been in actual 
connection with the European continent; and as a direct proof 
he adduces the fact that almost all the birds are of European 
species, and that all the European species of insects which are 
found on these islands are strong flyers, while, on the other 
hand, 45 per cent. of the indigenous species of insects cannot 
fly at all, being in fact wingless. He has still further con- 
siderably strengthened his views by an investigation of the 
peculiarities exhibited by the land mollusca of these islands. 
These, as I have already said on the strength of Dohrn’s re- 
searches, bear a typical European character, but not one species 
is identical with a European form. The most important means 
of transport for land Mollusca are, beyond a doubt, marine cur- 
rents ; the possibility of eggs being conveyed by adhering to the 
feet of birds does not here come under consideration. The 
direction of the currents in the Atlantic is, moreover, such that 
the conveyance of European land-snails to these islands might 
easily be possible. But it is evident that constant winds would 
be able to transport a much greater number of individual 
flying creatures within a given time than that of the land mol- 
lusca conveyed by currents. Hence these last would exhibit a 
considerably less variety of species than the former ; for we know 
that the greater facility for free crossing with the parent species 
renders the formation of new species more difficult, while it is 
facilitated when a variable species that has been introduced 
into a new home is by any means prevented from constant in- 
breeding with the parent form. And this is directly applicable 
to these islands ; there is no serious hindrance to the transport 
to them of flying creatures from Europe in great numbers, and 
accordingly we see that the good flyers among the insects of the 
Canary Islands are almost all identical with European species ; 
and it is in perfect agreement with this that the land mollusca 
which are difficult of transport have become differentiated into 
a number of new forms, since the greater difficulty of immigra- 
tion has prevented the crossing of these varieties with new 
individuals of the parent stock. 

This, of course, presupposes, or, rather, it follows from the 


MIGRATIONS OF BIRDS. * 997 


foregoing remarks, that the direction taken by the migrations of 
flying animals is, in a great measure, determined by the direction 
of the winds ; without this, Wallace’s explanation would remain 
as unsatisfactory as the hypothesis of the sunken Atlantis. Such 
effects ought, as we may suppose, to be most easily recognisable in 
the migration of those flyers which of their own free will make — 
long migratory and aeria] journeys; moreover, we might expect 
to find some reliable data in observations made on migratory 
birds. But, strangely enough, all the investigators of the 
phenomena of migration in birds appear to have taken no 
notice of this matter ; my closest researches have failed to find 
any data with regard to it, and the only remark which may 
have some value is that of Von Brehm, that migratory birds al- 
ways fly against the wind. The necessity for this is self-evident ; 
a bird which is driven before even a moderately strong wind 
blowing through its feathers, is prevented flying, and still more 
hindered in steering. Even in the most recent and very 
thorough researches as to the phenomena of the migrations of 
birds by Von Palmén,!'!5 this point is wholly disregarded, and 
though it must be admitted that several of the lines of migration 
laid down by him from many observations can by no means 
be brought into agreement with those of prevailing winds, on 
the other hand the number of cases is not small in which a 
very extensive agreement between the two is conspicuous ; this 
is the case, to cite a single example, in the west of Europe. 

If then the observations at our disposal afford no satis- 
factory information as to the question how far the influence, 
certainly exerted by the wind, affects migratory animals, it 
is still more difficult to trace its effects on aquatic animals, 
notwithstanding that they undoubtedly come under its in- 
fluence. This influence can naturally only be effective in two 
ways, either by flying creatures, like water-birds, carrying 
small animals or eggs on their journeys with them, clinging to 
their feet, or by the wind transporting these aquatic creatures 
directly through the air. Darwin has pointed out the possi- 
bility of the first mode in his discussion on the geographical 
distribution of fresh-water mollusca, particularly with a view to 
explaining the fact, recognised by him, that these animals, 


14 


298 THE INFLUENCE OF INANIMATE SURROUNDINGS 


unlike their congeners living on land, display an extraordinary 
range of identical species. Eggs or young individuals, so he 
argues, might make very long journeys adhering to the webbed 
feet of a migratory duck; and as such journeys must be frequently 
repeated, according to the constant direction of the migrations 
or circuits of the bird, numerous specimens of the same species 
of water-mollusc must traverse the same route. The accumu- 
lation of individuals of the parent species in the same colony 
thus caused would prevent the formation of a new species, since 
the selective influence of the struggle for existence in the new 
conditions of life would be constantly counteracted by the re- 
peated immigration of individuals of the parent form. 

This explanation is satisfactory, and in many cases it cer- 
tainly seems to be the right one, as, for instance, in the case 
of the distribution of the European fresh-water molJusca. Still 
very considerable difficulties stand in the way of its exten- 
sive application. It must, in the first place, be observed 
that the great similarity of the fresh-water mollusca throughout 
the globe, assumed by Darwin, does not exist to such an extent 
as might be supposed from what he says. This contradiction 
on my part requires some explicit verification. The genus 
Unio, for instance, is distributed almost everywhere on the 
face of the globe; it is absent only from a few tropical 
countries, as the Moluccas, the islands of the Pacific, probably 
New Guinea, and others; but the species of Unio are extra- 
ordinarily various; in North America almost every little 
stream has its own peculiar form, and the European, Asiatic, 
and Australian species are widely dissimilar. These differences 
may be even greater and more striking than we now suppose, 
for we have hardly exact knowledge enough of the organic 
characters of more than a few dozen forms to venture to 
pronounce a decisive opinion, while hundreds of species are as 
yet known to us only by their shells. The other genus of 
fresh-water mussels, Anodonta, has an even wider range, for it 
occurs even in those islands where Unio is wanting. But for 
this genus also these remarks hold good. Among the Univalves 
the genera Melania and Paludina (fig. 77) have a very wide 
distribution, and exhibit a considerable resemblance in the 


DISTRIBUTION OF UNIVALVES. 299 


characters of their shells; nevertheless, both have been sub- 
divided into a great number of different genera, and, as has 
been proved by a study of the animals themselves, in many 
cases this has been perfectly justified. But hitherto we do not 
know the anatomy of more than a few dozen of these genera, 
and so it is at least possible, if not probable, that a more exact 
investigation of the animals may demonstrate precisely the re- 
verse of any extensive uniformity in the structure of the species 
placed in these genera. Thus we have no longer the right to 
speak of the extensive distribution of the genera Melania, Palu- 
dina, Anodonta, and Unio, and it is consequently superfluous to 


Fig. 77.—T'wo operculated fresh-water univalves. a, Melania ; b, Paludina. 


seek an explanation of a fact which, though it cannot be shown 
to be false, is as yet ‘ not proven,’ but, on the contrary, cannot be 
brought into harmony with the few facts which are ascertained 
and established. . 

One more difficulty musi here be briefly alluded to. 
According to Darwin’s views it might be expected that all 
easily transportable kinds should show a greater uniformity of 
species than those which are less protected against the perils of 
a long journey. But the reverse is often the case with fresh- 
water univalves. The species of Paludina and Melania have 
an operculum which fits almost exactly into the mouth of the 
shell, so that the animal would seem to be effectually protected ; 


300 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


but the species of Lymnaea, Planorbis, Physa, and Suceinea, 
which also live in fresh water (see fig. 78), have no such pro- 
tection, and the mouth of the shell is remarkably large. 
Research has shown that, as a fact, operculated snails resist 
injurious influences far more successfully than those without an 
operculum. According to this the species of the inoperculated 
fresh-water univalves ought to exhibit a much sharper differen- 
tiation into separate species according to their habitat than the 


Fic. 78.—Various fresh-water snails. «, Luma; b, Succinea; c, Physa; @, Planorbis, 


opergulated forms; but the fact is precisely the contrary. 
Analogous examples of other fresh-water animals could easily 
be adduced. Thus, for instance, it is impossible to explain the 
existence of Temnocephala chilrnsis (see fig. 76) in Chili, the 
Philippines, and Java, by supposing it to have been carried 
thither by birds, for it deposits its eggs in its host; and these 
are creatures much too large to have been carried alive by 
birds across the ocean. 


It would certainly be, I will not say a grateful, but a very 


AFFINITIES OF ASTACIDA. 301 


important task to determine the share which can with any cer- 
tainty be ascribed to the effects of atmospheric agency in the 
transportation of fresh-water animals. At present we cannot 
do this even approximately. The investigation would be un- 
commonly difficult, for, in order to get a clear idea of it, it 
would be necessary to contemplate at the same time the ques- 
tions, first: Whether many nearly-allied forms, or forms which 
to our eye appear as identical, might not have originated in two 
or more distinct localities by what is known as polyphyletic 
descent, and secondly : How old the different forms may be his- 
torically in the development of the animal world. In every 
case we should thus be led to an exact inquiry into the genea- 
logical affinities of the animal. One example will suffice. True 
Astacide or river cray-fish occur in Europe, in America, and 
Australia, while they are absent from the intervening countries 
and islands. Now, it would certainly be more than bold to 
derive either of these groups directly from one of the others by 
any theory of transportation throngh the air on the feet of 
water-birds ; consequently the question at once arises, whether 
we here have an instance of polyphyletic descent or not. Now, 
so far as is known, there is no Crustacean living on land or in 
fresh water, in either of the three continents, which can be 
regarded as the parent stock of the Astacide living there. But 
in the different Oceans we do indeed find crustaceans—as, for 
example, the species of Paranephrops—which have been con- 
sidered as the nearest allies of the river crustaceans ; we will 
not here discuss whether with justice or no. Now, if these 
different marine Astacide had gone through the same migra- 
tions into rivers or on land, independently of each other in the 
three continents, and had passed through analogous modifications 
corresponding to those migrations, the extraordinary resem- 
blance of the river Astacidee at such wide distances from each 
other would be satisfactorily explained. But this method of 
explanation obviously presupposes that our views as to the 
essential affinity of the animals in question must be in fact per- 
fectly accurate.}!6 

Though, in the instances we have thus far been considering, 
it has been difficult, or even almost impossible, to point out the 


302 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


effects of wind on the migrations of fresh-water animals, there 
are other cases in which they can be recognised with the greatest 
ease. We know that our atmosphere is densely full of the 
desiccated germs of minute organisms which are most easily 
raised and borne hy the wind, but which fall to the ground as 
goon as the air is still again. We have learned from the highly 
important experiments made by Tyndall on lower organisms 
and their distribution, that the only unfailing method of freeing 
the air of such microscopic elements is absolute stillness. Thus, 
if it were possible to trace with any certainty this sediment of 
the atmosphere, so to speak, we should be in a position to deter- 
mine the direction which the different animals occurring in it 
must have taken through the air. 

But two conditions must be fulfilled in order that the distri- 
bution of animals may thus be effected: In the first place, the 


Fic. 79.—a, An Ameeba in its plastic state, with small powers of resistance ; 6, the same 
encyated, i.e, enclosed in an envelope which protects it against injurious influences. 


force of the air in motion must suffice to raise the organisms 
high up ; and, secondly, the organisms themselves must he capable 
of endming the associated desiccation. These conditions are in 
fact fulfilled, but only with microscopic animals and the eggs of 
minute Invertebrata. All Infusoria, for instance, have the power 
of enclosing their soft bodies in a firm envelope, the cyst (see 
fig. 79); this they do regularly before reproduction or when- 
ever the external conditions are too unfavourable. In this 
encystcd state they are able to endure desiccation without any 
injury to their vitality, and, what is more, they can lie dry for 
years—how long is not known—and then, after tens or perhaps 
even thousands of years, revive toa new life. In this state, being 
extremely light, they are naturally easy to transport, and it will 
therefore not surprise the reader to hear that Ehrenberg was 
able to detect, in dust collected in Germany at certain seasons, 


ABSENCE OF DATA. 303 


minute organisms of this class which demonstrably belong to the 
West Indian fauna. The only possible explanation of this fact 
is the assumption that these organisms were borne to us by the 
higher stratum of the returning trade-wind and deposited in 
Europe, where the trade-wind gradually sinks. In the same 
way all those higher organisms are capable of being conveyed 
through the air which I spoke of in a former chapter as being 
able to endure long periods of desiccation: the Tardigrada, the 
Rotatoria, and the eggs of various small Crustacea and Worms. 

If, in fact, the wind in this way fulfils the function 
of distributing such organisms, a]l such passively migratory 
creatures must exhibit a very wide range ; or else—which is the 
same thing—a great uniformity must prevail in the fauna of 
different countries as regards these forms, since the extreme 
facilities afforded to the migrated individuals for constant cross- 
ing with those of the parent species which are subsequently 
introduced, will easily prevent the rapid formation of new species 
in the new locality. The facts, so far as they are known, agree 
to a certain extent with these hypotheses ; but it is impossible 
at present to venture to offer any decided opinion in the matter ; 
the gaps are still too great in our knowledge of the distribution of 
these animals, the only creatures which for the moment concern 
us. We, as zoologists, may perhaps be blamed for this, since it 
is our duty to collect the observations bearing on the question ; 
but such a reproach does not touch us very deeply. Each science 
must determine its own course without regard to any collateral 
outside interest, and it may even occur that important questions 
should be for the time set aside from absolute necessity, if, 
within the province of the special science to which they apper- 
tain, no key as yet exists to their solution. And this is at 
present, or has hitherto been, in a conspicuous degree, the case 
with the point under discussion. So long as the vitality of our 
museums is kept up by the constant supply, year after year, of 
thousands of new butterflies and other insects imported from 
the tropics, so long as they can interest the public by the fact 
that so many new fishes or birds, bats or snakes, are described 
in them, so long, naturally, travelling naturalists will pay little 
heed to the search for those inconspicuous animals which are 


304 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


alone of any importance to the question here under discussion. 
I myself, having been one of those very travellers, must own 
myself guilty of such an oversight ; but if I nevertheless may 
venture here to avail myself of the few incidental observations I 
have made, they allow me to come to the following conclusion, 
the same that we are led to by Schmarda’s observations—that, 
in fact, in by far the greater number of the Infusoria, Rotatoria, 
Tardigrades, fresh-water Crustacea, and Worms, the European 
and American species are so extremely alike that they seem 
in many cases to be perhaps even specifically identical. If 
they were suddenly transferred to Europe, they would scarcely 
alter the character of the fauna of our lakes and rivers in any 
degree. 

I say scarcely, intentionally and with due consideration ; for 
a few exceptions, at present unfortunately too little known, 


Fic. 80.--Cupris sp., from the Philippines. 


seriously disturb this uniformity; or else forms are entirely 
wanting in other countries which, so far as our present ex- 
perience goes, belong to the characteristic fauna of the fresh 
waters of Europe. To this latter category belong the crustacea 
included inthe group of Phyllopoda, which, wherever they 
occur, live almost, or quite, exclusively in pools or sloughs. 
Apus (see fig. 33) and Branchipus are the most familiar of the 
European forms of this family. Quite similar species occur in 
North America, Australia, the Feejee Islands, and Africa; but 
for seven years I vainly endeavoured to discover any species 
whatever of this group in the Philippines; they are equally 
absent from the Pelew islands, and it would seem that they do 
not occur in the Malayan Archipelago. But the Daphnide and 
Cypride, which are associated with them in Europe and America, 
are nowhere wanting. I have found them wherever I have 
sought for them; nay, indeed, species which were deceptively 


ANOMALIES OF DISTRIBUTION. 305 


like our own. Thus the question arises as to the cause of this 
absence of Phyllopoda in countries where other forms can exist 
which live under the same conditions, since both belong to the 
animal group whose distribution on the globe seems to be caused 
essentially by the action of winds. We know that all these forms 
produce eggs which can be dried without losing their power of 
development, and that many actually require to have been dried 
before the young can develope and escape. These eggs too are 
minute and light, and certainly can be borne as dust before the 
wind. Why then are Apus and Branchipus absent in tropical 
localities where the other crustacea nevertheless occur? This 
striking anomaly is, however, as is easily shown, more apparent 
than real. The establishment of such forms depends not merely 
on the practicability of their germs being universally distributed 
in the manner above indicated, but also on the favourable 
accessories in the new conditions in which they find themselves. 
Hence it would be very possible that the eggs of Apus and 
Branchipus might reach the same tropical lands as those of the 
other crustacea, but not find there the circumstances that would 
favour their development. What the hindering causes may be 
it is difficult to say, for we are now only at the very threshold of 
our knowledge of the vital conditions of these creatures; but if 
it were allowable to generalise from Brauer’s elegant experiments 
we might say that perhaps it is the absence of winter-cold 
botween the tropics which constitutes this hindrance, for he has 
shown that the eggs of several Phyllopoda, at any rate, develope 
most rapidly, or perhaps only, when they have previously been 
exposed to a very low temperature, nearly down to the freezing 
point. 

The very general uniformity of the lower forms of fresh- 
water animals is, in the second place, interrupted by the occur- 
rence, among numerous species of typical European character, of 
isolated forms which appear perfectly foreign among their asso- 
ciates. Thus, among many Rotatoria in the Philippines which 

can hardly be specifically distinguished from the European 
species, there are a few quite divergent forms. The most 
remarkable of these is one named by me Zrochosphera cqua- 
torialis (see fig. 81) and which I described as long ago as 1872. 


306 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


I found this genus exclusively in Mindanao. What is the reason 
that it is absent from Luzon and Bohol, where the same external 
conditions of existence would seem to prevailas on the southern 
island? It is impossible to suppose that the eggs have not 
been able to find their way thither; but what the numerous 
causes may be which affect their development, in one place 
favouring and in another preventing it, is not at present known. 

A second example of the same kind is offered by the forms 
of Branchipus. Species of this genus live both in Europe and 
America. The species occurring in the two continents, though 
easy to distinguish, still are so similar that the American 
species might be transferred to Europe and vice versd@ without 
changing anything in the character of the fauna of either coun- 
try. But, associated with them, live a few other very divergent 
forms, particularly the very singular Thammnocephalus, whier. 


Fic. 81.—7rechosphera «quatorialis, a Philippine species (Rotatoria). 


disturbs the uniformity of the American Branchipoda by its 
occurrence in the south of the Union. Here also the causes are 
perfectly unknown which prevent this genus from developing 
fully in the higher latitudes of North America; but we are 
obliged to assume that there are such hindrances, since it is 
difficult otherwise to see why they should not develope in the 
north just as well as the eggs of the other Branchipoda which 
are distributed with great uniformity over the whole continent, 
and which everywhere develope in the same manner. It 
appears to me that Brauer’s researches—so often alluded to— 
if not as yet fully available, contain the germ of future and 
more fertile inquiry in this direction, and it is only to be wished 
that Brauer may not long remiin in sole possession of this field, 
for a combination of forces will in this, as in every case, lead 
sooner and more certainly to the desired result. 


CO-OPERATING INI'LUENCES. 307 


2. Currents and winds as limiting the distribution of 
species.— When we reflect on the mode in which alone winds 
and currents can possibly convey animals from place to 
place, it becomes self-evident that they must very frequently 
act also as hindrances to the distribution of species. Ships, 
drifting ice with the boulders, erratic blocks or drift-wood 
transported by it, trees uprooted from the land, and the leaves 
and dust often carried to great distances by storms—all these 
serve from time to time and with more or less frequency for the 
transport of many kinds of animals. And since storms, winds, 
and currents, in spite of many variations in their courses, are 
still on the whole very constant, it necessarily follows that those 
animals which either do not come within their range, or which 
cannot bear transmission by such means, are excluded from dis- 
tribution by these agents. Elephants cculd never be conveyed 
to any distance on floating trees, as small snails can, or even 
such mammals as live among their branches; wingless land 
birds, like those of New Zealand and Madagascar, are incapable 
of migrating to any distance; but still, in these and all similar 
cases, currents serve as a means of separation only because the 
nature of the animals concerned forbids their availing them- 
selves of them. Thus the action of winds and currents is 
dependent on that of the animals or co-operates with it. 
Even in cases where the currents appear to act quite indepen- 
dently, their influence is always dependent on other conditions 
which may be associated with them. Thus, for instance, many 
animals are extremely susceptible to variations of temperature ; 
consequently, if any warm-water animals are borne by a current 
from the region of warm seas into a cold one, they must in all 
probability perish very soon. Here, then, the currents might 
have acted as promoting and aiding distribution, but this 
result was completely neutralised by the contemporaneous 
action of a diminution of warmth. Inthe same way very often 
some animal may be carried by the wind from one island to 
another without any favourable issue ; for its establishment in 
the new locality depends, as we know, not merely on its safe 
arrival there, but also on the creature’s finding in its new home 


308 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


all the other conditions of life favourable for its living and re- 
producing its kind. 

Currents themselves must no less have a dividing ac- 
tion in some cases. It is well known that all floating ob- 
jects, such as drift-wood, leaves, trees, &c., gradually drift to 
the edge of the stream, even though they may have fallen into 
the middle of it. Every navigator is familiar with the pheno- 
mena resulting from this, and knows that the western and 
eastern limits of the Gulf-stream are both indicated by a broad 
band of accumulated sea-weed, wood, leaves, and other objects. 
This tendency of the current to clear itself—or clean itself—is 
stronger in proportion to its rapidity and strength. Hence, 
objects torn by a stream flowing between two islands from the 
one lying to the left of it, could be borne to that on the right 
side only under specially favouring circumstances ; and vice versd, 
those brought from the right could never, or very rarely, be 
carried to the opposite side. Thusa mixture of the faunas of the 
two islands might be hindered, or at any rate rendered extremely 
difficult, simply by the action of the current flowing between 
them. Only those free-swimming animals which might be 
able to overcome the mechanical resistance of the current to 
which they would be exposed in their attempt to cross it, would 
be in a position to escape its influence. That this action of the 
current is theoretically inevitable cannot be disputed ; still, 
the question may of course be raised as to whether actually it 
often comes into play. 

Certain phenomena attending the distribution or migration 
of animals do in fact leave no room for doubt that this dividing 
action may often be detected, above all in marine currents. We 
have already met with a few examples in previous sections. 
When we were considering the striking circumstance that the 
islands lying close to Africa have a quite different fauna from 
that of the neighbouring continent, we mentioned this as a factor ; 
for that fact was intelligible only on these grounds, and we 
pointed out, on the one hand, that the stream flowing from 
Europe, on the north, was, from the course it takes, able to in- 
troduce a quantity of European forms into these islands, while, 
on the otber any species of animals carried off from the African 


DIVIDING CURRENTS. 309 


shore must be deprived of every chance of reaching these islands, 
since a dividing current flowed between them. 1 have further 
remarked above, how sharp a contrast is defined between the 
marine fauna east and west of the meridian of the Cape of Good 
Hope; to the east, the animals of the Indian Ocean brought 
down by the Mozambique current, and a multitude of beautiful 
forms are abundant; to the west, there is the greatest poverty 
of animal life and a quite different set of species. No mixture 
of the two occurs, as it would seem; and this is confirmed by the 
statement—never, so far as I know, contradicted—that the whale 
of the Atlantic never crosses the meridian of the Cape, although 
it is certainly one of the strongest swimmers of the deep. I 
have before shown how difficult it must be for the larger land 
molluscs to cross arms of the sea, and this is visible even on a small 
scale. To the north of Luzon lies a small group of islands 
known as the Babuyanes. Their land-snails belong on the whole 
to the groups typical of the Philippines; true Cochlostyle 
for the most part, but quite different on the eastern and western 
islands. Species occur on the latter which bear a remarkable 
resemblance to those of the west coast of Luzon or are quite iden- 
tical, while on the former only such are found as are especially 
characteristic of the eastern side of that island. This may in 
part result from the faci that the vegetation of the eastern and 
western Babuyanes seems to be tolerably dissimilar, but this 
would not remove the influence of the currents flowing from 
the east and west of Luzon—an influence which is plainly 
discernible—it would only make it indirect. For on Luzon, too, 
the same difference is perceptible in the vegetation of the eastern 
and western portions ; in the east, forests without limit; in the 
west, cultivated land and pasture. It would be easy to cite a 
great number of similar cases in which the dividing action of 
marine currents would be more or less discernible, but it must 
suffice here to discuss in detail another of the more interesting 
examples, 

The difference between the fauna of the islands of the 
Malayan province and that of New Guinea and New Holland 
long since attracted the attention of naturalists. Schmarda 
defined the Australian region as in direct contrast to the Indian. 


310 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


Certainly he includes in the Indian the Sunda islands, which 
Wallace, who first attempted to explain this contrast, placed in 
the Australian region. The limit-line, which, according to 
Wallace, sharply divides these two regions, runs between the 
two islands of Bali and Lombok, close as these two lie to each 
other. He reckons the fauna of Bali with that of Java, while 
that of Lombok is said to he completely ditferent, and to belong 
to the Moluccas. From thence the limit between these two 
provinces runs somewhat to the north-east, between Borneo, 
which still belongs to the Indian, and Celebes; it then turns 
abruptly to the east; thus all the Philippine islands are thrown 
into the Indian region, while a few of the smaller groups, form- 
ing a connection between Mindanao and Gilolo and New 
Guinea, lie south of this limit-line and are thus included in the 
great Australian region. The line thus laid down has been 
designated as Wallace’s line, in honour of its founder. 

It cannot be disputed that this line seems, in fact, a very 
natural one, if only the birds and mammalia are taken into con- 
sideration and the insects not brought into the comparison. In 
the Australian region the Marsupials, birds of Paxadise, Mono- 
tremata, lyre-birds, cockatoos, cassowaries, and the very peculiar 
Trichoglosside ; in the Indian region, on the other hand, the 
apes, lemurs and flying squirrels, Ga/eopithecus, and many other 
Mamualia which are absent from the Australian region. Among 
birds, the Argus pheasant, the peacock and Huplocamus, the 
various pigeons, and of parrots the Loriculus and Paleornis, 
with many others, never occur in the Australian region. 
We must not, however, leave out of the question the fact that 
many of these forms, or of others equally characteristic, not 
rarely pass across into the neighbouring region, where the two 
come into contact. Wallace himself points this out. But the 
contrast is much less sharply defined in the Reptiles, Amphibia, 
and even the Insects; thus Pascoe, who has the most perfect 
knowledge of the Coleoptera of the eastern hemisphere, says 
that, as regards its beetles, New Guinea most positively belongs 
to the Indian region, and that they are quite clearly distinct 
from the Coleoptera of New Holland. Hence the contrast indi- 
cated is not absolute throughout, and Wallace himself, in his 


WALIACE’S LINE, 311 


bock on the geographical distribution of animals, repeatedly 
points out that many species encroach on the limits of the 
neighbouring province in a very singular and incomprehensible 
manner; and he justly infers that there must be some peculiar 
means of dispersal as yet unknown to us, by which these species 
are enabled to overstep the limits apparently assigned to them 
by nature. Moreover, he speaks of the fauna of the island of 
Celebes, included in the Australian region, as exhibiting so re- 
markable a mixture of animal types that it might just as well 
be included in the Indian region. But irrespective of these 
forms, which prove that Wallace’s line does not indicate an im- 
passable frontier, there yet remains so vast a number of ex- 
tremely different species, peculiar to each of these ragions and 
belonging exclusively to one, that the sharp distinction so long 
recognised between them appears fully justified. 

The question now is whether this distinctive contrast can 
be explained, and more particularly how it happens that two 
islands lying so close to each other as Bali and Lombok should 
by Wallace’s line be placed in two different regions of animal 
distribution. Wallace himself—who, so far as I know, was 
the first to attempt to explain this phenomenon—does so as 
follows :— 

He assumes that at some former period the Indian continent 
and the Indian islands as far as Java, Borneo, and the Philip- 
pines, were connected ; and that, in the same way, Australia 
with New Guinea, the Moluccas, and Celebes, were in connec- 
tion, and that only the group of islands from Timor as far as 
Lombok were perhaps excluded. This last must probably have 
been divided from Java by a deep sea ; and it was not till a later 
period that Bali by the side of Java and, the smaller islands 
as far as Lombok by the side of Timor, were raised from the 
bed of the sea. The differences of the fauna which nevertheless 
occur on the individual islands of each region he endeavours to 
account for by their separation at different periods from the 
Indian or Australian continents. 

Now, it certainly cannot be disputed that very many circum- 
stances in the distribution of the land animals on these islands 
argue in favour of this view; thus, for instance, the fact, un- 


312 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


known to Wallace but discovered by myself, that at a former 
period an elephant!!7 was found on Mindanao, the most 
southerly of the Philippines, can scarcely be explained except by 
the supposition that a direct connection existed between this 
island and the Indian continent, or an indirect one by a junction 
with the larger Malayan islands. For any transportation of 
this species, which is very nearly allied to the dwarf variety of 
Indian elephant, by a passage across the sea isnot to be thought 
of. Nevertheless, I believe that this hypothetical connection of 
the islands and mainland is not sufficient by itself to explain 
even those facts that are already known to us as to the distribu- 
tion of Indian and Australian forms on the islands lying between 
the two continents. Even Wallace himself falls back on a 
number of other causes, and in order not to abandon his general 
principle he suggests a hypothetical history of upheavals and 
subsidences, so numerous and various in the different islands 
that, in the total absence of all geological proof of them, we feel 
ourselves gradually withdrawn from the terra jirma of justifi- 
able speculation and floating in the clouds. It seems to me 
that there is a very general predilection for too readily construct- 
ing sunken continents. Whenever any extensive resemblance 
between the faunas of two distant countries is discovered, or 
even imagined, a bridge of mainland is always freely brought in 
as the only mode of accounting for this resemblance. No doubt 
it is the most convenient of instruments, and all the more easy 
to work with, z.e. to use as evidence for a theory, because it is 
absolutely impossible to prove the fallacy of the hypothesis by 
the method of observation, the only way open to the natu- 
ralist. 

But until the question is finally settled whether two parallel 
series of animal development might not have proceeded inde- 
pendently in two countries remote from each other, we can 
never venture to regard the resemblance of two faunas as con- 
clusive evidence of their primeval actual connection; nay, it 
even seems to me that the two historical series of species of the 
horse, recently discovered both in Europe and America, may on 
the contrary be regarded almost as a proof that each series was 
developed independently in the two continents and yet led to 


TRANSPORTATION OF FOOD-PLANTS. 313 


the same result: namely, the production of the horse. How- 
ever, I leave this an open question; thus much only I think 
may be insisted on: that in such speculations this possibility 
should never be lost sight of, and, at the same time, that all the 
different causes which may have had a share in influencing the 
distribution of animals must be fairly investigated and weighed 
before it is possible to set up any one special method of explana- 
tion as the only correct one to the exclusion of all others. 

At any rate it is perfectly certain that winds and marine 
currents sometimes promote and sometimes hinder the diffusion 
of species, both directly and indirectly. One species may be able 
to cross a pretty strong current at a sharp angle, while another 
may be prevented by a feeble wind or current from reaching an 
island lying very near ; certain kinds of seeds can only be trans- 
ported by the wind, others again only by the sea; where, on an 
island or a group, some particular plant is absent because its 
seeds could not reach it, there of course the animals also will be 
absent which depend on it for food and are monophagous. The 
relations thus occasioned between the faunas of islands in two 
contiguous regions, such as the Indian and Australian, are of 
course extraordinarily various, complicated and difficult to in- 
vestigate ; but this does not justify us in neglecting them. It 
may be more convenient to argue from upheavals and sub- 
sidences which may be imagined in any required number, or 
from hypothetical intervening continents whose former existence 
can be neither proved nor disproved; but the easiest method is 
certainly not always the most accurate—I may almost assert 
that it hardly ever is. But in this particular case it does not 
appear to me to be so exceptionally difficult to detect the rela- 
tive conditions if we do not wilfully shut our eyes to them. 

If we now reflect more particularly on what has been said 
here and in a former chapter as to the mode of action of cur- 
rents, the view is irresistibly forced upon us that it is possible 
that the differences here pointed out in the distribution of 
animals may, without exception, be explained by their agency 
without assuming any former material connection between the 
islands and the nearest continents. Granting that the larger 
number of the Malayan islands and of those in the vicinity of 


314 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


New Guinea did not rise from the sea until quite recent times, 
still the colonising of the islands from the neighbouring con- 
tinents might have taken place in such a way as to involve a 
distribution such as is actually presented to us there. All the 
larger Mammalia, being incapable of overcoming the strong cur- 
rents prevalent there, would have been excluded from immigra- 
tion into the newly formed islands; only the smaller species, 
that cling to trees, could have been carried across seas by 
those currents; and it agrees with this that we find all the 
Marsupials out of Australia, as in New Guinea, the Moluccas, 
and Celebes, belonging-exclusively to the climbing genera. And 
that these should not have succeeded in crossing Wallace’s 
limit-line is the inevitable and very intelligible result of the 
tendency of currents to ‘clean themselves,’ as before described. 
This tendency results from the circumstance that such a current 
is always a little higher in the middle than atthe sides. Hence 
objects floated off by the right margin of a current flowing 
through the straits of Timor or Celebes, or between Bali and 
Lombok, could reach the left shore only under some specially 
favourable circumstances; they would usually remain on the 
same side, particularly when they were passively borne along, 
as would be the case with uprooted trees and so forth, In 
looking at a map on which the currents in question are laid 
down, it is at once seen that the currents flowing from Australia 
and the southern part of New Guinea are suldenly diverted 
from their slightly westerly or quite northerly direction to a 
north-easterly or quite easterly flow, exactly by the very island 
namely Celebes—where the mixture of Indian and Austra- 
lian forms is most conspicuous. ‘Land animals—such as land- 
snails—of which the transportation can only be effected by cur- 
rents must have been subject to the same influence, and it is 
therefore quite intelligible when we find that two islands lying 
so close together as Bali and Lombok exhibit less similarity 
than, for instance, Celeles and Java; for the current that parts 
those two little islands is so strong that it must be quite impos- 
sible for molluscs, or other creatures that avail themselves of 
drifting trees for their voyages, ever to pass from one island to 
the other. On the contrary they might, under certain circum- 


WALLACE’S CURRENT. 315 


stances, easily cross the boundary originally set to their passage 
by the current on the longer voyage to Celebes ; very easily, 
indeed, when, by slight variations in the strength or direction of 
the monsoons and in the surface currents caused by them, a 
temporary change was produced in the direction of the normal 
current flowing between Celebes and Borneo—known as Wallace’s 
current—which is merged in the return current of the Pacific 
Ocean. All those animals, on the other hand, which might 
have other means of transport at their command, would be ren- 
dered independent of the agency of this current, whether in 
separating or in mingling the faunas; but of course only so 
far as they were not monophagous, and thus absolutely depen- 
dent for food on certain plants of which, again, the extension of 
range was subject to the action of the said current. In pursu- 
ance of this mode of viewing the matter we should then have 
to inquire whether those insects and birds which appear to have 
migrated from the Indian region to the Australian, and vice 
versa, way not be polyphagous and easily satisfied with various 
kinds of food; and, on the other hand, whether, as an inevitable 
corollary, those forms which are confined to particular islands 
or districts may not be monophagous or dependent on certain 
forms of food whose extension of range from one island or 
region to another is prevented by the agencies under considera- 
tion. 

This, however, is not the place for pursuing this inquiry in 
detail, nor do we as yet possess sufficient materials for it in the 
form of well-confirmed observations. But so long as the 
general observations we do possess allow of no positive conclu- 
sions, we are, on the other hand, not justified in rejecting any 
possibility as erroneous, and consequently Wallace’s hypo- 
thesis must for the present remain open to discussion ; the 
arguments here laid down in opposition to it are so too, in the 
same degree and for the same reason, and it must be left to the 
future to decide between them. Still, I am of opinion that the 
hypothesis I have put forward may claim the advantage of ap- 
pealing for proof only to such elements as can be brought 
under direct observation, while Wallace’s is intrinsically in- 
capable of demonstration by observation. 


316 THE INFLUENCE OF INANIMATE SURROUNDINGS, 


It follows from all this—as it seems to me—that the action 
of marine currents, as means of separation and amalgamation in 
the distribution of organic life, must be made to bear a larger 
part than it has hitherto done, in inquiries as to the origin 
of the present fauna of the globe from those of former periods. 
For if we conceive of this course of development as a mechanical 
process and make it our purpose to trace those determining 
causes which have been merely mechanically operative, this can 
never be done by propounding a more or less plausible hypo- 
thesis, but only by methodical investigation ; nay, only by the 
method of modern physiology—as much by a due reference to 
all the factors together which must be taken into consideration, 
as by successively identifying the influence which each of them, 
separately, may or must have had. 


CHAPTER X. 


A FEW REMARKS AS TO THE INFLUENCE OF OTHER CONDITIONS 
OF EXISTENCE. 


Besrpus those external conditions of animal life which I have 
treated of in the foregoing chapters, there are others of which 
the effects in certain cases may be of much greater consequence ; 
which may indeed not unfrequently neutralise the effects of 
apparently more important ones, while they may nevertheless 
at present escape any close investigation. Such, for instance, 
are the effects of gravitation or pressure, of electricity, of the 
aggregate condition of the surrounding medium, and many 
others. They are often apparently insignificant as compared 
with temperature, light, nutriment, éc., not because they are of 
themselves unimportant, but only because we know much less 
of their normal effects on the life and growth of animals than 
of the conditions we have hitherto been discussing. Their 
action almost entirely eludes those methods of research that I 
have hitherto employed and which alone I acknowledge as the 
right ones; consequently in the following brief discussion of 
these points I find myself wholly thrown back on hypothetical 
interpretations of such observations as have been incidentally 
made. Nay, the number of these observations is in itself so 
small, that in many cases they do not even suffice as a basis for 
such an hypothesis; and finally it must be acknowledged that 
sometimes the interpretation hitherto offered of certain facts 
has been founded on gross errors which, however, are widely 
diffused and, as it would seem, almost ineradicable. 

The effects of gravitation and pressure.—The selective 
influence of gravitation and its bearings on the organisation of 


318 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


animals are in many cases very conspicuous and intelligible. 
Thus, for instance, it is perfectly evident, and has long been 
acknowledged, that by it a certain standard is fixed for the bulk 
of the animal’s body which cannot be exceeded without en- 
dangering the life of the individual. If, for instance, we grant 
that the structure and specific gravity of any animal are factors 
bearing a relation to each other that does not allow of any con- 
siderable variation, and also admit the possibility of its growing 
beyond the normal standard of size, the animal would finally 
be so large that it could not move its own weight, since its 
gravity must increase in geometrical progression. Of course the 
maximum of height or length attainable by particular animals 
varies with their organisation, and hence must differ in different 
groups of animals, Birds are the most remarkable in this par- 
ticular; in them the standard of bulk generally attainable 
would be remarkably small with a specific gravity the same 
as that of mammals, and their life in the air. But there 
are in their organisation certain adaptations which make the 
maximum bulk they actually attain tolerably high ; these are 
the pneumatic bones and the air-cavities between the muscles 
and in the body, which are sometimes extensively developed, 
particularly in the strongest flyers, as for instance the Albatross. 
By these the bird is enabled to attain a volume of which the 
weight could not long be carried by the most powerful flyer if it 
corresponded to that which any quadruped of the same size 
would have to move. A very interesting illustration of this 
peculiarity is afforded by one of Professor Marsh’s latest dis- 
coveries in America. The wonderfully rich deposits of fossil 
remains in the Rocky Mountains have yielded to his search 
a Reptile which, according to careful estimates, from a restora- 
tion of its hind limbs, must have attained a height of at least 
eighty feet. 

According to the calculation made by a mathematician—a 
friend of Professor Marsh’s—this creature would in that case 
actually have exceeded the maximum size it could have con- 
trolled, under the supposition that in general organisation, and 
therefore in the specific gravity of its body and bones, it 
exhibited no deviation from that of the largest reptiles now 


PRESSURE AT OCEAN DEPTHS. 319 


living, the crocodiles. But in point of fact the specific gravity 
of its bones was less, for they are traversed by very large 
cavities which, Professor Marsh says, have all the appearance of 
having been air-cavities, and after careful investigation he does 
not hesitate to pronounce them decidedly to be such. JI, in 
company with him, examined these bones, though not, I must 
admit, at any great length, and I confess that from their struc- 
ture alone it did not seem to me possible to prove that this 
reptile had actually had pneumatic bones like those of birds. 
It is, however, possible that the calculation, which must have 
been intrinsically one of great difficulty, may have been erro- 
neous, and in that case, in my opinion, the most weighty argu- 
ment for Professor Marsh’s view would disappear. But if in 
fact his friend’s calculation as to the maximum size of a reptile 
having the specific gravity of the crocodile is correct, I believe 
also that the large cavities which undoubtedly exist in the 
bones of that fossil creature can have been nothing else than 
air-cavities, whose function it was to render the animal light 
enough for it to carry the still considerable weight inseparable 
from such an enormous mass. 

The media in which animals live also exert a certain pressure 
depending on their mass and specific gravity, and it is easy 
therefore to imagine that all creatures which either fly in the 
air, swim in water, or creep in mud, must be affected by the 
pressure of the superincumbent mass. This, of itself, is quite 
true; but this true view has, even in quite recent times, often 
led to perfectly false issues. The most striking instance of 
this perverted application of a true idea is offered in the case 
of animals living at great depths, in fresh as well as in salt 
water. It used formerly to be said, and the idea is not uncom- 
monly expressed even at the present day, that it was most won- 
derful that animals generally, and more particularly such deli- 
cate structures as Polypes, many Worms, Univalves, &c., were 
capable of enduring the enormous pressure of the vast volume 
of water above them, amounting in ocean depths to that of 
many atmospheres. But, put in this form, the idea is simply 
absurd ; for the soft-bodied creatures living at the bottom of 
the sea are no more conscious of the pressure above them of a 


320 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


column of water, theoretically estimated as that of many atmo- 
spheres, than we human beings, in the normal condition of our 
bodies, are aware of the weight above us of one atmosphere ; 
and simply for this reason, because the pressure is equal on all 
sides, and because they are themselves permeated by fluids 
which, as is well known, are almost incapable of compression. 
The weight of a high column of water can never affect an 
animal at the bottom, excepting when the animal has cavities 
in its body which are filled either with a fluid of less density 
than the water, or with gases. In the latter case particularly, 
the effects are, as is well known, easy to observe, since gases are 
in a high degree compressible. Divers who plunge into great 
depths—as, for instance, the pearl-divers in the Indian seas— 
or, on the other hand, people who climb mountains to a great 
height, often suffer severely from the difference of pressure in 
the external atmosphere and the tension of the air in their 
lungs, or the pressure in the internal vessels: the divers 
because the increased pressure causes compression of the air in 
the lungs; the climbers because, on the contrary, the heavier 
air in the lungs tends to expand under the reduced atmospheric 
pressure at a great height. When a man has accustomed 
himself to the lighter atmosphere of high mountains—i.e. to the 
smaller pressure—he frequently finds it more healthy and agree- 
able than that of the plain, or at least equally so. Birds, which 
often come down to the plain from the giddiest heights with 
extreme rapidity, must evidently be capable of accommodating 
themselves much more promptly than man to the alteration of 
pressure, since not their lungs merely but their pneumatic bones 
and all the other air-cavities of the body are filled with air. 

On the other hand, there are animals which have cavities 
filled with gases in their body, but which are not capable of 
effecting a change from the compression which must be the 
condition of such gases at considerable depths, so rapidly as 
birds nor even as man; not so rapidly indeed as quick alterna- 
tions in the external pressure would require. This is the case 
with fishes provided with a swimming-bladder. An interesting 
instance is afforded by the little fish of the Lake of Constance 
known as the Kilch. These fish, allied to the Trout family, 


AIR IN THE BODY OF FISHES. 321 


are a favourite article of food. They are caught in nets and 
brought to the surface of the water; they come up invariably 
with the belly much distended; the air in the swimming-blad- 
der, being relieved from the pressure of the column of water, 
has expanded greatly and occasioned this unnatural distension, 
which renders the fish quite incapable of swimming. Under 
these conditions the fish is naturally unable to live for any 
length of time. But the fishermen of the lake have a very 
simple remedy; they prick into the air-bladder with a fine 
needle ; the air escapes with some force, the distension subsides, 
and the fishes are enabled to live under totally changed condi- 
tions as to pressure, even in quite shallow water and at the 
surface, swimming quite as freely as their companions, the 


F13. 82.The Kilch of the Lake of Constance (Coregonus h'emalis), showing the 
distension caused by the expansion of the air in the swimming-bladder. 


natives of the surface water. Hence the Kilch is confined to 
a certain depth, because it is not capable of accommodating the 
tension of its swimming-bladder to the change of pressure in 
the column of superincumbent water. Since, moreover, in the 
Kilch the pressure from within outwards is the same as the ex- 
ternal pressure, or must at any rate be very nearly the same, 
the mechanical problem stated above has no existence for this 
fish, nor for any other creatures living under the same condi- 
tions. It can arise, in fact, only in those uncommon cases—snch 
as would seem to be offered, for instance, by the whale—where 
an animal furnished with internal air-cavities plunges from the 
surface of the sea down to considerable depths and remains 
there for some length of time; in these it is evident that some 


15 


322 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


contrivances must exist which neutralise the ill effects of the 
compression of the contained air—which must undoubtedly take 
place. 

In the instances here adduced, and in other similar ones, of 
the action of gravitation on animals, the effects are obviously 
merely selective ; all the individuals which are not qualified to 
accommodate themselves to the actual conditions of pressure must 
perish or seek a more suitable habitat. But gravitation may 
perhaps have also a direct determining action, perhaps in a 
mode analogous to that by which the growth of the roots of 
plants, or the structure of the underside of leaves, and other 
things may directly depend on gravitation. The theoretical 
possibility of this influence is beyond dispute; but we know 
very little of its actual effects and extent. Nor can there be 
any doubt that animals, in consequence of their greater freedom 
of movement, are in a great degree independent of it; and any 
extensive influence of this kind, such as is undoubtedly mani- 
fested in plants, must be out of the question, except as regards 
sedentary animals, such as corals, sponges, bryozoa, &c, How 
far, in such creatures as these, gravitation may have an effect in 
determining the general form of the colony, or of the individual 
animals and their organs, is perfectly unknown, and it is diffi- 
cult to see by what means it would be possible to ascertain 
experimentally the effects of gravity upon such animals. For 
all those contrivances which have been successfully employed 
on plants, to allow gravitation to exert a perfectly independent 
influence on their growth, cannot be applied to animals, and, 
so far as can be seen, we can only fall back on the interpre- 
tation of those experiments which Nature herself performs on 
growing animals under the normal conditions of their existence. 
It is evident that we can thus only arrive at more or less bold 
ov plausible hypotheses ; for the fact cannot be too often insisted 
on that experiment alone can ever enable us to explain the 
causes lying at the root of any particular phenomenon in the 
development of an animal. All theories deduced only from the 
visible phenomena without the counter-check of experiment 
are mere clever suggestions, which only serve to conceal our 
ignorance, and in fact hinder any advance. Thus, for instance, 


MODIFICATIONS FROM PRESSURE. 323 


it has been asserted that gravitation has an influence in deter- 
mining the direction of the growth of the embryo in Mammalia. 
But it seems to have been forgotten that in most cases of 
viviparous animals the position of the embryo in the uterus is 
by no means constant, but alters in many ways during its deve- 
lopment. If, in fact, gravitation were here of so much impor- 
tance as has frequently been assumed, scarcely any normally 
developed animals would be born; for we know, from the re- 
markably complete experiments of Marcel de Serres, that such 
embryos as normally assume a position of equilibrium for their 
development in the ovum—as, for instance, those of birds—are 
invariably deformed in the most irregular manner, if they are 
constantly moved into other positions and so the original equili- 
brium is disturbed. In the ova of many invertebrate creatures 
the developing embryo floats in a surrounding fluid; so that 
under any inversion of the whole ovary or cluster of eggs all 
the embryos recover the same position, since the centre of 
gravity, lying out of the centre of the body, always sinks to the 
bottom. In such cases as these gravitation evidently acts to 
prevent any disturbance of the equilibrium; but it is very 
questionable whether at the same time any effect is thus pro- 
duced on the process of development of the embryo itself within 
the egg. That the limbs—as our arms and legs—are suhject to 
the conditions of gravitation allows of no doubt, and its effects 
may very possibly tell on the same parts of the body in the de- 
veloping embryo while still in the uterus. But how far this 
influence, which, no doubt, actually exists, may contribute to 
determine the normal formation of animals and their orgaus, is 
perfectly unknown, and cannot be ascertained by any merely 
theoretical discussion. 

Darwin adduces many instances which prove that even the 
bones of the skull may be modified by gravity, or by pressure 
in whatever manner exerted. Burns and repeated convulsions 
in certain muscles have heen known to affect the form of the 
bones of the face; the same effects have been produced when 
human beings during youth have been forced to keep the head 
constantly in a fixed position; it is supposed that in certain 
persons—for instance in shoemakers—who are obliged to keep 


324 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


their head bent down for many hours together, the forehead 
acquires a prominent development of the frontal bones. Darwin 
has shown, moreover, that the forward lop of one ear of the 
long-eared rabbit induces a corresponding forward growth of 
almost every bone of the skull on the same side, so that it is 
perfectly asymmetrical. Nay, even the growing brain appears 
to exercise a decided influence by the pressure it exerts on the 
shape of the surrounding bones. But in all these, and many 
other cases which might be enumerated, the only fact ascertained 
with any certainty is that the growing parts themselves, as well 
as other organs connected with them, may be modified by pres- 
sure and by their own weight; no determined standard for 
estimating this influence is in any instance fixed, and we learn 
from them absolutely nothing as to how far this influence may 
be efficient in determining the production of the normal types 
in animals now living. So far as I know, as yet only a few 
attempts have been made to refer the normal form of the skulls 
of vertebrate animals to the effects of such constant pressure ; 
the most important of these are certainly those of Lucae in his 
researches as to the skulls of mammalia, and those of Gudden in 
his investigations as to the growth of the skull of the rabbit. 
Lucae, however, altogether disregards any experimental treat- 
ment of the question ; and what we learn from Gudden, valuable 
as it may be to physicians, physiologists, and anthropologists, 
is of no present value as affording any methodical standard for 
our inquiry. 

The influence of solid bodies —The aggregate condition of 
solid bodies must decidedly have a certain influence on the 
animals whose life is passed in digging or boring into them. 
The highly sensitive nose of the mole, for instance, must certainly 
exhibit a quite different structure from that of the prairie-dog, 
or Dipus, which uses it, as I myself have seen in tame indivi- 
duals, to beat the earth down firmly in its dwelling. The 
various and extremely dissimilar structures which occur as 
organs for burrowing in both vertebrate and invertebrate 
animals, are so perfect in their adaptation to their function that 
from the structure of the legs, which are the limbs most com- 
monly employed for this purpose, it is easy to infer the mode of 


PRESSURE OF RESISTANCE. 325 


using them; nay, the whole form of the body may be thus 
determined, as in the turnspit, a dog with a propensity for 
digging, or in the cylindrical form of the boring beetles of the 
genus Bostrychus. It is clear that these adaptations of the 
organisation of certain animals to the resistance offered by the 
conditions under which they live must be a powerful means of 
selection by which all the individuals which are not strong 
enough to overcome the obstacles opposed to them must be 
eliminated. And since any variation in the aggregate condi- 
tions of the soil, wood, or stone, in which such creatures live, 
must be either very insignificant or perhaps wholly absent, 
the point of ‘ stable equilibrium’ must soon be reached between 
the strength of the digging organs on one side and that of the 
obstacle to be overcome on the other ; the variations which easily 
may occur in other surrounding conditions—as in the tempera- 
ture, moisture of the atmosphere, salt constituents of the water, 
nutriment, &c.—can in such cases but very rarely be efficient 
causes in effecting any modification in animals exposed to the 
former influence. Nevertheless, such modifications of the 
organs of many animals as were dependent on the aggregate 
conditions of solid bodies must have occurred in the course. of 
their phyletic or generic development; for, if we are to suppose 
that boring or burrowing animals descended originally from 
such as did not bore or burrow, in the process of modifying the 
organs adapted for motion above ground into such as were 
fitted for subterranean progress, organs which were originally 
destined and contrived for walking, running, or swimming, must 
have abandoned these functions to assume new ones, and thus 
have been so far modified in structure that they could be used 
in the best way for the purpose. But we know nothing as to 
how such alterations in the habits of certain animals—urged 
by some inward prompting—may have been able to occasion 
these modifications in the structural characters of the organs in 
question. It is well known that a determined mode of motion, 
or on the other hand the strength of the obstacle to be con- 
quered, may have a certain effect on the strength of the muscles 
called into play, and, through them, on the power of resistance 
in the fulerum joints of the bones, and ultimately on the 


326 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


structure of the skeleton. But between a merely mechanical 
modification of a bone during the lifetime of an individual and 
the differentiation which the leg of some primeval mammal 
must have undergone before it could have given rise to such a 
strikingly peculiar limb as the foot of the mole, there lies a 
great gulf which no experience hitherto attainable enables 
us to bridge over. The most we can say is this: That, beyond 
a doubt, some cause unknown to us must exist—or must have 


a ee 
LD \ Og S02} 
Cod P8ie, 
A (Jan, 


ine. 


Fig, 83.—A piece of wood bored by Limnoria terebrans, from Heligoland. 


existed—in the nature of different animals, which has occasioned 
them sometimes to abandon their original habits or to alter 
them. 


Fig. 84.—A piece of solid limestone bored by Limnoria terebrans, from Ireland. 


The two woodcuts here given illustrate a very striking case 
in point. It has long been known that a small Crustacean, 
Liranoria terebrans, attacks the hardest kinds of wood—like the 
well-known ship-worm, Veredo navalis—and pierces it in all 
directions with its cylindrical galleries (see fig. 83). But it is 
perhaps less well known that the same species attacks solid 
limestone in the same manner. The stone of which the an- 
nexed cut shows a small portion (see fig. 84) IT myself picked up 


ACCOMMODATION TO PRESSURE. 327 


in Ireland, one of thousands, both lerger and smaller, which 
were pierced in the same way by myriads of these small boring 
animals, Unfortunately it was: impossible to examine the 
animal very minutely at the time, and I could only ascertain 
that it certainly was the Limmoria, so familiar to me on the shores 
of Heligoland and elsewhere, which in Ireland had not disdained 
the hardest limestone. Very likely closer investigation might 
reveal certain differences in the different individuals of the same 
species boring in wood and in stone, as to the structure of the 
organs used in boring, and which might show an evident relation 
between those organs and the hardness of the substance bored 
into. It is known, too, that certain species of sea-urchin some- 
times bore into very hard rocks, while in other places they do 
not. In other instances the form of the animal shows the 
direct. effects of contact with solid bodies; this is the case 
with many sedentary animals or those enclosed in a solid shell. 
Many oysters, and such shell-fish as establish themselves on 
rocks or wood or in fissures, frequently adapt their shells very 
exactly to their position, and the form thus given to them might 
easily become constant, and even a fixed specific character, if a 
similar position were adopted by all the individuals of the species. 
But here again, we do rot know how far the determining 
influence of the solid object, which in some measure mou'ds the 
shell, may extend, for no experiments exist which can conclu- 
sively prove that in such cases the moulding power of the posi- 
tion has been exclusively effective with no assistance from 
inheritance, and we therefore are not in a position to assert 
that this explanation of the observed cases, though in itself 
extremely plausible, is in every respect the right one. In order 
to verify this usual and apparently correct interpretation of 
these forms of shells, it would be necessary to prove by experi- 
ment that species which had hitherto lived in some particular 
situation lost the form thus impressed on their shells as soon as 
their young were compelled to establish themselves in a position 
different in character ; and, yet more, that they could be made 
regularly to assume quite different shapes according to the 
differences in their new habitat. Experiments of this kind that 
have heen made with oysters undoubtedly prove that a certain 


328 THE INFLUENCE OF INANIMATE SURROUNDINGS. 


and somewhat extensive influence is exerted upon them by the 
conditions of their position. Nevertheless, they yield no satis- 
factory conclusion as to the amount of these mechanical effects, 
though their existence cannot be disputed. 

And we are still worse off with regard to other influerces, 
which certainly exist, although they remain up to the pre- 
sent time perfectly unintelligible to us. Thus it can hardly 
be doubted that the electric tension of the atmosphere or of 
terrestrial magnetism must have some effect on animals, and 
attempts have not been wanting to refer certain phenomena of 
animal life to these causes. Thus, for instance, at one time the 
view was put forward that migratory birds directed their flight 
towards the magnetic poles in the Old and New World; an 
opinion which certainly seems to have been based on the tempt- 
ing but usually misleading principle of Post hoe, ergo propter hoe. 
We may, however, congratulate ourselves that the latest and 
most thorough work on the migration of birds—that of Palmén 
—sets aside this mysterious power of the magnetic poles as 
purely fabulous. Even with regard to the effects of atmospheric 
electricity, we may surmise their existence, but cannot grasp 
it, and remain unable to come to any decision ; that the dis- 
charge of electricity in the form of lightning can occasion death 
is all that is certain. But this, of course, is of no great impor- 
tance, as it never can be a factor in the origination of a new 
species or the extirpation of an old one; with regard to such 
results as these, only the feeble electric teusion of the atmosphere 
need be taken into consideration—and this probably is not even 
perceptible to most animals—since that alone is capable of 
exerting any constant influence. T.ately, and particularly in 
France, naturalists have begun to investigate the effects of 
electric currents on animals and on their development, and have 
already arrived at very remarkable results. Thus, Onimus 
found that the ova of frogs developed more rapidly at the nega- 
tive pole of a constant current than at the positive pole; 
‘Wagner supposes that he has observed an effect of electricity 
in altering the colours and the form of the wings of butterflies. 
These and the observations of Pigeon, Chauveau, and others 
do not, however, at present, allow of our applying them to 


POSSIBLE EFFECTS OF ELECTRICITY. 329 


the question of immediate interest to us here; for our pur- 
pose it must first be proved, or at any rate shown to be proba- 
ble, that electricity generally may exert a determining and 
ascertainable influence on the normal growth of animals, and 
this has not been done by any experiments hitherto made. 


330 THE INFLUENCE OF LIVING SURROUNDINGS. 


SECTION IIL. 


THE INFLUENCE OF LIVING SURROUNDINGS. 


CHAPTER XI. 


THE TRANSFORMING INFLUENCE OF LIVING ORGANISMS ON 
ANIMALS, 


Introductory Remarks.—It is self-evident that all animals, 
without exception, are to a certain degree simultaneously depen- 
dent on various other animals as well as on plants; for even 
such species as appear to depend for their nourishment exclu- 
sively on a particular kind of vegetable food also come under 
the indirect influence of other organisms, often of a great num- 
ber, by reason of that very limitation. Examples of such 
highly complicated relations and interdependence are familiar 
to all, and this relieves me from the necessity of repeating here 
all that has been said so often and with so much emphasis by 
Darwin and others on this part of my subject. 

Occasionally alterations taking place in these complicated 
relations may even lead—as is actually known—to the destruc 
tion of a species. For instance, if the food-plant supplying a 
strictly monophagous animal is by any circumstance extirpated, 
that species must inevitably die out. When plants on which 
any species of animal is dependent for food are affected and 
modified by any accidental variation that may occur in the 
temperature, the moisture of the air, or the nutrition they 
derive from the soil, the animals living on these plants will 


RECIPROCAL INFLUENCES. 331 


necessarily be sensible of the changes and possibly perish in con- 
sequence. Again, if certain insects whose function it is to 
fertilise certain plants die out, others living as parasites on 
those plants would necessarily suffer in a variety of ways. A 
few species, being wholly monophagous, would perish com- 
pletely ; polyphagous species, as it would seem, might be able 
to adapt themselves, without suffering in the least, to a changed 
form of nourishment and mode of life; others again, if they were 
not extirpated, would probably find themselves affected in some 
way or other by the change of food, and the effects might very 
likely exhibit themselves by a certain modification in the struc- 
ture of their organs. We have mentioned some cases of this 
kind in the chapter on Food ; it will therefore be superfluous to 
dwell on them here, and all the more so as I have dealt with 
food as one of the inanimate conditions of existence. 

But the reciprocal action of living organisms gives rise to 
yet other modifications which can be in no way connected with 
those occasioned by nutrition, even though this may be derived 
from the organic kingdom. Thus, for instance, there are highly 
intimate relations between the different individuals of the same 
species and between the two sexes of a species; many animals 
are directly dependent upon others, although not using them in 
any way as food ; to these belong the commensals or messmates, 
which do not feed on their host or on its organs, although they 
make use of them as a means of procuring the food that suits 
them. 

As arule the species thus thrown into juxtaposition are 
reciprocally dependent on each other. If a parasite only 
destroys the internal germ-gland of its host—as is not unfre- 
quently the case with parasitic worms and crustaceans—the 
host will not be rendered incapable of living, but only of propa- 
gating its species. Thus I have never yet heard of the hermit- 
crab, Pagurus, veing found infested with parasitic Cirrhipedia of 
the genus Peltogaster and at the same time carrying eggs on its 
jifed feet ; nevertheless, the crabs continue to grow, and are to all 
apuersucs healthy. In a similar manner the peculiar larve of 
certain Trematode worms eat away the reproductive glands of 
pond-snails so completely that they are incapable either of fer- 


332 THE INFLUENCE OF LIVING SURROUNDINGS. 


tilising or depositing their eggs ; in spite of this they live just 
as well as the individuals not attacked by parasites, and it may 
be inferred from the size of their shells that they live equally 
long.!!8 

Now, it certainly is a fact that the influence exerted by one 
living creature over another is in most cases selective only. A 
transforming influence, on the other hand, occurs, and can occur, 
only when the two species come into direct bodily contact. But 


Fic. 85.—Part of the stem (c) of a hydroid Polfp, Campanularia, with closed pear-shaped 
galls (6) within which lives the larva (a) of a sea-spider, Pycnogonum, 


even then a selective influence is the first to come into play. 
For example, if a certain species of hermit-crab were an equally 
good host for the larve of all the parasitic Crustaceans which 
float or swim in the sea, it would in all probability soon be 
wholly exterminated ; consequently the continued existence of 
the parasites themselves depends not merely on their being able 
to attach themselves to a suitable host, but also on the selection 
which the host himself may be able to effect among the guests 


CASES OF CONSTANT ASSOCIATION. 333 


to whom he offers a residence—though he cannot be said to 
invite them. 

The modifying influence of living organisms on living 
animals.—In a former chapter we have already become 
acquainted with a case coming into this category; namely, the 
cysts or galls formed on certain corals by the presence of 
crabs. In these cases the crabs do not seem to be particularly 
affected by their host, unless we are disposed to attribute the 
flat shape of Hapalocarcinus or the cylindrical form of Litho- 
scaptus to the direct mechanical influence of their peculiar 
dwelling-places. The corals, on the contrary, exhibit such great 
and peculiar deviations from their normal growth that the 
effects of the parasites on their host are plainly perceptible, 
both as stimulating and as checking its growth. There are a 
not inconsiderable number of such cases known. Certain sea- 
spiders, Pycnogonide, produce exactly similar galls, but com- 
pletely closed (see fig. 85), on the stems of a small polyp—as 
Hodge informs us. All the larve of our fresh-water mussels, 
after leaving the parent, require to attach themselves to the 
skin of a fish before they can develope any further ; there they 
occasion an excrescence which gradually swells to a capsule 
visible even to the naked eye, and in this cavity the larva lives 
for months and goes through its metamorphosis into a true 
bivalve. We wust include in the same category the gall-flies 
forming galls on extremely various plants. 

We are now accustomed, in all such cases of deviation from 
the normal growth, to regard them pathologically, as the result 
of disease, and certainly not altogether erroneously, since we 
know that they constitute more or less frequent exceptions. 
But supposing that the reciprocal relations between two animals 
or an animal and a plant were of such a character that each 
was dependent on the other in an equal degree, so that neither 
could exist without the other, any deviation from the normal 
growth must evidently no longer be regarded as indicating dis- 
ease. We must even consider. the apparent abnormity as a 
peculiarity or character of the species, since it must necessarily 
occur in every individual of the species, The constancy of the 
causes which first led to the association of the two kinds of 


334 THE INFLUENCE OF LIVING SURROUNDINGS. 


animals would inevitably result in the constancy of the de- 
viation, and consequently the transformation of a pathological 
phenomenon into a normal character would depend solely on the 
uninterrupted constancy of the active causes. We arrived at 
the same result when investigating those modifications of struc- 
ture in animals which were occasioned by the first class of ex- 
ternal conditions of life; every variation induced by a change 
in temperature or nutriment, in the direction or strength of a 
current, or in the salt constituents of the water, must always 
recur, and thus become constant or even be increased, so long 
as the efficient causes remain unchanged. Now, in point of 
fact, several cases have long been known to us of pathological 
changes in animals which have become normal modifications, 
and the causes of which can only consist in the association of 
two species of animals. I will proceed to investigate these, and 
a few others which are new or have met with less attention. 

A very singular genus of small corals, called Heteropsammia, 
is found living in tropical seas (see fig. 86), of which each in- 
dividual regularly harboursa worm, Aspidosiphon, belonging to 
the class of Sipunculide. It is difficult to understand what 
advantage each animal can derive from their association ; yet 
some must exist, for a coral is never found without a worm. I 
myself have fished up numerous specimens of Heteropsammia 
Michelini in the Philippine seas, and never found one without a 
worm; and in every representation and description of all the 
species of this genus, the dwelling of this companion of the 
coral is always found. Now, the presence of the Sipunculide is 
the cause of certain very conspicuous deviations from the normal 
structure of the corals they live in—peculiarities which have 
indeed been regarded and described as specific characters of the 
species or genus. In young specimens the base of the free- 
growing coral is scarcely larger than the circumference of the 
cup; in fully-grown ones, on the contrary, it is much larger. 
This is the first generic character which appears to be occasioned 
by the presence of the stranger. For the intruder settles on the 
base of the quite young coral and grows along with it; but, as 
it would seem, quicker than the coral, so that the worm, in 
order not to outgrow the base in its rapid progress, has to curl 


PARASITES ON CORALS. 335 


itself round in a spiral. At the same time it appears so to 
stimulate the base of the coral that it grows faster than the cup 
itself, and thus the base gradually but conspicuously outgrows 
the cup. Many corals are affected in a perfectly similar manner 
by parasitic crustaceaus; Deaseris Preycineti by certain Cirrhi- 
pedia of which the shells often greatly outgrow the foot of the 
Diaseris, though this too is abnormally extended. Certain 
species of the genus Heterocyathus also are infested by Sipun- 
culide just like Heteropsammia, and their growth is modified 
by them. Even in fossil species of this genus holes are often 
observable in the foot which can scarcely have been anything 
else than the dwellings of Sipunculide.'!® There are also, it is 
true, some species of the same genus, Heterocyathus, which 


Fig. 86.—Heteropsammia Michelini. a, seen from above with the broad base! 6, from 
below, with the tube of the Aspidosiphon partly opened ; ¢, also from below but intact, 
showing the large entrance of the tube and the small side openings. 


establish themselves on the shells of univalves; the animals 
which have formed these are invariably dead, and the cavity of 
the dead shell is always occupied by another of the Sipuncu- 
lide ; but in this last case the worm has had no effect on the 
growth of the coral, which has followed the normal course of its 
species and is at most so far modified that the coral, in order to 
obtain as secure a hold as possible, has extended its base rather 
more widely over the surface of the shell than seems to be its 
natural habit. 

In the genera Heteropsammia and Heterocyathus, in the 
second place, another generic character becomes modified in a 
very peculiar manner by the Stpunculide. All the species 
attacked, of both genera, display both on tbe under side of the 


336 THE INFLUENCE OF LIVING SURROUNDINGS. 


base and at the sides a very variable number of perfora- 
tions which, in all the works that treat of them systematically, 
are described and particularly pointed out as specific or even as 
generic characters. But these holes do not in any way agree 
with the peculiar characteristics of the families to which these 
genera belong; for in Heterocyathus the side walls of the coral 
ought properly to be quite without perforations, and in Hetero- 
psammia, which belongs to the group of corals with porous walls, 
the holes which we find are quite different from those proper to 
the coral itself. In both cases these perforations, which are 
clearly visible in the illustration (see fig. 86), are occasioned by 
the worm, as is plainly shown by their irregularity of number 
and arrangement. They open directly into the spiral cavity 


Fic. $7.—a, a’, Helerocyathus philippensis, a fully-grown specimen. At a the hole 
formed by a Sipunculus is visible in the otherwise solid wall of the cup. 6, a young 
individual showing the terminal hole of the tube of a Sipunculus ; here it is visible in 
the side wal], but the growth of the coral pushes it to the bottom. c, Heterocyathus 
parasiticus, established on a species of Cerithium. 


in which the worm lives, and correspond exactly to its growth; 
that hole which is nearest to the opening of the tubular dwell- 
ing, out of which the worm protrudes its head, being situated 
exactly as it must necessarily be with reference to the position 
of the anus of the worm to serve as a passage for the ejection 
of excrement. Finally, these holes have no connection with 
the cavity of the coral itself. 

Thus the enlarged foot and the large holes observable at the 
base or in the sides of the corals, have originated in the same 
way as those pathological deformities in animals and plants 
previously mentioned, and this is more particularly proved by 
the fact that no such holes are formed in the Heterocyathus, 
which establishes itself on the shells of univalves. But as 


A CORAL AND A MOLLUSC. 337 


they are perfectly constant—no specimen having hitherto been 
found without such holes, sufficiently proving that a Sipunculus 
lives as commensal in the coral—they here bear the aspect, or 
disguise, of a true specific character. 

Many years since, a very interesting case of association or 
commensalism between a mollusc and a coral was described by 
Steenstrup. The young of Rhizochilus antipathum (see fig. 
88) have all the appearance and characters of a true Buccinum. 
When they have reached a certain size they attach themselves to 
the slender branches of a horny coral known as Antipathes, and 
at once so modify their normal growth that it is quite impossi- 
ble to name any other mollusc in which any alterations of 
growth at maturity are to be met with in the least resembling 


Fic. 88.—Rhizochilus antipathum, Steenstrup. To the right the young shell exactly 
resembling Buccinum ; to the left the old shell firmly attached to the branches of Anti- 
puthes by itsirregularly formed margin. 

it. The shell throws out processes in every direction, by which 

it clings to. the coral, in the mode here shown in a woodcut 

copied from Steenstrup, till at last the mollusc loses all 
power of motion and lies, anchored as it were, to the Anti- 
pathes. Of what use this can be to the animal it is difficult to 
say; but we may venture to put forward the hypothesis that 
it must be, and is, of some service, and also that an originally 
accidental connection and growth of some true species of Buc- 
cinum with the slender branches of a Gorgonia must have given 
rise to this extraordinary habit. 

Certain parasitic Crustaceans offer another example of such 

a peculiar action of one animal on another. The species of Pel- 

togaster often live attached to the hind part of the body of the 

hermit crab, and they then assume the form necessitated by that 


338 TIE INFLUENCE OF LIVING SURROUNDINGS. 


of their host and of his dwelling. Pachyddella lives in preferences 
on the hind part of other crabs, and particularly on the under 
surface, which, as is well known, the crab always carries folded 
in under its body towards the front; it occurs almost exclu- 
sively on the abdomen of the female crabs. The parasite always 
(see fig. 12) consists of a somewhat flattened sac, adhering closely 
on both sides to the surface of the crab’s abdomen ; the side 
edges of the parasite, when seen in its natural position, perfectly 
agree with the form of the crab’s body and correspond exactly 
with the lateral symmetry of the crab itself. The structure of 
one surface of the Pachybdella almost always differs from that 
of the other, and one of these surfaces has hitherto been always 
regarded as the natural hinder side of the Pachybdella and the 
other as the front or belly ; but this is quite an error, as has 
been proved by the careful researches of Professor Kossmann. 
The old view seemed quite established by the presence of a large 
opening which was generally recognised as the mouth, and it 
cannot be denied that a median line of the body seemed to be 
indicated by this opening, and by the situation, exactly in a 
line with it, of a style by which the animal attaches itself to 
the abdomen of the crab, and these allowed of our dividing it 
into two symmetrical right and left halves analogous to those 
observed in most other animal forms. But after Kossmann’s 
observations and an exact investigation of its internal anatomy, 
previously but little known, there can no longer be any doubt 
that the flat surface is in fact only one side of the body, and 
that the two corresponding halves constitute the front and 
back. Thus, in form, this animal reminds us somewhat of the 
laterally compressed flat-fishes, in which the back and belly form 
two edges while the right and left sides are broad and flat; and 
like them it always lies on one side, sometimes the right and 
sometimes the left. 

T cannot resist the temptation to attempt to explain this 
extraordinary condition of things by an hypothe.is put forward 
by Kossmann. The larve of all the Cirrhipedia—to which 
Pachybdella belongs—are distinct from those of their nearest 
allies among the Crustacea by the circumstance that they must 
pass through a second larva-stage before they can assume the 


FALSE SYMMETRY. 339 


form of the fully developed sexual animal; during this stage 
they have two shells, connected, like those of bivalve mollusca, 
by an elastic ligament at the dorsal margin. It is probably at 
this stage that they attach themselves to the abdomen of a 
crab ; this compels the Cypris-like larva to lie on one side, 
since there is no room for it to occupy a perpendicular position 
between the thorax and the folded-up abdomen of the crab. 
Since, moreover, they establish themselves, almost without excep- 
tion, in the centre of the crab’s body, and ere long cast off 
the hard shells ofthe larva form, to allow of the growth of the 
somewhat soft permanent skin, the back and front of the 
Pachybdella will continue to grow in similar directions, and it 
is easily explicable on mechanical grounds that the two edges 
must be symmetrical in their growth, in consequence of the 
symmetry of the form of the crab’s abdomen. In most animals 
it is not the back and belly that are symmetrical, but usually 
the right and left side. Now, since the left side of the larva of 
the Pachybdella is applied to the thorax of the crab and the 
right to the surface of the abdomen, it need not surprise us to 
find that the two sides of the body, which in other animals are 
usually symmetrical, have become dissimilar in consequence of 
the unusual pressure upon them, and so their normal symmetry 
is lost. This is actually what, takes place, as Kossmann has 
also proved; the markings and the hairs of the skin of many 
species of Pachybdella are extremely different on the right side 
and on the left. This renders the comparison with the unsym- 
metrical flat-fish—as Plaice—even more striking, but in them 
the false symmetry of the front and back is less distinctly 
marked, than in the Pachybdella. 

The method here suggested as an explanation of the false 
symmetry of Pachybdella presupposes, however, that only one 
individual at a time shall have attached itself to the crab, for 
in that case only can the abdomen ‘of the crab be capable of 
pressing the Pachybdella, as it grows, into this particular form. 
And indeed only one individual is commonly found on each 
crab. But when—as sometimes, though very rarely, happens— 
more larve than one establish themselves almost simultaneously 
onacrab’s tail, these must mutually hinder each other in the 


340 THE INFLUENCE OF LIVING SURROUNDINGS. 


form of growth, otherwise determined by the pressure of the 
crab’s abdomen, and we might expect to find that such false sym- 
metry would disappear, since the mechanical causes determining 
the growth of the Pachybdella wou!'d no longer be able to act in 
the same way. This anticipation is not, however, justified ; in 
the cut here given the abdomen of a crab is shown which bore 
on it three such uninvited guests, and although a certain 
irregularity is plainly perceptible in the form and dissimilarity 
of the three parasites, the false symmetry is quite normal and 
well developed in all three. This proves that in this case the 
false symmetry induced by pressure has already become an heve- 


ee ec 
Soe AES 


= ae 


Fia. 89.—A specimen of Carcinus mernas, from Heligoland, with three parasitic specimens 
of Sacculina carcini, All three, in spite of their irregular growth, exhibit the false 
symmetry proper to the genus. 


ditary character of the species; otherwise it must have dis- 
appeared. Thus in this case, what was originally an abnormal 
and pathological character seems to have become a normal 
specific character, transmissible by inheritance, 

A still more wonderful instance of the same kind was long 
since described by Count Pourtalés. During his dredging 
expedition in the West Indies he discovered a horny coral 
(see fig. 90) invariably associated with an Annelid. The worm 
lives in a tube formed by the abnormal growth—which in this 
species has become normal—of the slender branches of the coral 3 
they grow together into a rather fine network, and thus form 


A CORAL AND A WORM. 341 


a cylindrical cavity lying parallel to the main stem of the 
Antipathes. Among the numerous specimens found by 
Pourtalés not one occurred that had not a tube, and thus 
the same thing has happened here as in the other instances 
adduced, An abnormal peculiarity caused by a modification in 
the mode of growth has, by the constant recurrence of the 
exciting cause, become a distinguishing mark of the species. 

In conclusion I cannot refrain from mentioning one more 
very singular and hitherto little-noticed case of the association 
of two organisms which seems closely allied to the well-known 
case of Lichens among plants. These, according to Schwen- 
dener’s researches, are to be regarded as colonies of true one- 
celled Algse and Fungi, and though individual botanists still 
raise their protest against this view, the latest investigations on 


RAs hints 
What 


t 


Fic. 90.—Antipathes filix, Pourtalés, a horny coral of the West Indian seas, which by 
constant association with an Annelid has been forced to form a tube for the worm. 
Deep-sea corals. g 


the subject seem to prove that they are no longer justified in 
doing so. 

The Sponges are now universally classed with animals; 
their soft parts consist exclusively of cells which are scarcely 
ever co-ordinated to form special organs such as occur among 
the higher animals. These soft portions are usually strength- 
enedand supported by a network of fibres secreted from the cells 
and extremely variable in structure. In the forms which are 
generally regarded as the simplest and most typical, all the parts 
unite to form a funnel attached by the pointed end, and of which 
the free end has a large opening leading into a central cavity ; 
this, for brevity, we will call the mouth of the sponge. But, 
besides this, the internal cavity communicates with the sur- 
rounding water by a system of fine canals which penetrate the 
lateral portions of the sponge funnel in every direction. A 


342 THE INFLUENCE OF LIVING SURROUNDINGS. 


stream of water, produced by the cells of the sponge, cir- 
culates through the system of tubes thus formed, and this, it 
would seem, supplies the animal with food, consisting of micro- 
scopic organisms. By a course of growth and subdivision, 
after the manner of plant-growth, a compound sponge is fre- 
quently formed; one, that is to say, which has a number of 
mouths, more or less, and in which the central cavity—which 
in calcareous sponges is often quite simple—is transformed into 
a highly complex structure of internal canals and cavities. 
These soft and perfectly harmless organisms, sometimes, how- 


Fig. 91.—a, longitudinal section through a calcareous sponge, showing its simple central 
cavity. 6, the sponge uninjured. (From Haeckel.) 


ever, growing to an extraordinary size, offer a welcome shelter 
in their innumerable cavities to a host of other creatures, which 
retire into them, as I might say, for rest and refreshment, and 
can easily find in their labyrinthine passages a place of con- 
cealment from the pursuit of their enemies; sometimes these 
are true parasites, sometimes only commensals, which establish 
themselves there. Such a specimen of sponge freshly dredged 
up from the sea offers to the collector a rich mine of Annelids 
aad Planarians, Nemertide and Polypes; Crabs of every kind, 
various Mollusca, and even Fishes, may be found, and Plants, 


A SPONGE AND A FLORIDEA. 343 


as Aloe and Fungi, also establish themselves there. Some of 
these last have very singular habits ; it was Lieberkiihn, so far 
as I know, who first pointed out the fact that certain alge—the 
Foridee—are invariably associated with certain species of 
sponge, and that they grow, not on the soft portions, but on the 
hard fibres. Other alge, again, serve as a base for the sponges 
which cover them like an incrustation. Both of these cases 
might be designated as instances of parasitism if we knew thas, 
in the former, the Floridez, living in and penetrating the fibres, 
derived their nourishment from the sponge; or that, in tho 
second, the alge supplied food to the sponge growing upon it. 
But we have no certain information on these points 


Fig. 92.—Spongia cartilaginea, Esper. Half the natural size. The holes in several of the 
branches are the mouths or stomata of the sponge. By far the greater portion of the 
substance of the broad branches is composed of the matted filaments of an alga, Floridea. 


While following up the question as to whether such pecu- 
liarities of structure might not in fact be more common than 
was supposed, and what the nature might be of the reciprocal 
relations between two organisms thus associated, I unexpectedly 
met with an object which at the first glance has all the normal 
appearance of a highly ramified species of sponge. It seems to 
have been so described already by Esper, and I believe I am cor- 
rect in designating the organism represented in the cut (see fig. 
92) as the Spongia cartiulaginea of that writer. The branches, 
which are sometimes cylindrical and sometimes flat, divide in 
one place and reunite in another, thus forming an irregular net- 


344 THE INFLUENCE OF LIVING SURROUNDINGS. 


work, with the meshes and branches spreading almost in a 
plane; such forms are tolerably common among the true 
sponges. Besides this, all the branches have large perforations 
on one side, which in living examples—if I may judge from the 
points of attachment—appear all to be directed upwards. If 
such a branch is cut across, certain peculiar thick transparent 


Fic. 93,—Spongia cartil.rginea, Esper. Sections of branches showing the skeleton formed 
hy the filaments of the sca-weed and the stomata, a, of the sponge. The spiculse and 
protoplasmic tissuc are cnly visible under a high magnifying power. 


fibres appear which do not greatly resemble the usual fibres of 
sponges, and which penetrate the whole organism in every direc- 
tion and through all its anastomoses. Still, when we again 
study the uninjured organism, even with a lens, we feel once 
more inclined to agree with Esper in regarding it asa true 
sponge, 

But a more minute investigation with the microscope shows 


THEIR INTERNAL STRUCTURE. 345 


us, beyond the possibility of doubt, that this body consists of two 
distinct organisms, namely, of a sea-weed associated with a 
sponge exactly as is the case with the lichens among plants, and 
it is impossible, with only specimens preserved in alcohol, and 
without investigating their vital properties, to decide whether 
the form of the whole mass owes its origin to the sponge or to 
the vegetable growth. The thick and somewhat vitreous, trans- 
parent branches of the internal network (see fig. 93), which 
give rise where they anastomose to the broader branches, on one 
side of which we find the stomata of the sponge, are un- 
doubtedly filaments of a sea-weed—probably an undetermined 
species of Floridea; and the spaces between these internal 
branches of the sea-weed lead directly into the cavities which, 
on one side of the main stem, pass into the stomata of the 
sponge. Hence the margins of these are actually composed of 
the filaments of the alga. On the other hand, the soft tissue of 
the sponge proper lies in a very thin layer on each filament ; 
tbe sponge has no true fibres, though it has spicule which are 
scattered through the soft substance. Unfortunately the spicule 
are so far from characteristic in their structure and their arrange- 
ment that it is impossible to determine the genus in which the 
sponge should be systematically placed. If we assume that its 
normal growth is neither hindered nor modified by its association 
with the sea-weed, it may with some probability be included in 
the family of the Chaline. 

But it is highly probable that in point of fact both the or- 
ganisms are to a certain degree reciprocally influenced and 
modified by their association. Although I have examined very 
numerous examples of these colonies, I have never succeeded in 
detecting the smallest trace of fructification on the filaments of 
the Floridex ; they even seem not to grow in the usual manner 
of the Florides, so far as I could determine, and I am supported 
in this view by those specialists whom I have asked among 
botanists. In the first place an internal union, by secondary 
coalescence, occurs between the large primary branches, which 
continue growing at the ends only; that they are truly coales- 
cent and not merely superficially connected is proved by the fact 
that in many places the original scar or line of contact is still 


16 


346 THE INFLUENCE OF LIVING SURROUNDINGS. 


visible, while in others the union has proceeded to such a point 
that even the cell-walls have merged in one. This appears to 
me to be a very conspicuous deviation from the normal growth 
of the Florides, for, so far as I know, two separate filaments or 
branches never, or most rarely, anastomose in these marine 
alge. Even the absence of all fructification seems to prove 
that the sea-weed is forced to an illegitimate mode of growth, 
so to speak, by its association with the sponge. 

But it is, moreover, very probable that the sponge, on its 
part, is affected by the Floridea. The greater part of its stomata 
occur only on that side of the broad primary branch which is 
directed upwards, but sometimes we meet with some which, by 
a twisting of the branch, have quite lost this normal direction. 
Now, if the sponge alone could determine the direction of 
growth, all the mouths would be turned in one direction, and, 
as this does not occur, the presumption is obvious that this de- 
viation from its normal behaviour is occasioned by the influence 
of the sea-weed on the sponge. As it would seem, the direc- 
tion of growth of-the stomata is determined by that of the 
filaments of the alga; this extends its blunt tips—which, as is 
well known, are its growing points—in every direction. They are 
found every where, at the broad free end of the primary branches, 
as well as within the oldest portion and all round the mouths 
of the sponge. As the encrusting layer formed by the sponge 
is excessively thin, it may be supposed that round about the 
stomata the growth of the sea-weed is as vigorous as that of 
the sponge, or even stronger ; it is then forced by the filaments 
of the alga into a direction of growth perhaps not originally 
natural to it. Be this as it may, in any case the compound 
organism I have here described must be of the highest interest, 
and I make no doubt that a careful investigation, not of 
dead specimens, but of living individuals on the spot, with an 
inquiry into their mode of life and physiological characters, 
will furnish an answer to the question whether, as I believe, 
the organisms—sea-weed and sponge—have a reciprocal influence 
analogous to that which it has been proved that the alge and 
fungi have in the compound organism known as a Lichen. 

The degeneration of the organs of parasites. Besides all 


DEGENERATION OF UNUSED ORGANS. 347 


these influences to which two organisms living in association are 
exposed, there are yet others which exhibit their results in the 
degeneration of the organs of true parasites. Most parasites 
dispense entirely with many organs indispensable to the 
existence of free-living species; or else they possess them in so 
undeveloped a state that they appear almost incapable of ful- 
filling their functions. They are then called rudimentary 
organs. It is usually said that this extinction of organs which 
in other cases are so important, is the result of the parasite’s 
mode of life ; but this is only a brief way of stating the fact 
that such extreme degeneration has been observed to any great 
extent only in those true parasites which actually feed on the 
juices of their host. The statement is in no way an explana- 
tion of the phenomenon itself, and up to the present time we 
are absolutely unable to assign with any certainty the causes 
by which, in any individual case, an organ may have been 
affected in such a way as to reduce it to the rudimentary condi- 
tion or to cause it to disappear altogether. It is self-evident 
that all sedentary parasites, or such as live in the organs of 
other animals, must lead a free life while young, in order to 
secure the perpetuation of the species by seeking a new host. 
In accordance with this, we see that all those parasites with the 
history of whose development we are acquainted, go through a 
stage of free larva-existence during which the Jarvee live under 
the same conditions as other independent creatures. Such a 
free existence is, of course, impossible without organs of locomo- 
tion, such as legs, fins, and so forth; these again would be use- 
less if they were not under the control of volition and its 
auxiliaries, the organs of sensation; all these organs must be 
supplied with nourishment, which the free-swimming larve 
could not obtain unless they had grasping organs to seize their 
prey; and finally, they could not digest the food thus obtained 
if they were not endowed with organs proper for that function. 
Vessels or other contrivances must be present in them which 
may convey the food-juices to the remotest part of the body ; 
other internal parts must fulfil the task of carrying away all 
the products of that decomposition of the food-constituents 
which results from the process of combustion carried on in the 


348 THE INFLUENCE OF LIVING SURROUNDINGS. 


body, since these waste products are in the highest degree pre- 
judicial to life. Parasites permanently attached to their host 
and living on its juices have no need of most of these organs, 
aud, in fact, in all such parasites all or most of them. have 
totally disappeared or are extremely degenerate; the degree of 
degeneration is, however, certainly very different in different 
species of parasites. 

With reference to this it will suffice to select a few of the 
best-known and more instructive examples from the abundance 
at our disposal. Among mollusca there is first and foremost the 


Fic. 94.—Entoconcha mirabilis, Miller. 8, when sexually mature, in the form of a spiral 
worm-like creature in the body cavity of Synapta digitata. B, the larva of the 
molluse. 


well-known Lntoconcha (see fig. 94); this consists of a simple 
sac containing nothing but the hermaphrodite organs and 
the embryos of a univalve mollusc. These embryos have 
precisely the form and structure of the ordinary larve of 
univalves adapted toa free existence; an oval shell with an 
operculum to fit the mouth, an organ for swimming—known as 
the velum, such as occurs in many similar larvee—a brain and 
auditory organ, intestines, gill cavity and all the other parts. 
But all the organs here enumerated are entirely lost when the 


DEGRADATION OF THE PERFECT PARASITE. 349 


creature is transformed into a mere parasitical pcuch-Jike 
molluse living in a Holothurian. The Cirrhipedia, Copepoda, 
and Isopoda among the Crustacea offer numerous examples of 
equally extensive degeneration. The extraordinary Zhomp- 
sonia globosa, described by Kossmann (see p. 47), is nothing 
more than a small perfectly closed sac attached by a short stalk 
to the leg of a crab, Melia tesselata ; it contains larve in the 
Cypris-stage without a trace of any other organ whatever, and 
the other two above-named orders of Crustacea contain several 
equally degenerate forms. In the same way the larve of many 
parasitic worms are often more highly organised than the 
fully grown and sexually mature individuals, and in many 
other groups of animals between those here mentioned ‘ degene- 
rate metamorphosis ’ often appears simultaneously with a para- 
sitical mode of life. 

Now, at the first glance, it seems tolerably easy to explain 
the gradual disappearance of many organs in these different 
creatures by the principle of disuse. We know that a muscle 
which is not constantly exercised in the proper way gradually 
loses its power and precision and at the same time materially 
diminishes in size. The organs of sensation may be rendered 
keener in their perception by use, and our mental activity in- 
creases with exercise and diminishes by lack of employment. 
Thus, applying this principle to the foregoing cases, we might say 
that the Entoconcha or parasitic crustaceans had lost their 
organs of motion because, after attaching themselves to their host, 
they no longer used them. In the same way we might under- 
stand the disappearance of a true stomach in Sacculina and 
Thompsonia, since it becomes useless from the moment when 
the animal establishes itself in the cavity of its host and by 
plunging a sucker into its body is enabled to suck up the absorb- 
able juices of its host, and so to convey them into its own body 
cavity, without any circuit vid a stomach. Eyes and ears, 
brain and nerves, muscles and other similar organs, dependent 
on the will of the animal, might in the same way easily have 
become extinct from desuetude. But, plausible as all this 
sounds, certain not unimportant difficulties seem nevertheless to 
stand in the way of this methed of explanation.. An investiga- 


350 THE INFLUENCE OF LIVING SURROUNDINGS. 


tion of these will once more set some of the general principles 1 
have already laid down in a clear light. 

If desuetude were invariably to be regarded as a primary 
cause of the disappearance of organs no longer in exercise, it 
would be very difficult to understand why, under apparently 
identical circumstances, identical results should not follow, .e. 
the disappearance of an organ. All the free-swimming larve of 
the lower Crustacea have similar swimming organs, namely legs, 
and all alike are thrown out of use by the settlement and 
attachment of the parasite. In spite of this, the legs are by no 
iwneans universally absorbed in the same way. In some species 
one disappears first, in other species another; sometimes too a 
few limbs are spared and remain attached to the body, though 
perfectly useless. Hence the same cause affects the same organs 
very differently in different species ; and this proves that the 
absence of a disused organ is not a mere mechanical result of 
desuetude, but, on the contrary, is subject to other determining 
influences according to the peculiarities of the animal whose 
organs of motion are no longer exercised. We arrived at the 
same conclusion in a former section when considering the inani- 
mate conditions of existence, and I will endeavour in this 
chapter to illustrate this point more fully by a few other striking 
instances. 

Asa rule a tolerably sharp distinction is made between ecto- 
and endo-parasites ; the former being such as live on the outer 
skin of animals, e.g. the louse, the latter living in the interior 
organs. It isalso regarded as an almost universal rule that 
ecto-parasites are of less degraded forms than endo-parasites ; 
however, there are some very striking exceptions to this rule. 
The most remarkable exceptions known to me are the following, 
which I myself observed in the Philippine Islands. 

Holothurians, like all animals, are infested by a great num- 
ber of various parasites. Besides the Mierasfer (a fish) and 
Pinnotheres (see p. 80) which live in the water-lungs, other 
parasites, molluscs, and worms are found on and in them. 
Among the former Hulima occurs very frequently on the skin 
of the Holothurize (as also on that of Star-fishes) ; it exactly 
resembles other univalve mollusca, and its parasitic mode of life 


EULIMA ON AND IN HOLOTHURIA. 351 


has only led to its losing the organ for gnawing and masticating 
which is universal and peculiar to univalves. It does not need 
it, for it seems to suck up the slimy secretion from the skin of 
the host. Hence Hulima has never been included in the 
category of true parasites; and rashly dogmatising from this 
view, the actual observation of Mr. Cuming (the well-known 
traeeller and conchologist), who found similar specimens of 
Eulima inside the stomach of Holothurians, was at once rejected 
and explained away by the quite unfounded assertion that the 
univalves had only been eaten by the Holothurians; but 
Cuming was perfectly correct in his statement, for I myself 
have found living Eulime in the intestine of large Holothurie, 


Fic. 95.—T wo undescribed species of Zulim7. u, lives creeping freely in the stomach of 
a Holothurian. 0 is sessile on the skin of a Holothurian, through which it plunges 
its sucking proboscis, c, the front of the proboscis with its simple mouth. 


and that very often and by no means as a great rarity. Here 
they creep about rapidly on their broad foot, on the wall of 
the intestine, and they have, moreover, all the crgans proper to 
univalves, as a nervous system, organs of sensation, an intes- 
tinal canal, &e., exactly like the form living on the outer skin ; 
the only organ wanting is, in the same way, the masticating 
organ, rachis or tongue, as it is called. With these, certain small 
flat worms live in the same intestine ; these have the internal 
structure of the Trematoda, but glide along the intestinal 
canal after the fashion of the Planarian worms, by means of 
the cilia on their skin; and, lastly, a few species of minute 
Crustacea, belonging to the Copepoda, float and crawl within 


352 THE INFLUENCE OF LIVING SURROUNDINGS. 


the intestinal cavity of the Holothurie. These have recently 
been described by Kossmann under the name of Lecanurtus ; 
but these small creatures have not assumed the organisation 
of true and degenerate endo-parasites, but possess all those 
organs which are found in free-living species or in ecto-parasites 
and which enable them to change their dwelling-place rapidly 
and at will.}2° 

On the other hand, I found on the skin of the very same 
species of Holothuria which harbours in its intestine the crea- 
tures just described, a Eulima which is far more degraded in 
structure than any other species of the genus. The front of the 
head which bears the mouth is drawn out into an extremely 
long proboscis, which pierces quite through the very thick skin 
of the Holothuria, and the mollusc is just as securely anchored 
by it as is the Pachybdella by its style or holdfast. But this 
proboscis must also act as a food-sucker, since it bears, at the 
end it inserts into its host, a simple mouth without any gnawing 
apparatus. The foot, which in other species living on the skin is 
well developed, has here wholly disappeared (see fig. 95), and 
eyes are likewise wanting. Thus we perceive that the effects 
usually produced by the condition of living in the intestine, in 
this instance have not been able to impress the character of endo- 
parasites on these living in the Holothuria ; while, on the other 
hand, a true ecto-parasite has been modified in the way com- 
mon to endo-parasites, although it belongs to a group of animals 
of which the numerous species live, without exception, on the 
skin of Echinodermata, but nevertheless are thereby modified 
to so insignificant an extent that their parasitic nature has even 
been altogether denied. 

The causes which have so far come under our consideration 
as lying within the agency of living organisms and occasioning 
modifications in animal forms, sink altogether into the back- 
ground as compared with one now to be discussed, and about 
which much has been written, and not a little that is false; 
namely, Hybridisation. This word signifies the fertile union of 
two individuals which according to our systematic classification 
are supposed to belong to two different species, and which are 
supposed, by a certain school of naturalists, never to have becn 


HYBRIDS IN CONFINEMENT. 353 


intended by nature to be capable of actual sexual union. Hence 
the opponents of Darwin’s theory—maintaining, as they do, the 
immutability of species—have always denied the occurrence of 
hybrids, or have declared that even if they could occur the off- 
spring of such an unnatural union must inevitably prove barren, 
or, finally, that the fertility of the progeny of a successful case 
of hybridisation affords a proof in itself that the animal forms 
previously regarded as distinct species must henceforth be con- 
sidered a mere variety of the same species. 

The importance of the subject induces me to pass the facts 
briefly under review. 

In the first place, it must be stated that those persons who 
deny, even now, the general possibility of hybridisation, by that 
very denial display their ignorance of the subject. It is simple 
folly, in the face of the increasing number of cases of successful 
hybridisation in our Zoological Gardens, to insist on maintain- 
ing this negative position. Hybrids are already known to us in 
the most widely dissimilar classes of the animal kingdom. 
Among apes, Cynocephalus mormon and Macacus cynomolgus 
have crossed and produced young; the hybrid race of Leporide, 
a cross between the rabbit and the hare, is very generally 
known ; the tiger has bred with the lion, the leopard with the 
jaguar, the polar bear with the brown bear, the masked pig 
with the common Berkshire pig, Dama vulgaris with Dama 
mesopotamica, Equus onager with Equus hemippus, Equus 
Burchelli with the common horse, and then again with the 
common ass and Equus hemionus, and all these crossed couples 
have repeatedly given birth to offspring. Quite lately a 
hybrid snake was born in the Zoological Gardens in London, a 
cross between Chilobothrus inornatus and Epicrates angulifer. 
Among ducks hybrids are extremely common; Anas sponsa 
crosses with Pudicula ferina and nyrocca, Anas boschas with 
Anas crecea, Among fishes hybridisation between two species 
of carp or of trout is very easily effected.* Hybrids of inver- 
tebrate animals are less common ; this, however, may be because 
they have been less experimented on by man, or because they 
have generally attracted less attention than the vertebrata. 


* Salmo salvelinus and 8S. furio; Cyprinus carpio and C. carassius. 


354 THE INFLUENCE OF LIVING SURROUNDINGS. 


Among insects, however, there are several species known which 
readily brecd together and produce hybrid offspring; thus a 
cross has been produced between the larger and smaller peacock 
moths and between the willow and the poplar hawk-moths. I 
have intentiovally mentioned here only some of the more recent 
cases (there is a mass of well-established older examples which 
may be found briefly enumerated in the fourth edition of Claus’s 
‘Lehrbuch der Zoologie ’).!?! 

This must suffice; the fact that two different species can 
unite and be fertile must be regarded as established, for in most 
of the cases here enumerated the individuals breeding together 
stand so far apart in the systematic scale that no systematic 
zoologist—not even the most virulent anti-Darwinian—-could 
venture to assert that they were only varieties of one and the 
same species. 

However, to deprive these numerous instances of hybridi- 
sation of any universal application and value, it is further 
asserted that the newly originated hybrid forms are always, or 
almost always, sterile. But even this statement must be 
declared to be inaccurate in its naked and literal form. The 
race of the Leporide is, as is well known, perfectly fertile, and 
has produced other cross-breeds with both the rabbit and the 
hare. Hybrids between the dog and jackal or between the dog 
and wolf remain fertile for many generations; those of Phasta- 
nus colchicus and torquatus are perfectly fertile; so are the 
well-known hybrid geese, and those between Cervus vaginalis 
and Cervus Reevesia; a female muJe in the Jardin d’Acclima- 
tation at Paris has produced two foals to a horse and two to 
an ass ; in the Zoological Gardens (London), a hybrid female of 
Bos indicus has had young by a male of Bos frontalis. Newton 
states that a hybrid female between the common duck and Anas 
boschaus proved fertile with a male AMareca penelope, and I have 
no doubt that many similar cases have escaped my notice. 
The infertility of hybrid races is certainly not a universal law, 
fur besides those cases which are always, or under certain cir- 
cumstances, infertile, we meet with others, as we have seen, 
not less numerous, of which the undiminished fertility is un- 
doubtedly established by reliable observers. We may conse- 


HYBRIDS IN A STATE OF NATURE. 355 


quently assume it as proved that cross-breeds may originate 

_ from hybridisation which are both fertile and capable of trans- 
mitting their characters to their descendants, but that certainly 
there is no must in the case. 

An attempt may, however, be made to invalidate this posi- 
tion by the assertion that, although the formation of such hybrid 
races may be possible, it can only succeed with domestic animals, 
and never in the freedom of nature and without the co-operation 
of man, since in all the examples here adduced the cross-breed- 
ing has been intentionally effected by man in animals kept in 
captivity. I may at once admit that though I have so far men- 
tioned none but such cases, it is not because cases of hybridisa- 
tion in a state of nature have not been observed; I have pur- 
posely reserved the mention of them, and will now first briefly 
allude to some rather doubtful examples. elis torquata, 
described and correctly drawn by Cuvier as a distinct species, 
appears to be a hybrid, produced in a free state, between the 
domestic cat and Felis bengalensis; Anas bimaculata is a freely 
engendered hybrid between Anas boschas and Anas crecca ; 
Tetrao medius, in the same way, between 7’. wrogallus and TL. 
tetrix. Siebold’s view is certainly well founded, that the vast 
number of intermediate forms which constitute the erua of the 
zoologist who endeavours to determine the species of the fresh- 
water fishes of Germany, must have originated from cross-breed- 
ing in a state of nature; this naturalist, in his well-known 
work on the fresh-water fishes of Germany, enumerates no 
less than eight hybrids, most of which have been described 
by other zoologists, even as types of special genera. According 
to Von Loewis, Lepus timidus and Lepus variabihs not un- 
frequently produce hybrids. Dr. W. Wurm states that he 
has often seen cross-bred partridges. J. von Fischer * asserts 
that the polecat and ferret are two different species and 
produce hybrids in a free state. The case noted by Mr. Buxton 
is perfectly verified, of a male white cockatoo and a female rose- 
coloured Leadheater’s cockatoo, which had never bred in con- 
finement, and which when set at liberty in the woods near that 


* Director of the Zoological Gardens at Cologne. His sketches of 
animal life are well known in Germany 


356 THE INFLUENCE OF LIVING SURROUNDINGS, 


gentleman’s house bred two years in succession. The processes 
of such attempts at hybridisation have not unfrequently been 
detected. Thus, G. Koch observed the union of Zygwna peuce- 
dani with Zygena trifolit, of Zygena minos with Zygena loni- 
cere and of Smerinthus popult with Smerinthus ocellata. A. 
Meyer detected that of various species of Phryganide ; Peragallo 
that of Luctola lusitanica with Ragonycha melanura; Kuenckel 
that of Strangalia melanura with Leptura livida; Gerstaecker 
that of Lipula oleracea with Pachyrhina scalaris. Heynemann 
iso tells us that Lymncea stagnalis and Lymncea wuricularis have 
bred together. Of course the question remains unanswered as 
to whether in all these cases the union led to the production of 
offspring ; but the mere fact that in a free state of nature such 
attempts at hybridisation are certainly made, renders it in a 
high degree probable that they may frequently lead to such a 
result, and we can no longer doubt the possibility of hybridisa- 
tion in a free state of nature. 

Now, I began by saying that such hybridisation might be 
one of the means employed by nature for originating new forms, 
that is to say, for producing offspring, and, moreover, fertile off- 
spring, which varied from their parents in form, colouring, and 
other characters, thus offering to Selection fresh material to ex- 
periment upon. To justify this statement it will be sufficient 
to examine one or two of the above-quoted instances rather 
more minutely. 

The hybrid cockatoos which I have mentioned were distin- 
guished from their parents very conspicuously, for while one of 
these was white and the other rosz-coloured, both the broods of 
young birds had large orange-coloured tufts. All the hybrids 
of fishes spoken of by Siebold display a peculiar mixture of the 
characters of both parents, besides others which cannot be 
referred to either with any certainty. The descriptions given 
by many systematic naturalists of recognised ability, of various 
hybrids as distinct species, prove that in these cases—as, for 
instance, in Felis torquata, Anas bimaculuta, and others—cha- 
racters occur which do not positively belong to either parent. 
The hybrid between the masked pig and the Berkshire pig was 
black with white feet, and the hybrid hear born at Stuttgart, 


COLOURING OF HYBRIDS. 357 


which was recently described, the offspring of a cross between the 
brown bear and the Polar bear, is described as follows.* ‘The 
change of colour undergone by the hybrid young was very in- 
teresting. All four came into the world quite white, but pre- 
sently assumed a silvery grey or bluish hue, and at the age of 
three months were of a dark-brown colour, shot, as it were, with 
a blue gleam. They at no time showed any trace of the white 
necklace which is characteristic of the young of the brown bear. 
The two that are six months old are at present for the most 
part greyish-brown, but not uniform in colour ; all about the 
throat they are conspicuously lighter, almost white. The two 
that are eighteen months old are much lighter altogether ; the 
backs and sides are a light bay-brown ; a dark median band in 
one of them is tolerably broad and extends all down the back ; 
in the other it is only faintly indicated in the fore-part; the top 
of the head is light brown, the under part and the rump 
whitish, all four extremities a rather dark brown.’ It is easily 
seen from this description that in this case changes in the 
colouring have been produced in the hybrids which exhibit a 
very considerable deviation from that of the parents. 

At the same time this last-mentioned instance is particularly 
adapted to bring into prominence another phenomenon which is 
at least as conspicuous in the hybridisation of animals as of 
plants, namely the mixture in the hybrid young of the colouring 
of both parents, particularly in the hybrids of insects and birds. 
In these a very distinct combination of part of the colouring of 
the female with differences taken from the male is regularly re- 
produced, as the mixture of colours in the hybrid (or mule) of the 
canary-bird and goldfinch ; in the hybrids above mentioned, 
between the poplar and willow moth, the peculiar marks on the 
hind wings of the former may be plainly seen overlying the eyes 
of the latter. But the extent of this mixture is extremely dif- 
ferent in the individual progeny, as has been evident in the 
minute description of the hybrid bears ; sometimes the colours 
of the female predominate, sometimes those of the male, and this 
may occur in different young of the same brood. Thusit is evi- 
dent that hybridisation does not result merely in an aggregation 


* In a German periodical, ‘The Zoological Garden.’ 


358 THE INFLUENCE OF LIVING SURROUNDINGS. 


of new characters in addition to suchas are already present in 
the parents ; but these too are rendered to a cortain degree fluc- 
tuating, so that any exact, or even approximate, repetition of them 
in the offspring is put out of the question. The rigid constancy 
of the parental characters maintained by interbreeding is so 
completely interrupted by hybridisation that the organisms 
thereby exposed to the means of selection acting on the young 
animals are to a certain extent perfectly new. Certainly, we as 
yet know very little concerning the mode in which hybridisation 
affects any other characters than the colouring ; the comparative 
difficulty of breeding such hybrids, and the short series of years 
that have elapsed since more attention has been paid to such 
cases than formerly, leave no room for surprise that available 
material is so extremely scanty in this direction. But this 
cannot alter the results that have been attained so far; for if 
after longer investigation we should be brought to the conclu- 
sion that in the animals at our command for experiment the 
colouring of the skin, or of its covering, still seems especially 
adapted to exhibit the effects of hybridisation, while other organs 
as the skeleton—for instance—are not affected at all, or ina very 
insignificant degree, neither of the principles above laid down 
would be disproved, but merely restricted in their application. 
If we now compare and contrast inbreeding and hybridisa- 
tion—cross-breeding—it is well known that the very essence of 
the former process is the union of very closely allied individuals 
of the same race or species, while that of the latter, on the con- 
trary, is the union of individuals very distinct from each other. 
And since this contrast, so far as we learn from the numerous 
experiments now within our knowledge, points to one and the 
same cause as that by which the difference in the results of 
the fertile union is determined in each case, we may deduce 
another general principle which will lead us to still wider con- 
clusions : namely, that the more remote the systematic affinity 
is of two animals that unite to produce young, the greater is the 
probability that, together with a perfectly undetermined mix- 
ture of the parental characters, new characters may arise which 
do not occur in either parent. The cause of the disturbance 
tkus arising in the constancy of the specific character is the act 


VARIATION OF SPECIFIC CHARACTERS. 359 


. 


of sexual union. Hence we may further infer that sexual union 
is in itself an auxiliary to the interruption of the constancy of 
species, even when it is effected between two individuals of the 
same species; only in this case it is worked out by nature with 
far less violence, as I may say, than in cases of true hybridisa- 
tion. And this agrees with the results of investigation in the 
domain of botany, for Sachs, in his ‘Text-book of Botany,’ 
expressly says that there is no essential difference between the 
self-fertilisation of pure species or varieties, and fertilisation by 
other species or varieties, and that in the case of true hybridisa- 
tion many peculiar characters attributable to sexual differentia- 
tion and agreement are brought into greater prominence. From 
all that we know at present the interruption of the constancy of 
specific characters may be regarded as one of the most conspicu- 
ous of these peculiarities. 


360 THE INFLUENCE OF LIVING SURROUNDINGS. 


CHAPTER XII. 


THE SELECTIVE INFLUENCE OF LIVING ORGANISMS ON ANIMALS, 


In the foregoing chapter we have seen that two organisms com- 
ing into physical contact may be able to exert a permanent 
transforming influence over each other. But this purely 
mechanical transforming or modifying process must always 
have been preceded by selection; for if all the larve which 
creep or swim on the earth or in the water were equally capa- 
ble of settling on any plant or animal that accidentally came in 
their way, these species would certainly be extirpated. Thus, in 
order that such animals may continue to exist as are capable of 
affording shelter or food to a certain number of others, they 
must be enabled to make a selection between the species which 
crowd upon them as commensals or as parasites. This selection 
may under some circumstances have been already effected by 
the other conditions of existence, as we saw in the first section ; 
but a second process of selection may be performed on those 
forms which have been able to outstep the limits thus imposed 
upon them, by the animal they choose to settle upon. This is 
of course always undesigned. A very striking example of this 
selective power of individual animals on the larve of parasites 
is offered by the different forms of the family of the Bopyrida 
among the Crustaceans. Many of the species, and particularly 
those of the genus Bopyrus (see fig. 38), live in the branchial 
cavities of crabs or of tailed Crustaceans, in which they always 
produce an enlargement—sometimes a very considerable one—of 
the branchial cavity. We may suppose that the young larve 
puss into it with the current of water which enters the branchial 
cavity close to the mouth to supply the gills with fresh water, 


INDIRECT SELECTION. 361 


for there is no other and certainly no more convenient way. 
This stream enters both the gill-cavities at once ; it is therefore 
easy to understand that the larve sometimes get into the right 
and sometimes the left cavity, and also that in the course of 
their growth, which is probably very rapid, they must hinder 
other larve from settling in the same spot, or perhaps they feed 
on the later arrivals as an easy prey. But it is highly remark- 
able that when one individual has established itself in one bran- 
chial cavity it is impossible for another animal of the same 
species to settle in the other unoccupiedgill-cavity ; so al least 
we may conclude from the fact that hitherto no case has been 
described of the simultaneous occurrence of two individuals 
in the gill-cavities of the same Crustacean. I myself, though 
I have collected and studied hundreds of these animals, have 
never met with one exception to this rule, and my observations 
have been confirmed by so distinguished a student of the 
natural history of Crustaceans as Professor Gerstaecker.* So 
far as I can see, there is only one possible way of explaining this 
striking fact. The guest already in possession of one of the 
cavities—let us say the left—cannot of course directly prey 
upon or turn out the larve brought by the current into the 
other unoccupied cavity; it can only do this indirectly, by so 
influencing its host that after the establishment of the first in- 
truder it is no longer fit for the reception of a second.!#? 
Where, as in this case, the two different animals do not 
come into actual contact and yet exert a definite influence on 
each other, this under all circumstances can only be a selective 
action, and this selection, as is well known, may he effected in a 
variety of ways. With regard to the end I have in view, it 
would be superfluous here to discuss all or even most of these 
ways; the general result will be set in a full light by consider- 
ing a small number of examples. But before we more closely 
examive the means which nature has bestowed on animals to 
give them the advantage in their relations towards others, it will 
be desirable to say a few words in general consideration of those 
mutual relations of any two animals which are called forth 


* The well-known editor of the section on Crustaceans in Bronn’s 
great work on General Zoology. 


362 THE INFLUENCE OF LIVING SURROUNDINGS. 


by their respective struggles for the same conditions of 
existence. 

The competition for similar conditions,—It is self-evident 
that different animals, or different individuals of the same 
species, must often come into antagonism in their search for food 
or for other things. In such cases the struggle will be the 
severest when they belong to the same species; for as they then 
will have approximately the same aims and about the same 
strength, skill, and powers of resistance, the combat for the 
hunting ground, the fémale, or for dead prey must be more 
severe than when the antagonists, belonging to different species, 
have in consequence different needs and tastes, and exhibit a 
conspicuous difference in their strength of body or in their 
weapons of offence and defence. In the latter case even, under 
certain circumstances, the struggle to obtain possession of the 
same object may come to an issue without any personal combat ; 
for if the two creatures attack the prey in a different manner 
both may be satisfied before they come into collision, and a 
personal combat will be averted. When, on the contrary, two 
individuals of different species can apply the same, or nearly 
similar, means for appropriating and keeping possession of the 
booty, just as virulent a contest must ensue as between two 
individuals of the same species. 

Any such direct battle between two animals, whether of the 
same or of different species, must always result in selection. 
The phrase ‘ Survival of the Fittest’ is a happy one, but it is a 
somewhat rough and not perfectly exact expression of the out- 
come of such cases; for it is certainly not always the fact that 
a species which is not qualified to conquer in such a personal 
contest with one species not its own, must be equally incapable 
of triumphing in a struggle with another, and so inevitably 
perish. This could be the invariable result of such a struggle 
only when the life of an individual or the existence of a species 
depended solely, and in every particular, on those conditions 
which had occasioned the strife of the two combatants. 

But, besides this, as has already been remarked on many 
sides, this selection by direct personal combat does not depend, 
as many imagine, exclusively on it and on the mode in 


THE DIRECT STRUGGLE BY COMBAT. 363 


which it is more or less successfully conducted by this or that 
individual, but also on the reaction of those very conditions for 
the possession of which the struggle took place. An example 
will here be appropriate. Supposing that two suitors of the 
same distracting beauty agreed to fight a duel for the possession of 
the fair one, the surviving lover could be designated as fortunate 
only in the event of its having been understood that he would 
thus win the lady’s affections. This example illustrates, with a 
little exaggeration of course, the indisputable fact that in every 
case of a competition between two animals for one and the 
same object, this object itself must have a certain, and indeed a 
selective, influence on the ultimate results of the contest. Sup- 
posing that two larve of dissimilar species of Bopyrus should 
meet at the entrance of the same branchial cavity and begin a 
struggle for the possession of it, under certain circumstances the 
parasite to which the cavity did not properly belong might 
come off victorious, but this would avail it but little, since it 
could not live in the branchial cavity of which it had obtained 
possession. The larva of an insect boring and living in wood 
might perhaps in the same way be able to conquer another 
which was accustomed to live and bore in leaden bullets ; but 
it is extremely doubtful whether the wood-borer would thus be 
enabled to live in the lead. Exactly-the same thing occurs 
between two individuals of the same species. We might 
perhaps be inclined to assume that the tendency of two indivi- 
duals to possess themselves of the same object was of itself a 
satisfactory and sufficient proof that both animals must be 
equally well fitted for the condition of life in dispute, but 
nothing could be further from the truth than this assumption. 
If, for instance, two men contend for the same office, a presi- 
dency or a professorship, under all the given circumstances one 
will be better fitted for the post than the other, and if among 
men this better qualification were the sole condition of selection 
the post would always be filled by the best candidate; the post 
itself would effect the selection by the qualifications which it 
demands. It is precisely the same with animals. When two 
lions or wolves fight for a prey, the stronger will undoubtedly 
win; but if the conqueror happens to have a bad digestion, in 


364 THE INFLUENCE OF LIVING SURROUNDINGS. 


spite of his victory he will not be able to derive the same 
benefit from it as the weaker might have done in the same cir- 
cumstances, 

The capability of an animal for winning a suitable position 
in life can never depend on one qualification alone, as for instance 
the possession of powerful weapons. If we suppose that all the 
Nauplius-larvee of a Pachybdella were simultaneously produced, 
and that they consequently all started simultaneously on a race 
for their place on the abdomen of a crab, the victory would 
naturally rest with the best swimmer. But the very organ 
which had secured it this success would subsequently be of no 
further use to it, since its powers of attachment and therewith 
its final success in life depend on the clinging antenne of the 
larva. Supposing, then, that the individual arriving first were 
ill-furnished in this respect, all the benefits of the victory would 
be lost to it, since the crab might possibly be able, by a 
vigorous movement of its hinder parts, to get rid of the unwel- 
come guest. If the conquered laggart and inferior swimmer 
were to arrive at this moment, and if it possessed better organs 
for clinging than the foremost one, it might, though originally 
beaten, finally become the possessor of the field—on the abdo- 
men of the crab. 

Thus the selection effected by the issue of a direct combat 
for any particular condition of existence can only lead to further 
results when the surviving party is also qualified to secure the 
benefits of the victory. This point, as it seems to me, has not 
seldom been lost sight of by Darwin’s followers, and still more 
by his opponents, who have frequently designated direct combat 
as the only means of selection. Darwin himself, I am perfectly 
convinced, never meant to say that a direct struggle between 
two individuals was the only or even the most important means 
of selection made use of by nature in the process of natural 
selection. Nevertheless, the struggle for existence cannot but 
be rendered more severe by the occurrence of such personal 
combat than it already is; and since this will chiefly occur when 
the individuals are as nearly alike as possible, it must, no doubt, 
very frequently be a means brought into play by nature to 
effect a selection between several varieties of the same species 


MEANS OF DEFENCE OR ESCAPE. 365 


lying at her disposal for the end and purpose of forming a new 
species. But this must depend not only on the reaction of the 
object or condition fought for, but also on the qualifications 
which the individual—or the variety—brings into the contest ; 
and it will be advisable here to examine somewhat more closely 
the relations thus originating between different animals. 

The relations of the pursued and the pursuers.—The 
means adopted by nature to give one form the victory over 
another are endless in variety. Muscular strength and power- 
ful natural weapons give the advantage in some cases ; swiftness 
of limb, nimbleness and skill in others. Neither cunning nor 
instinct, nay, not even really intelligent characteristics, are 
always capable of contending successfully with antipathy or 
sympathy; persevering inactivity or even overweening stu- 
pidity sometimes secures a great advantage in such a contest. 
The proverb is a familiar one, ‘The gods themselves cannot 
contend against stupidity ’—a firm belief in any dogma, be it the 
most stupid in the world, gives it a certain power and affords its 
adherents the confidence they need under the attacks to which 
it is subjected. The weak or the timid will often find a protec- 
tion in the semblance of courage which they may be able to 
put on. In short, there is no quality of body, of mind, or of 
instinct which may not, on occasion, prove a powerful weapon 
of offence or of defence. The very various external weapons used 
by animals in personal combat are familiar to every one. One 
species uses its teeth, another its feet, or both together, like the 
elephant ; the bird uses its bill, wings, or legs and spurs; the 
apparently ponderous horn on the nose of the rhinoceros is a 
formidable weapon, and the rattlesnake has one no less danger- 
ous at the end of its tail; the abdominal glands of scorpions and 
ants, or the poison glands in the mouth of venomous snakes and 
in the maxillary glands of spiders and centipedes, the foetid oil- 
glands of many caterpillars, of bugs and beetles, and of many 
feetid species among the Vertebrata, are all so many weapons 
available alike for defence or attack. Many of them—probably 
by far the greater number—are very constant in their occurrence, 
and in their structure, size, and form ; other characters, as those 
known as secondary sexual characters, particularly in insects, 


366 THE INFLUENCE OF LIVING SURROUNDINGS. 


are frequently very variable in their peculiarities. This last 
circumstance has given rise quite recently to an objection to the 
applicability of Darwin’s principles which I will here take the 
opportunity of discussing briefly. 

Kramer asserts, on the ground of a very elaborate mathe- 


Fic. 96.—Cludognathus dorsalis, Erichson. In the four corners are four different forms 
of the male ; the lower form on the left lias mandibles hardly larger than those of the 
female in tle middle. Natural size. 

matical calculation on the method of the doctrine of chances, 

that, assuming Darwin’s principles as the basis of such a calcu- 

lation, the extreme forms of a series of varieties must be less 
numerous than the intermediate forms, and in the same way that 
the production of excessive deviations must also be possible ; 


KRAMER’S CALCULATION. 367 


the result, would be a chaos of male forms gradually passing into 
each other and all belonging to a single female form. And he 
adds that this result, mathematically calculated, stands in tren- 
chant contrast to the facts, for it is known on the contrary that 
the secondary sexual characters of male beetles, in by far the 
greater number of cases, are extremely constant. 


Fie. 97.— Chalcosoma atlas, from the Philippines ; the upper one to the left is the horned 
form of male (C. Atlas, Linn.), the upper one to the right is the almost hornless form 
(¢@. Phidias, Blain.), Below is the female. Half natural size. 


But Kramer has not taken one element into his calculation 
which ought, under the circumstances, to have been duly con- 
sidered ; he calculates only the results of an unlimited capacity 
for variation in the males, while it is precisely this variability 
which, according to Darwin’s views, will be limited by sexual 


368 THE INFLUENCE OF LIVING SURROUNDINGS. 


selection, though not wholly prevented. Thus Kramet’s calcula- 
tion only holds good for those cases in which the causes that gave 
rise to the occurrence of deviations from the parent species con- 
tinue to act unchecked and uniformly, and alone determine the 
final result. He denies, indeed, that such cases can oceur, but, 
as it seems to me, without reason. For, particularly among 
tropical species, there is a not inconsiderable number of such 
instances, one of which is exhibited in the accompanying cut (see 
fig. 96) of the extreme forms of the male Cladognathus. Here, 
according to my enumeration, the varieties standing about mid- 
way between the two extremes represented occur in by far the 
greatest numbers ; towards each extreme the number of indivi- 
duals gradually diminishes, till at last, of the two extreme forms 
only one of each was captured among hundreds of others. 

The Lamellicornes in general, beetles with flat leaf-like 
antenne, exhibit a marked difference in the form of the two 
sexes; the males very frequently have large or small horns on 
the head, while the females are usually devoid of them. This 
is most conspicuous in the Goliath beetles, as they are called, of 
which many species are very common in the tropics. Some- 
times the horns of the males vary in a quite extraordinary degree ; 
as an illustration I here subjoin the extreme forms of the male 
of Chalcosoma atlas, a species mentioned by Darwin (see fig. 
97); one form is the original typical Chaleosoma atlas, of Evich- 
son, the other, smaller one, C’. Phidias of Blainville. I have 
captured many hundreds in the Philippine Islands, and from 
among them have selected the two specimens here accurately 
drawn from nature. They are the two most extreme forms of a 
quite distinct series, and I can positively assert that they are 
both of the same species. It can be seen that the larger indi- 
vidual has four large horns, one of which belongs to the head 
and three to the prothorax ; the smaller specimen shows only 
a trace of the horn on the head; the middle horn on the pro- 
thorax has disappeared entirely, while the two lateral horns are 
not merely absolutely smaller, hut much smaller in propor- 
tion. 

Darwin has adduced similar cases, and not only of insects, as 
Kramer seems to suppose, but of Crustacea, Spiders, Birds, and 


SELECTION A SECONDARY AGENT. 369 


Mammals. All these examples consequently display, in a con- 
spicuous degree, those very phenomena on which Kramer calcu- 
lates, but with the omission of an essential factor, namely, that 
very factor which, according to Darwin, is, under the form of 
‘ sexual selection,’ one of the chief agents in effecting the selection, 
i.e. the preservation of extreme and particularly favoured varie- 
ties. If, on the other hand, we take this factor into the calculation, 
the numerous cases so strongly insisted on by Kramer, of great 
constancy in the distinctive characters of male beetles, may be 
very well explained by Darwin’s principles. According to these 
the selection effected in these instances by the physiological 
bearings of the organs in question has already acted, and has 
selected, and consequently rendered constant, those species 
which had special advantages in the struggle which led to the 
selection ; while in those cases of still prevailing variability for 
which Kramer's calculation holds good, no such physiological 
bearings can have been at work. For it must not be forgotten 
that neither natural nor sexual selection can originate new 
characters, but can only come into play when some active 
mechanical causes have given rise to such modifications in 
organs already existing, as are capable of introducing some new 
physiological correlation, So long as this does not take place, 
the force which originally gave rise to the deviations, 7.¢. the 
variability, will still be able to act unchecked. Now the ques- 
tion as to how an old organ may come to have a new physio- 
logical value, in relation either to the other organs of the same 
creature or to the external conditions of existence, is evidently 
one of great importance; and this seems to me a suitable oppor- 
tunity for studying it with reference to an actual example, 
namely, the eyes recently detected by myself on the back of an 
Onchidium, a naked mollusc. 

It is universally known that almost all univalve Mollusca 
have two eyes either at the tip of the tentacles or at their base. 
These eyes are extremely different as to structure from those of 
the Vertebrata. Inall eyes, without exception, the optic nerve 
is gradually merged in a layer of tissue which includes the ter- 
minating fibres of the nerve and is known as the retina ; theso 
fibre ends are the rods and cones, or columnar layer. Jn Verte- 


17 


370 THE INFLUENCE OF LIVING SURROUNDINGS. 


brata (see fig. 98, 6) the optic nerve penetrates the outer skin of 
the eye, and spreads out on its inner surface between it and the 
lens in such a way that these ends of the nerve are turned away 
from the lens and thus have their free ends directed outwards, 


Fri. 8.—Sections of eyes (a) of a univalve. sis the layer of rods and cones enclosed in J, 
the fibrous layer of the retina, 6, the eye of a vertebrate animal at the spot where 
the optic nerve enters it. The nerve traverses all the layers and spreads out, forming 
the fibrous Inver £3 the colunmar layer lies outside it, and thus in the reverse 
position to what it occupies iti the eye of the mollusc. 


In the cyes found on the tentacles of snails (see fig. 98, «) these 
rods are in the contrary position ; the surface of the tips is turned 
towards the lens. Thus, in the former, the layer of rods and 
cones itself is pierced hy the opt’c nerve, and in that spot of 


DORSAL EYES OF ONCHIDIUM. 371 


course none can be formed ; hence vertebrate animals are blind 
in that particular spot, which is in fact known scientifically 
as ‘the blind spot.’ Such a spot is absent in the eyes of the 
second category ; the optic nerve extends over the outside of the 
eyes, and the rods and cones situated within cover the whole 
inner surface of the retina uniformly. 

So far as concerns the eyes placed on its head the genus 
Onchidium in no way differs from the allied groups. But the 
greater number of species in this genus are further distinguished 
by having other eyes situated on the shell-less but coriaceous 


Fic. 99.—Section of the dorsal eye of Onchidium verruculatum, letters as in fig. 98. 


back of the animal. These dorsal eyes (see fig. 99) are extremely 
interesting, for, simple as they are in structure, they are iden- 
tical in type with those of the vertebrata. A comparison of 
the two sections here given of the eye of a vertebrate animal 
and of one of these dorsal eyes will suffice to exhibit the 
resemblance; in both there is the ‘blind spot,’ because the 
optic nerve must pierce the external layer of the retina; and in 
both the layer of rods and cones forms the outer layer of the 
retina. This is the only example hitherto known of an eye: 
so constructed in an invertebrate animal. 


372 THE INFLUENCE OF LIVING SURROUNDINGS. 


It is evident that these eyes must be of the greatest im- 
portance in the life of the animal possessing them. In the 
first place it is simply inconceivable that well-developed eyes, 
capable of fulfilling their functions, should be useless. If eyes 
occurred in other univalves in the same place as in Onchidium, 
we should naturally think at once that the dorsal eyes of 
Onchidium were degenerate eyes; but they are found exclu- 
sively in this genus, whence we may infer with considerable 
confidence that they must have originated in it. But suppos- 
ing they were, nevertheless, rudimentary eyes inherited from 
extinct ancestors !73 of this family, they must in some way 
prove themselves to be such; we should expect to find some 
part absent—the lens, or the rods and cones, or the pigment of 
the retina. All these parts, however—recognised as being 
essential to the normal use of the eve—are present in the dorsal 
eyes of the Onchidium, and not in one species only, but in above 
twenty forms that I myself have examined. Finally, all these 
twenty different varieties of dorsal eyes represent an unbroken 
series, from those of very low development up to the highest, 
and they all exhibit the essential parts of a seeing eye, varying 
in arrangement, it is true, but quite normal in structure. This 
irrefutably proves that these eyes have originated independently 
in thefamily of Onchidiade, and that they are no doubt of 
great importance in the life of the mollusc. 

During many years of travel in tropical regions these eyes 
were perfectly unknown to me; but on other grounds I had 
devoted much attention to the mode of life of the Onchidia. 
They live exclusively on the seashore or in brackish marshes ; 
they creep along close to the edge of the water, hiding in clefts 
of the rocks or under large stones. Together with them, in the 
same spots, live numerous specimens of two genera of fishes, 
Periophthalmus and the nearly allied Boleophthalmus ; these 
skip along the strand with long leaps, evidently seeking their 
food, which, besides insects, consists principally of this very 
genus of mollusca. This, as it seems to me, affords a way of 
accounting—though only hypothetically, it is true—for the 
development of these dorsal eyes. The Onchidia are terribly 
slow creatures, perfectly incapable of escaping or of withdrawing 


ASSOCIATED WITH CONTRACTILE GLANDS. 373 


rapidly into a rift for shelter. They eat nothing but sand, 
which they shovel into the gullet through the mouth, as do 
some of the Echinoderms; of course they only digest the nutri- 
tions organic particles which are mixed with the sea-sand. 
Thus, in order to find suitable nourishment, they must often 
be exposed to the gaze of the swift fish that leap rapidly 
along the edge of the sea, as well as to that of other animals. 
Flee they cannot ; a house into which to creep—lke many other 
molluses living equally exposed—they have not; they have 
neither spines nor jaws with which to defend themselves ; and 
the eyes on their back, which are capable only of warning 
them of the approach of danger, do not avail to provide them 
with protection; in short, even with these dorsal eyes they 


Fig. 100.-- Periophthatmus Koelreuteri, a fish which pursues Onchidium—a land mollusc— 
on the sea-shore. The large ventral fins serve for a forward leap. 


seem to be perfectly defenceless against their pursuers. Still, it 
would certainly be very strange that such eyes should be deve- 
loped here, and only in this particular genus, without being 
qualified to be of any real advantage to them, since they cer- 
tainly cannot require eyes on their back—useful, no deubt, for 
looking up to heaven, but quite useless for looking down on 
earth—in order to find their food, which lies close before them 
in the sand under their mouth. 

Hence, if these eyes were in fact to be of some service to the 
mollusc, it must have been also provided with some sort of 
weapons, and in point of fact such weapons do exist in every 
species that has such eyes. The skin of the back is very thickly 
set with minute glands, of which the contents are not perfectly 


374 THE INFLUENCE OF LIVING SURROUNDINGS. 


fluid, but rather a kind of concretion, and the pores for its 
emission are excessively fine, so as to be hardly discernible. 
Moreover, these are closely surrounded by a circular muscle, so 
that by its contraction the openings of the glands are easily 
closed. Feeble contractions of the skin, such as inevitably 
occur in the act of creeping, cannot consequently express the 
minute globules of the secretion out of the glands ; the moisture 
cannot exude. But supposing that a Periophthalmus approaches 
suddenly and with rapid leaps (see fig. 100) ; it rises—as I have 
often seen—several inches into the air, and may thus not un- 
frequently throw a shadow from some distance off, on the back 
of the slowly creeping Onchidium, and of course alarm it greatly. 
The mollusc has all its eyes—and I have positively counted 
ninety-eight on one specimen—turned upwards in various 
directions; suddeuly aware of the fish, or of its shadow, it 
quickly draws up its whole body, thus contracting the glands in 
the skin on all sides with considerable force. Granting that 
this force is sufficient to express the globules of secretion from 
the pores of the glands, these, as the skin contracts, must inevi- 
tably be expelled from them ; instead of flowing over the skin of. 
the creature’s back, they will be shot into the air in hundreds 
—or thousands—towards the pursuing fish; the fish, now 
alarmed on its part, and hit by the shower of minute shot, 
which may be in some way injurious or offensive, retires from 
the pursuit, and the Onchidium is safe, 

Of course, as I have already said, this is merely an hypo- 
thesis ; the question nevertheless arises whether it may not be 
possible to show by indirect evidence that it is extremely 
probable. 

It would, no doubt, be quite conceivable that the Onchidium 
night be able to defend itself in the mode I have suggested, not 
merely against the Periophthalmus and Boleophthalmus, but 
against other foes. But if, as I believe, these two fishes are 
actually the only, or at any rate the most dangerous, enemies 
it has to dread, and if the eyes and glands thus serve as 
weapons of defence against these fishes only, we must expect 
that wherever these fishes occur Onchidia with dorsal eyes will 
be found. This is in fact the case. The Periophthalmus is 


DISTRIBUTION OF ONCHIDIUM. 375 


found in North Australia and the western portion of the 
Pacific, in the whole of the Chinese Seas as far as Japan, in the 
Indian Ocean and the Malayan Archipelago, and on the east 
coast of Africa. The Onchidia with dorsal eyes have precisely 
the same distribution. 

Only one exception existed until lately. On the coast of 
Congo, where, from Giinther’s catalogue of recent‘fishes, Peri- 
ophthalmus has long been known to exist, no Onchidium had 
been found, and even in the latest list of African land-mollusca, 
which we owe to the industry of Martens, no Onchidium is in- 
cluded. However, in answer to an inquiry from me, I was 
informed by that admirable malacologist that he was in posses- 
sion of several species which had lately been brought back from 
Congo by the German expedition to that coast. He was good 
enough to send me a specimen to examine; but unfortunately 
it reached me in so bad a condition that it was impossible to 
arrive at any positive conclusion with regard to the presence or 
absence of dorsal eyes. 

It therefore is possible that the Onchidium found on the 
West African coast may not have such eyes, and thereby a 
strong argument would be raised against the hypothesis I have 
put forward. For, according to that view, we should be inclined 
to regard the dorsal eyes as an organ indispensable to the genus, 
since by them only could its extirpation by the fish be pre- 
vented. However, there are spots where the fishes which we 
regard as the chief enemies of the Onchidium do not live, and 
where nevertheless the molluscs occur and are by no means 
rare. One species, long since described by Cuvier as Onchidium 
celticum, is found on the Atlantic shore of England and France ; 
another occurs in America, on the high northern coast; others 
again live on the west coast of both North and South America ; 
the Galapagos Islands have their peculiar species, and in the 
exstern parts of the Pacific, as in New Zealand and Australia, 
many species are found. In all these places the fishes are 
absent, and all the species of Onchidium here mentioned— 
almost all of which I have examined anatomically—are devoid 
of dorsal eyes, and at the same time of the glands which, acting 
as weapons, can alone serve to demonstrate the use of the dorsal 


376 THE INFLUENCE OF LIVING SURROUNDINGS. 


oyes themselves. Here, where the molluscs seem to be exempt 
from pursuit, the eyes and the weapon would alike be useless, 
and it is quite intelligible that they should not be developed on 
the back in these species. It is easy too to understand that 
they must have degenerated if ancestral Onchidia provided with 
eyes migrated to these regions, where, in consequence of the 
absence of the fishes, both the organs for defence and those for 
warning immediately ceased to be of use. In this way the 
absence of dorsal eyes in the species living in localities where 
there were no hostile fishes would seem to be a confirmation of 
the view suggested: That the eyes of those species furnished 
with them are of use in the way above described. One single 
difficulty, however, remains ; the West African Onchidia perhaps 
have no dorsal eyes, and one single species living in the Western 
Pacific certainly bas none, though it lives associated with those 
fishes, the hereditary foes of its race. But even this exception 
may easily be explained by a somewhat closer consideration of 
the structure of the genus, and of the mode of development of 
the dorsal eyes. 

In a former chapter I have already pointed out that every 
living cell or group of cells must possess every attribute of living 
protoplasm; they must be able to move or change their form ; 
they must be capable of assimilation, reproduction, respiration, 
and secretion, and finally they must be capable of elaborating 
external impressions, and, if I may say so, of transmitting 
them to their consciousness. We know, moreover, that a lens 
has the property of collecting in a focus the different rays which 
combine to make white light, and hence it follows of course 
that in every papilla of an animal’s skin, that is either spherical 
or formed on any other regular curve, a similar conver- 
gence of the rays falling upon it must ensue if these portions of 
the skin are only sufficiently smooth and transparent. The 
chemical rays or heat-rays which are thus concentrated by the 
papilla ou any point lying within the skin, must be able to act 
upon some of the cells they impinge upon differently to others, 
since the reaction of two contiguous cells must always be 
slightly different. Thus certain cells will be particularly stimu- 
lated to an increased exertion of the secretive action which is 


THEORETICAL DEVELOPMENT OF THE EYES. 377 


common to them all, and these will become gland-cells; while 
others will not be stimulated at all, or be modified in some other 
way. Now, supposing that the cells, as yet unmodified, and 
lying in the focus of the lens or the papilla, were to come 
into contact with a sensory nerve, they might easily be con- 
verted into true sense organ-cells, since they, as living cells, 
possess the inherent capacity of reacting on external impressions 
in the way to which we give the name of sensation; in the first 
instance, no doubt, these sense-cells could only transmit general 
sensations, and in this respect would at most be distinguished 
above the other contiguous cells by possessing that general 
sensibility in a somewhat higher degree. If now, in conse. 
quence of any influence, the cells of the epidermis subjacent to 
the lenticular prominence became surrounded with pigment, so 
that the rays concentrated by the lens could penetrate no 
deeper, it would seem that the first impetus would be given 
towards the development of a true eye. It may be assumed 
that this primitive eye would in the first instance only be capa- 
ble of distinguishing different shades of light and darkness—a 
full light from a shadow. From this, by itself alone, the 
animal would derive no advantage, since this eye, though sensi- 
ble of the shadow of a pursuing fish, would only give warning of 
the danger. But the same cause which originated the eye—i.e. 
the regular convex curving of the prominent part of the skin— 
will also have been capable of modifying some of the cells of 
that primitive cellular mass into gland-cells, and thus a weapon 
will have been formed. Not till then could Natural Selection 
step in with its stabilitating and extending influence, and develope 
from this simple eye, capable only of distinguishing light and 
shade, a perfected organ qualified to give an exact image of the 
fish in pursuit ; and the glands when necessary, in the same way 
could become more effective weapons than they were at their 
first origin. 

A pretty theory ! may perhaps be said—but unfortunately 
merely an hypothesis. Granted. Nevertheless, I was able, in 
the course of my investigations of the development of the dorsal 
eyes as it takes place in nature, actually to observe all the’ 
stages, just as I have here deduced them from an hypothetical 


378 TUE INFLUENCE OF LIVING SURROUNDINGS. 


construction of the possible process of development of such an 
eye from the simplest conditions. A short account of these 
observations will here be of service. 

All the species of Onchidium ob:erved by me, the blind as 


Fic. 101.—The development of the eye of Onchidiam. Above, to the left, the first stage; 
small vesicular cells («) close under the cpidermis cells of the prominent point of 
the papilla. Above (right) a larger mass of these vesicular cells, which gradually 
increases and grows spherical. Below (left), this ludy of homogeneous cells is 
enclosed in pigment layer, Below (right), the eyeball thus formed communicates 
with the optic nerve (op/), and its cellular mas3 has Leen differentiated into a large 
lens lying in front, and retina-cells behind it. 


well as those that can sev, are covered with a great number of 
tubercles of various sizes, of which the surface is everywhere 
curved very regularly and is at the same time quite smooth. 


The intervening portions of the skin of the back are, on the 
contrary, distinguished by much roughness and granulation or 


OBSERVED DEVELOPMENT. 379 


wrinkling ; hence in these intermediate spaces no uniform re- 
fraction of rays of light can take place in any one point, while 
it can in the smooth rounded papille. These vary greatly in 
size, and they constantly increase in number with the age of the 
animal. The smallest have beneath the cuticle, or outer skin, 
merely a simple cellular layer—the epithelinm—like all uni- 
valves. The next in size show, exactly in the centre of the 
papille, a cellular mass growing inwards and downwards from 
the epidermis (see fig. 101, the upper fig. to the left), in which 
one or two cells may already be discerned as the basis of future 
gland-ceils; in the next this group of gland-cells are pushed 
aside by another cellular mass proceeding in the same way 
from the epidermis at the summit of the papilla (see fig. 101, 
right, top fig.) and of peculiar aspect. Subsequently the inner- 
most cells of this last-named mass become conspicuously modi- 
fied, they increase much in size, their contents become peculiarly 
granular, and their circumference highly refractive, and then a 
fine nerve may be seen proceeding from the interior of the skin 
towards this cellular mass. In still larger papille we find 
roundish cell-bodies which are in direct communication with a 
nerve, and which at the back are already partly surrounded by 
pigment (see fig. 101, bottom, left fig.). At the spot where the 
nerve enters, and where the pigment layer is not altogether 
closed up, there are a few peculiar cells which are of precisely the 
same size and aspect as the cells above mentioned in the largest 
papille without pigment. Finally, the pigment layer closes 
round the central cellular mass, and the primitive rudimentary 
eye is complete. 

The structure is not, of course, thereby definitely completed. 
Very striking modifications now take place in the central 
bodies, composed of homogeneous cells and enclosed in pig- 
ment; one of them, lying nearest to the prominent surface of 
the papilla—the cornea—grows more than its neighbours ; soon 
others do the same, and a true lens, consisting of at least four 
cells, is thus formed. The still unmodified cells, lying between 
the lens thus formed and the pigment sheath, now are trans- 
formed into a retina of which the structure has been above 
described. The process and conditions here described, and con- 


380 THE INFLUENCE OF LIVING SURROUNDINGS. 


firmed by actual observation of the development of one of the 
most highly developed dorsal eycs of the Onchidium, coincide— 
as I think must be allowed—with the phases of development 
previously set forth on hypothetical grounds as being those 
proper to the formation of an eye, if we assume that by the 
simple property of a regularly curved and smooth surface of a 
ppilla of the skin, all the light, heat, and chemical rays of a 
beam of light must be made to converge in one point. 

If the foregoing chain of argument is correct, as I cannot 
doubt, we have here detected an organ of extremely complicated 
structure in the very process of formation; and this instance 
proves, if indeed further proof were needed, that, as Darwin has 
often insisted, an organ can never be ereated by natural selec- 
tion, but can only be modified and improved by it. Sometimes, 
no doubt, expressions are used, even by naturalists, which 
might lead us to suppose that they were of opinion that an 
organ might originate from its use—thus, in this case, an eye 
from the act of sight; this, of course, is absolutely erroneous. 
Sight, on the contrary, cannot occur till from other causes— 
as in this instance the direct stimulus to the skin—those parts 
have been formed which must be in existence before sight is in 
any way possible. Tere, in the dorsal eyes of Onchidium, in 
’ the first instance it was the concentration of light on a certain 
point within the skin, which was occasioned by the form and 
structure of the papille, and the consequent modification of 
certain cells, which gave rise to a primitive organ of sight, and 
this organ was capable of still further development through 
natural selection, since, from the very first, it contained within 
itself the elements of further perfectibility and modification. 

It follows, moreover, that if at any time such a primitive eye 
were developed in an animal which was not exposed to this 
process of natural selection by its association with a fish that 
preyed upon it, the organ, being useless, would disappear or at 
any rate degenerate. And this actually is the case, as we have 
seen; for all the species of this genus which live where the 
Periophthalmus does not, are devoid of these eyes. Only oneas- 
cortained exception is as yet known—the above-mentioned species 
of Onchidium which lives associated with species that can see 


FORMATIVE AND SELECTIVE ACTION, 381 


and with the preying fish, though it is itself blind. But this 
species is found exclusively on the outer limits of the region 
inhabited by Periophthalmus, namely, on the south-east coast of 
Australia and in the central region of the Pacific. Also the 
species spoken of as living on the West coast of Africa, and 
which is probably blind, belongs to such another frontier dis- 
trict. Now it is, of course, not much to be wondered at, that 
in these frontier districts a blind form should occur among 
those that can see. But there isa much more striking argu- 
ment for allowing this exception again to serve as confirming 
the rule. While all the truly blind species of those regions 
whence Periophthalmus is absent, are devoid, not merely of 
developed eyes, but of the first elements for forming them, and of 
the accompanying weapons—having no dorsal glands whatever, 
since without these the eyes would be useless—the blind species 
of the Pacific has both the glands and the cell-groups inside the 
papille agreeing precisely in structure with a middle stage in the 
series exhibited in the formation of the very highly developed 
eyes of Onchidium verruculatum. Thus here we seem to have 
a species in which either eyes formerly existing have begun to 
disappear from desuetude, or the construction of a true organ 
of sight has been begun by the coincidence of the three factors 
which would give rise to it.!*4 

I have treated of this example somewhat in detail, on purpose 
to define as clearly as possible the limit-line where the external 
modifying causes which we discern as giving rise to a new 
organ—or rather as transforming one already existing into 
another—cease to be effective, and where those selective influ- 
ences begin to act which determine the further perfecting of a 
newly developed organ. 

I am, moreover, convinced that in every case, by a corre- 
sponding method of research, the same limit-line will be easily 
detected. That it is not yet recognised, nor even in most cases 
supposed to exist—so that it is often difficult to avoid a con- 
fusion between the transforming causes and the purely selective 
ones—is, in my opinion, entirely due to our having hitherto 
been satisfied with mere general speculations on Darwin’s theory, 
and having neglected to investigate the innumerable problems 


382 THE INFLUENCE OF LIVING SURROUNDINGS. 


which it suggests, but which can only be solved by experiment. 
I shall here discuss another important instance of the same kind 
which is very much to the point—the Means of Protection, 
namely, bestowed by nature on both pursuers and pursued to 
enable them to attain their end, that is to say to capture their 
prey on the one hand, or to escape capture on the other. 

Protection by imitation of surrounding objects.—It is well 
known that a great number of living animals are enabled to 
‘escape their pursuers by their more or less strong resemblance 
in form and colour to the objects among which they live, in 
which case the resemblance is protective; while others, on the 
contrary, are specially qualified by the same circumstance to 
pursue their prey with success. Thus a crowd of new agents of 
selection among forms are added to those already discussed in 
the former chapters. For it is self-evident that every alteration 
that takes place in the co-ordination of the conditions which 
surround any given species of animal must deprive it of the pro- 
tection it derives from its resemblance to a particular plant, let 
us say, if that very plant is exterminated ; and in the same way 
that an accident of colouring, which has hitherto occasioned no 
special resemblance to any object, may suddenly become a 
powerful instrument in facilitating attack or self-defence. Thus 
a selection will be effected between different forms which had 
previously been equally protected.!?° 

It is, of course, impossible to investigate in this place the 
greater number of the known cases of such protective resem- 
blance, and it will suffice to discuss a few of the most instructive 
examples. I shall here adopt the arbitrary division of these 
cases into two classes, which has become general: into those, 
namely, of the resemblance of animals to inanimate ohjects—as 
stones, sand, the soil, and living plants—and those of resem- 
blance to other living animals; bub merely for convenience sake, 
since [ can find no principle on which such an artificial division 
can be based. 

It has long been a well-established fact that very many 
animals are effectually protected against their enemies by their 
resemblance to stones or sand, lichens, leaves and twigs; and 
every one who has at any time been engaged in collecting 


PROTECTION BY MIMICRY. 383 


insects is familiar with numerous instances of this kind. Many 
birds and quadrupeds that are regularly hunted by men have 
become extremely rare in many places, because, though they are 
to a great degree protected against their enemies among other 
animals, by the resemblance of the colours of their feathers or 


Fic. 102.—Grasshoppers that are protected by their resemblance to leaves. a, Phyllium 
siccifolium, feeds on leaves and mimics fresh leaves. 6, Acanthops sp.. one of the 
Mantide, feeding on creatures which it captures among dry vegetable matter ; itexactly 
mimics dry leaves. From the Philippines. Half natural size. 


fur to the objects among which they live, man can employ a 
variety of means of attack or pursuit against which the protec- 
tion of resemblance is ineffectual. The zoologist who should 
attempt to capture the perfectly transparent creatures which 
swim at the surface of the sea without using a net for straining 


384 THE INFLUENCE OF LIVING SURROUNDINGS. 


the water, would certainly catch but a very few individuals 
with a glass vessel, for only under the most favourable circum- 
stances would they be visible to him. A resemblance of colour 
to that of various parts of plants is in many cases increased in 
efficacy by the habit many creatures have of squatting close 
and motionless when they are pursued, so that their resemblance 
to a leaf or branch ig greatly increased. The caterpillars of 
several species of butterflies are familiar to every one, as well 
as the ‘ dry leaf insect’ (see fig. 102); this belongs to the class of 
leaf-eating grasshoppers, to which also belong the Phasmide, 
walking-stick insects (see fig. 32). They are perfectly harm- 
less, and their resemblance to the objects which surround 
them is evidently only a means of escape from their pursuer. 
It is quite otherwise among the predatory Mantide, the best 
known of which is the ‘ praying mantis’ (Afantis religiosa). In 
the accompanying cut I have represented a species of the genus 
Acanthops which has an extraordinary resemblance to dry 
leaves, but in this case the resemblance must be available in 
facilitating attack. Thus the same character may conduce to 
two different ends, attack or defence. 

Besides the cases of protective resemblance in form and 
colour, which I have here briefly indicated, there are others 
which we might feel disposed to regard as the exaggeration of 
such a means of protection. In all the instances here mentioned 
the character of the form and colouring serves for purposes of 
concealment, irrespective of whether the ultimate end is offen- 
sive or defensive. But in many instances of brilliant and 
conspicuous colouring the case is quite otherwise, particularly 
among insects ; their colours are so splendid, and the markings 
on the wings or body so striking, that they must inevitably 
attract the gaze of every insectivorous creature. Thus they 
would seem to be actually forced on the attention of their ene- 
mies, and itis probable that not one of these vividly painted 
forms could long escape annihilation if they had not some other 
means of protection; but this, in fact, seems to be invariably 
the case, as we learn from Wallace’s admirable and exhaustive 
researches into this subject. Thus the gaudily striped bees and 
wasps have a sting connected with a poison-gland; other insects, 


SELECTION IN COLOURING. 335 


as the stingless Chrysidide and many proboscidian beetles, are 
protected by a strong coat of mail; bugs, lady-birds, and many 
butterflies have dermal glands from which the secretion—as 
every one knows in the case of the bug—is excessively objec- 
tionable to the pursuer, or even in some way injurious; others 
can escape pursuit by extreme rapidity of flight, while others 
again assume a peculiar posture by which—as it would seem— 
they can actually frighten away their enemies. It is in connec- 
tion with these facts that, according to the statements of this 
distinguished naturalist and of other inquirers, such brilliantly 
coloured insects are usually, if not invariably, avoided by the 
generality of insect-eaters ; birds—as well as frogs and lizards— 
showing a preference for the dull-coloured over the gaily 
colouted species. This view is strengthened by tke fact adduced 
by Wallace that those insects or larvae which are inconspicuous 
in colour are commonly devoid of any kind of defensive weapons 
such as are found in the more splendidly coloured species. 
Hence, according to Wallace, the use of the bright colours is 
evident ; the creatures which pursue these insects soon learned 
by experience, and communicated to their progeny, the useless- 
ness of pursuing these harlequin insects, as any attempt to 
attack them might be bitterly avenged. 

At the first glance this view seems a striking one. It may 
be the correct one in many or all of the cases here adduced, but 
it is certainly open to doubt whether it can be unhesitatingly 
applied—as has sometimes been done—to every case of splendid 
colouring in the skin of animals. Darwin has already raised 
this question as against Wallace, and he proposes to substitute 
the view that all or most cases of brilliant colouring have 
originated from a variety of natural selection which he terms 
Sexual Selection, According to this view, colouring has resulted 
from the determining selective influence of the sexes and their 
preference for certain colours and modes of colouring. Still, 
Darwin himself had already mentioned, though only inciden- 
tally, that there are many animals characterised by their 
splendid motley or metallic colouring which could not have 
preserved it through sexual selection; for example, all the 
different Polypes, and more particularly the sea-anemones and 


386 TUE INFLUENCE OF LIVING SURROUNDINGS. 


true corals, are conspicuous for their colours. The surface of a 
reef lying just under water has often been compared to a gay 
garden of flowers, and the splendour of such a ‘bed’ of animals 
is in fact quite astonishing. It is as though Mother Nature had 
here given free play to the fancy she is elsewhere compelled to 
restrain in some degree, by indulging her delight in lavishing all 
the colours of the rainbow, and by inviting a motley company 
of creatures to disport themselves among the flowers and fruit 
of her submarine garden—blue and red star-fish, Holothurie of 
every hue, and gaudily painted fishes. 

The fishes, in which the sexes are separate and which swim 
about freely, may perhaps have preserved their brilliant colour- 
ing by sexual selection, or even in the way put forward so 
emphatically by Wallace; but neither hypothesis suffices as a 
satisfactory explanation of the equally bright colours of polypes. 
No kind of sexual selection can here come into play, for the 
simple reason that the sexes do not seek each other ; they are all 
sessile animals, male and female alike, and are obliged to throw 
off the sexual elements into the water and leave it to chance, or 
rather to the currents, whether fertilisation is effected or not. 
Wallace's explanation is equally inapplicable. All polypes 
are predatory creatures, feeding on fishes, crabs, worms, &c. ; 
hence their striking and ornamental colouring would seem 
rather to be a disadvantage to them, for, since they cannot move 
about, they are fitted to catch such animals as approach too near 
to them with their long arms and the weapons with which these 
are furnished; and their colouring, therefore, would seem cal- 
culated to warn all creatures swimming in the sea, even at a 
distance, against coming within range of their perilous embrace. 
This apparent disadvantage might perhaps be outweighed by a 
greater advantage connected with this bright colouring, namely, 
that it warns the fish that prey upon them not to approach— 
which, of course, presupposes that those enemies have real 
cause to dread the weapons of the polypes. This, however, is 
by no means the case ; the fishes which feed on the true corals 
—as the Scarideamong the Labride (Wrasses) and the Dz- 
dontide among the Plectognathi—are perfectly indifferent as to 
whether the creatures they feed on try to clutch them with 


USE OF COLOURING OFTEN UNKNOWN. 387 


their tentacles or pierce their skin with their microscopic dart- 
like stinging-threads. It is impossible—so far as our present 
knowledge extends—to discover the faintest trace of usefulness 
in the brilliant colours of the polypes, and it is highly probable 
that they are in fact perfectly unimportant as regards the 
selective influence caused by their reciprocal relations with 
other animals. To this example of a mode of colouring which 
is insignificant with regard to selection, we might add many 
others, particularly of invertebrate animals; and the question 
even arises whether in many cases the distribution of colours, to 
which we are at present disposed to attribute a marked value 
in the process of selection, may not be considerably overrated 
in this respect. 

But it follows from all this that not the cclour or pigment 
itself merely, but its distribution —z.¢, the markings of the animal 
—may under certain circumstances have been produced by other 
causes than those on whose effects selection seems to depend. 
It is perfectly evident that under no conceivable circumstances 
can the pigment, the colouring-matter itself, originate from 
selection ; this point has already been gone into in Chapter ITI. 
(see pp. 99 and 115). It was there shown that the origin of the 
pigment must depend on physiological processes acting in the 
body of each individual, and which seem to be of the greatest 
importance to the healthy life of each. Hence the particular 
mode of its distribution throughout the skin must in the first 
place be the result of causes acting entirely in the animal itself, 
perfectly regular from the very first, or, it may be, wholly 
irregular ; and this will depend on whether the internal physio- 
logical causes have determined the deposition of the colouring- 
matter in the skin in a certain regular order or no.!76 If this 
order is very sharply defined, the distribution of colour must, of 
course, be extremely regular, and many of the characteristic 
markings in Actinie, corals, and the shells of Mollusca may 
have arisen in this way. But, on the other hand, selection 
can control this colouring of the skin, and can confirm any par- 
ticular arrangement which is especially advantageous to the 
creature in the struggle for existence, can make it more regular 
or enhance its brilliancy. The possibility that selection may 


388 THE INFLUENCK OF LIVING SURROUNDINGS. 


control a mode of colouring that has originated by a physio- 
logical process, by no means proves that in every case without 
exception the distribution of colouring in the animal world 
must have originated in the same manner, any more than the 
established fact that chlorophyll is formed in most green-leaved 
plants under the influence of light, alters the other fact that in 
certain cases, as in the Conifers, the same matter can be elabo- 
rated in the dark. 

A very striking instance of protective colouring is exhibited 
in the so-called Chromatic Function which has only recently 
been made the subject of exact investigation. It has already 
been discussed in the chapter on the influence of light (see 
p- 92 and note 7), It consists in the power possessed by many 
Fishes, Crustaceans, Amphibia, and Iteptiles, of adapting their 
general colowing—often by extremely rapid alternations—to 
the colouring of the surrounding objects, so that they seem to be 
helped by it in the pursuit of their prey, or especially protected 
against the attacks of their enemies. 

Hence it is perfectly evident that all such adaptations of 
colouring to that of surrounding objects must be a powerful 
instrument of seiection. Those individuals which are_ best 
qualified in this respect must have a conspicuous advantage 
over their less well-fitted companions in the struggle for exist- 
ence. Thus every cause which might give a species the capa- 
bility of rapidly assuming certain changes of the colour of the 
skin bya contraction of the chromatophores, would indirectly be 
the cause of a further perfecting of this capability by natural 
selection ; but this selection could not come into play till the 
contractile power of the chromatophores was actually existent 
and a protective mode of colouring was already produced by it. 
Neither selection, nor the struggle for existence, could in this 
case, any more than in any other, by itself effect a modification of 
the functions or of the morphological peculiarities of an animal. 
And the same is evidently the case in those instances, now to 
be considered, of protective resemblance in form and colour 
which is commonly known by the name of mimicry. 

The mimicry or imitation of one animal by another.—Bates 
and Wallace gave the name of mimicry to all those cases of 


MIMICRY IN INSECTS. 389 


protective resemblance in which a creature, otherwise defence- 
less, imitates the form and colouring of another which has 
some special means of protection, and thereby, as is probable, 
escapes the pursuit of its enemies more easily than it could 
without such a disguise. But here again the protection 
obtained may benefit the pursuer as well as the pursued; the 
former by disguising it in the eyes of the alert prey, the latter 
by protecting a defenceless animal which mixes with the better- 
armed species whose aspect it has borrowed. I need not insist 
once more that the words here used must be taken in a figura- 
tive sense, since it is clear that no animal can ever be capable 
of designedly mimicking another. 

We owe the most important researches that have yet been 
made on this interesting subject to the above-mentioned 
judicious and acute travellers. No doubt we have long been 
acquainted with insects living here, in Europe, which in form, 
colour, and mode of flight bear a great resemblance to others of 
different species; I may mention the Sesi# among the butter- 
flies, which greatly resemble bees, an owe to this resemblance 
many of their specific names.!?7_ Formerly, little attention was 
paid to this circumstance ; at most it was incidentally noted that 
those butterflies were apparently protected by this resemblance, 
but any attempt to explain this mimicry was never even 
thought of before Darwin and Wallace; and it was partly the 
new views which, from Darwin, rapidly extended among z0o- 
logists, and partly the vast number of striking examples of 
such resemblances in tropical Brazil, which led Bates in the first 
instance to examine the relations of these cases more exactly, 
during his many years’ residence in South America. 

A brief enumeration of the most important examples of 
such mimicry will here be desirable ; but a complete list is all 
the less necessary because the labours of Wallace, Bates, and 
Trimen are easily accessible, and popular essays on the subject 
have appeared in many periodicals. Among the American 
butterflies the species of Leptalis, Hrycina, and Ithomia mimic 
the Heliconiade, which are distinguished by a sharp and un- 
pleasant smell. In the same way the Danaide and Acreide of 
the eastern regions of tropical America are protected hy foetid 


390 THE INFLUENCE OF LIVING SURROUNDINGS. 


glands, and here again there are certain species of these 
families which are imitated by the defenceless species of Papilio 
and Diadema. In North America, Dunais archippus, a very 
common butterfly, is closely copied by Limenitis archippus ; 


vy nh i ‘A 


Fic. 103.—a, Doliaps sp.mimics b, Pachyrhunchus orbifer ; c, Doliops curculionoides mimics d, 
Pachyrhynchus sp.3; e, Seepastus pachyrhynchoides (a grasshopper) mimics f, Apocyr- 
tus; g, Doliops sp. mimics h, Pachyrhynchussp. ; t, Phoraspis sp, (a grasshopper) mimics 
k, a Coccinella. All from the Philippines, of nat. size, It is evident that the great simi- 
larity of the creatures to those they mimic is less conspicuous in the engraving than 
in real life, since the exact correspondence in the colouring cannot be given here. 


species of Sesiw and of .Zyertidea so closely resemble small 
wasps that every one fears to handle them, but they have no 
sting like wasps, and are in every respect perfectly harmless. 
Among beetles, the Hispide and Humorphide, which are pro- 


IN SNAKES 391 


tected by feetid glands, are imitated by various species of stag- 
horn beetles ; other tropical staghorn beetles look extremely 
like certain Curculionide—the Pachyrhynchide (sea fig. 103)— 
which have an integument so hard that insect-eating birds 
avoid them, probably on that account; other beetles, as, for 
instance, Charis melipona, resemble true bees; Odontocera 
odynerovdes resembles a wasp of the genus Odynerus ; the grass- 
hopper Condylodera tricondyloides is wonderfully like a beetle, 
Tricondyla of the family of the Cicindele. Many flies are 


Fie. 104,— Spiders which mimic ants and live associated with them ; it is very difficult 
to distinguish them. 


very like wasps ; spiders which live associated with ants have 
assumed the form and colour of the ants (see fig. 104), and Bates 
mentions a singular instance when a large caterpillar fright- 
ened him extremely by its extraordinary resemblance to a 
poisonous snake. Even among Vertebrata, such cases are not 
rare. Wallace tells us that several species of the poisonous 
genus Elaps (snakes that are common in Brazil) are closely 
imitated by quite harmless snakes—thus laps fulvius is 
copied by Pliocerus equalis; a variety of Hlaps corallinus— 


392 TINE INFLUENCE OF LIVING SURROUNDINGS. 


known as the coral snake—by Homalocranium semicinctum ; 
Llaps lenniscatus, of which the bite is said to be absolutely 
fatal, by Pliocerus elapoides ; and all again areimitated by various 
species of the quite harmless genus Oxryrhopus, which live asso- 
ciated with the poisonous kinds. The two cases, communicated 
by Wallace, of birds which mimic other birds, seem certainly to 
come under this head; the Tropidorhynchus, dreaded for its 
strength, is mimicked hy the helpless J/imeta, and Acciptter gale- 
atus, a bird of prey which feeds on other birds, exactly resembles 
the imsectivorous vulture Harpagos, wherever the two species 
occur together. But the only instance adduced by Wallace of 
mimicry among quadi upeds—the resemblance of an insectivorous 
Cladobates to the squirrels (rodents)—seems to me, on the other 
hand, to be included with less reagon in this class of resem- 
blances. The assumption seems to me without foundation that 
the squirrels are harmless creatures and cannot alarm the 
insects around them by their movements, so that the Insectivora 
which resemble them easily capture their food. The European 
squirrel, at any rate, is omnivorous, as are many rodents; ana 
granting even that they never eat insects, it does not appear to 
me to be by any means established by observation that insects 
would remain motionless when the squirrel, as he leaps from 
bough to bough, shakes every leaf and twig. 

Thus, omitting this case of the imitation of a squirrel by an 
insectivorous animal, the cases of mimicry that are here men- 
tioned seem to be well established on the whole. In each case 
it can be shown that the mimicked species is in some way very 
effectually protected by an offensive smell, weapons of some 
kind, a hard skin, a powerful frame, &c., while the mimicking 
form is, without exception, weak and devoid of defence, so as to 
be greatly in need of protection. It can, moreover, be proved 
that in many cases, if not in all, the mimicking species live 
associated with those they resemble. 

From the facts thus established by observation, Bates and 
Wallace argue as follows. They show that all mimicking 
forms have acquired, by their disguise, an undeniable advan- 
tage in the struggle for existence over those less well equipped, 
since, in consequence of the disguise, either they escape 


IN LAND MOLLUSCA. 393 


notice by the animals they pursue, or they are no longer 
liable to pursuit, because the predatory species to which they 
might perhaps afford a dainty morsel regard them as being— 
like the creatures they resemble—bad eating or even injurious. 
In this case the prey deceives the pursuer ; in the former case, 
on the contrary, the pursuer deceives the prey. The mode of 
origin of this wonderfully strong protective resemblance can be 
explained by the well. known principles of selection ; protective 
resemblances, at first small, have been developed by elimination 
toa greater, and at last to a perfect pitch of mimicry in form 
and colour, and also in mode of life. This theory seems 
extremely plausible, and I believe that in many cases it is the 
right one; whether it is in all is another question. Under no 
circumstances can this theory account for the first appearance 
of the resemblance, as seems to be tacitly assumed by many 
writers. But before I enter on any further discussion of this 
point, I will describe a few new cases of mimicry observed by 
myself. 

True cases of mimicry among Mollusca have not yet—so 
far as I know—been observed, although instances of protective 
resemblance are not rare even among them. This is surprising, 
since we might suppose that mimicry might originate where 
a protective resemblance to inanimate objects or plants already 
existed, for there seems to be an @ priori probability that 
mimicry may have been developed from this. Perhaps this and 
other gaps result from our very meagre knowledge of the 
habits of life of the animals, particularly the invertebrate 
animals, of other countries. 

Before describing the cases observed by me of mimicry among 
land mollusca, however, I must make a few remarks on the 
system of classification of land mollusca now in vogue. The 
system according to which they are classified is based almost 
exclusively on the practical requirements of the collector, ¢.e. on 
the comparison of the empty shells; on the other hand, the 
investigation of the animals themselves has until quite recently 
been very much neglected. But the anatomical researches carried 
on during the last ten years—to which I believe I have contri- 
buted a no inconsiderable share—prove that the shells of such 


18 


394 THE INFLUENCE OF LIVING SURROUNDINGS. 


land-snails as have, like the great vino snail (Helix pomatia), 
a wide mouth to their shells are extraordinarily variable. 
Genera which had been constituted mercly on our knowledge of 
these variable shells, such as Helix, Bulimus, Vitrina, Nanina, 
&e., have proved quite untenable, and we now know that species 
which, by a comparison of the animals, must be placed actually 
in different families, often have shells so exactly alike that 
conchologists and paleontologists—the latter having, of course, 
nothing but the shells to judge from—have placed them in the 
same genus. 

These recent investigations have, moreover, proved that the 
great majority of the genera of land mollusca have very narrow 
limits of distribution, so that with regard, for instance, to the 
numerous shells resembling Vitrina which have hitherto been 
described, their local origin supplies a far surer index as to 
their affinities of relationship to this or that genus than the 
characters of the shell itself. Setting aside a small number of 
cosmopolitan and for the most part minute forms, most of them, 
and particularly the larger kinds, are highly characteristic of 
the different countries where they are indigenous. Thus the 
three genera, Cochlostyla, Helicarion, and Rhysota, are quite cha- 
racteristic of the Philippines ; for only a few of the species extend 
into the neighbouring islands of the Moluccas, while they occur 
in a very great variety of differently characterised species in the 
Philippines themselves. Pfeiffer, who as a conchologist was 
beyond a doubt the highest authority, included the species of 
Cochlostyla in three different genera, and those of Helicarion in 
two; but anatomical investigation has proved to me that the 
species of these three genera, in spite of the great diversity of 
their shells, are quite as much alike as the different races of the 
Germanic or Romance nationality. 

The species of these three characteristically Philippine genera 
are mimicked in a very remarkable manner, both in form and 
colour, by species of other genera which are not characteristic of 
the Philippines only, but of the neighbouring groups of islands 
as well; one of these cases is, beyond a doubt, one of the most 
striking instances of true mimicry. 

The animals of the species of Helicarion (see fig. 105, ¢)— of 


MOLLUSCA OF TIE PHILIPPINES. 395 


which the nearest allies are found in Australia and the islands 
of the Pacific—are easily recognisable at the first glance by the 
mantle lobes, which cover the thin transparent shell, and by their 
remarkably long, narrow, high-ridged tail, which ends abruptly 
in a gland; a kind of horn, sometimes of some length, projects 
from the tip of the tail. The numerous species—of which the 
various distinguishing characteristics are much more conspicuous 
in anatomical details than in the shell—live on trees in damp 
woods, oftenin great multitudes ; they ave very active and creep 
about with considerable rapidity upon the twigs and leaves of 


Fic. 103.—a, Rhysota Antonii, a land mollusc of the Philippines, mimicked by }, Xesta 
mindanaensis, which lives associated with it; ¢, MZelicuron ligrinus, mimicked by 
Xesta Cumingii from the same locality. Half nat. size. 


the trees. Every species that I personally examined possessed 
the singular property, which many lizards have—particularly 
the Geckoes—of shedding their tail when they are seized some- 
what roughly, at a little way behind the shell. This they do by 
whisking the tail up and down with extraordinary rapidity, 
almost convulsively, till it drops off; if the creature is held by 
the tail, it immediately falls to the ground, where it easily hides 
among the leaves. If it is laid flat on the hand, the rapid 
wagging movement is strong enough to raise the body with a 


396 THE INFLUENCE OF LIVING SURROUNDINGS. 


spring into the air, so that it falls over on to the ground. 
These snails at first constantly escaped me and my collectors in 
this way, and not unfrequent'y we had nothing but the tail left 
in our hand. According to Guilding’s observations the same 
peculiarity of parting with the hinder prolongation of the foot 
characterises the West Indian snail Stenopus. I ascertained hy 
further investigation that in a freeestate of nature such self- 
mutilation not unfrequently occurs, for out of about a hundred 
specimens of [elicarion gutia, which is extremely common in 
the north-east of Luzon, I found perhaps ten individuals that 
had shed their tails, or, to speak more accurately, the hinder end 
of the foot, and had the stumps partly healed, or the foot to 
some extent grown again. Now, this hinder portion of the foot 
is the most conspicuous part of the snail’s body, and it may be 
supposed that it is, in most cases, the part first seized by the 
reptiles or birds that prey upon them; but, startled by the 
escape of the body itself, they would soon learn to recognise, by 
the form of the tail, those species which were capable, by this 
self-amputation, of depriving them of the larger and probably 
the only valuable portion of the prey. In this way the species 
of the genus Helicarion can escape the pursuit of their enemies 
better than they otherwise could on account of their exposed 
mode of life, and it is in agreement with this fact that, in the 
spots where they occur, they are commonly to be fownd in 
numbers together. But other Jand-snails which are not in fact 
protected by any such peculiarity might very well be equally 
effectually protected by mimicry of the appearance of a Heli- 
carion, since they might thus be mistaken for them. 

One single species of snail does, in fact, exist in the Philip- 
pines which actually bears an extraordinary resemblance to the 
species of Helicarion, although in internal structure it is, syste- 
matically speaking, very far removed from it, and does not even 
belong to any of the genera which are characteristic of these 
islands; this is Vitrina Cumingit, a species long since known as 
having been discovered by Cuming in Mindanao (see fig. 105, ). 
An examination of the living animal convinced me that it 
belonged neither to Vitrina nor to Helicarion, but to a 
thoroughly Indian genus, -Vesta, of which the very numerous 


XESTA AND IELICARION, 397 


species which live in the islands of the Malayan archipelago are 
distinguished by their very thick and brilliantly coloured shells. - 
Nesta Cumingit has, on the contrary, a thin transparent shell, 
which during life is covered by the lobes of the mantle 
in the same way as the species of Helicarion ; indeed, it is on 
the ground of the form and texture of the shell that it has 
hitherto been placed in the genus Vitrina. I+ has, moreover, 
the long foot which distinguishes Helicarion, and, in fact, I 
supposed this species to be one of that Philippine genus, until I 
had examined it anatomically, and so had convinced myself that 
it belonged to the Indian genus, though it has almost no exter- 
nal resemblance whatever to the other species. Thus Xesta 
Cumingti has assumed to a very great extent the appearance of 
a Helicarion; moreover, it lives,as I can aver from my own 
experience, in precisely the same spots as Helicarion, namely on 
the upper side of leaves in daimp woods, and so mixed with that 
genus that it frequently happened that I captured a specimen 
of Xesta when I thought I had a species of Helicarion. But 
it does not possess the power of self-amputation which is charac- 
teristic of that genus, and hence, when caught by the tail, by a 
snake or any other creature, it cannot escape in the same 
manner. 

It follows from all this that we may assume on good grounds 
that we have here a case of true mimicry—at least it seems 
hardly possible to think that it is only a singular coincidence. 
Even Wallace’s test and criterion of true mimicry is perfectly 
applicable here; while the model form (as we may call it) 
which has a real means of protection is extremely common, the 
imitating and defenceless form is found in solitary specimens. 
Hence, the question arises, whether, perhaps, there may not also 
be in the West Indies land mollusca mimicking the Sternopus— 
which is provided with the same means of protection as the 
species of Helicarion in the Philippines—so as to be protected 
in the same way as Xesta Cumingii. If this should prove to 
be the case, it would afford, as it seems to me, a strong argument 
for the accuracy of the view here put forward, that esta 
Cumingit is, in fact, effectually protected by the disguise it has 
assumed in imitating the species of Helicarion.1%8 


398 THE INFLUENCE OF LIVING SURROUNDINGS. 


Moreover, there are very many instances of similarity of 
colouring between two creatures vey remote from each other, 
and in which it is very difficult to discover any relations 
between the two animals thus characteiised. Thus, many 
Annelida, Mollusca, Planarie, and Ophiuride live on the stocks 
of the keratose corals, which they resemble greatly in colouring 
though not in form. In the same ‘way all sorts of creatures 
may be found, on the disks of star-fish and Comatul or on the 
spherical] shells of Echinide, which have perfectly assumed the 
colour of the animal on which they live. Here, certainly, we 
cannot speak of mimicry in the strict sense ; it is far more pro- 
bable that this resemblance serves only to enable these creatures 
to escape detection, living, as they do, exposed to a certain 
degree of danger on the surface of others. But, in the case I 
shall now describe, it appears to me that it can be of no use 
even in this way. 

Together with Xesta Cumingii a second species of the same 
genus lives in Mindanao which differs quite as much from Nesta 
Cumingii as this species does from those inhabiting Java or the 
Moluccas. On the other hand, its shell looks exactly like those 
of the species of Rhysota, w genus in the highest degree charac- 
teristic of the Philippines (see fig. 105, a), These have shells 
of a uniform brown colour, often wrinkled and somewhat 
depressed, and not overlapped at all by any marginal develop- 
ment of the body—the lobes of the mantle. The foot is flat, 
broad and short, and bears at the end u gaping gland. All the 
species of Rhysota live on the ground under trees; and when, 
on particularly damp days, they quit the ground, they never 
climb trees, but only low plants growing in deep shade. It was 
from its sharing these characteristics that I regarded the new 
Xesta—which I called Xesta mindanaensis, from the locality 
where it exclusively occurs—as being a true Rhysota till I had 
the opportunity of exaniining it anatomically. 

Nesta minlanacnsis and Yesta Cumingit ave the only two 
species of this Indian genus that are widely distributed in the 
Philippines ; the latter is found from Mindanao as far as Bohol 
and the southern part of Leyte; the former occurs exclusively 
in Mindanao, the southern island. But while their nearest 


FALSE INFERENCES FROM FOSSIL SHELLS. 899 


allies in Celebes, the Moluccas, Java, and elsewhere, exhibit all 
the typical Indian characters both of the shell and of the animal, 
both the species that have migrated into the Philippines are so 
completely metamorphosed as to the form and colour of both 
animal and shell that it would have been impossible to guess, 
without the closest anatomical investigation, that they did not 
belong to the genera proper to the Philippines. With regard 
to Xesta Cumingit the explanation is available that here we 
have a case of true protective mimicry ; but how can we account 
for the even more striking resemblance between Yesta minda- 
naensis and a Rhysota? This resemblance cannot certainly be 
regarded as protective mimicry, since the species of Rhysota 
have absolutely no peculiar property by which they are better 
protected against their enemies than other mollusca, and conse- 
quently an imitation of their appearance can be no sort of 
advantage to the other imitative species. 

The difficully which is evident in the case just described is 
still further enhanced by the fact that other similar cases are 
known, and actually in the Philippines. By far the greater 
number of the brilliantly and variously coloured land-snails of 
these islands live on trees and helong to the highly charac- 
teristic genus Cochlostyla. The forms of their shells, too, are so 
extraordinarily various that they have hitherto been included 
in three or four genera (or sub-genera), and I am convinced that 
any paleontologist to whom the various species—akove 200— 
should be submitted in a fossil state would distribute them into 
at least six, or more, genera. Anatomically, however, they are 
so nearly alike that it may be confidently asserted that there is 
no other genus of land mollusca at once so rich in species and 
so exclusively distinct as this of Cochlostyla; also, with the 
exception of six, or at most eight, forms, which occur on the 
small islands in the vicinity, they are confined to the Philip- 
pines. % 

With then, there occur there only two small groups of true 
Helicide ; one of which, CAlorca, lives on trees, while the other, 
Dorcasia, is found among grasses and low-growing plants, or 
even half buried in the soil. Neither genus includes many 
species, and the greater number of them occur in the northern 


400 THE INFLUENCE OF LIVING SURROUNDINGS. 


part of the island of Luzon; not a single species of Chlorcea is 
found in Mindanao, and only one of Dorcasia. Now, the anato- 
mical investigation of several species of both groups (see fig. 73) 
has proved that they are very nearly allied to each other, and, 
at the same time, to one of our commonest European land- 
snails, Helix fruticwum. But as regards their shells they differ 
so widely that in systematic classification Chlorea is placed far 
from Dorcasia, andalso from Lruticum. The shell of Dorcasia, 
however, is easily recognised as allied to Fruticum from its 
resemblance to that type in colour, form, and marking, so that 
it is to me quite incomprehensible why conchologists should 
hitherto have disregarded so striking a similarity iu the shells. 
On the other hand, all the shells of the group of Chlorea so 
singularly resemble those of Cochlostyla, of which the animals 
follow the same mode of life as the species of Chlorea, that they 
have hitherto been generally regarded us species of the Philip- 
pine genus, and in many of the species it is, in fact, quite impos- 
sible to decide which they belong to, so long as the shells alone 
are compared. Thus, on one side, a few species have preserved 
their resemblance to /ruticum, and with that the same mode of 
life; and on the other, a few have assumed the aspect of another 
genus and live on trees associated with certain species of it. 
Hence Helix fruticum ought properly to be included in one 
genus with Chlorwa and Dorcasia, and thus combined they 
would be closely connected with the genus Cochlostyla. Sup- 
posing now that a form represented by Melia fruticum were the 
original form—as there are various reasons for supposing— 
some few species of that genus, when they migrated into the 
Philippines, would have preserved their original external appear- 
ance and at the same time their old habits of life; while others, 
acquiring the habit of climbing trees, would have become so 
much modified in form, colouring, and sculptured marking, that 
the closest scrutiny of the shells alone would not suffice to 
decide the question whether they belonged to Cochlostyla or 
not. We might then easily be tempted to attribute this simi- 
larity to a process of protective mimicry ; but such an assumption 
would be at once contradicted by the fact that the species of 
Chlorea and Cochlostyla which most resemble each other, do 


“SPURIOUS MIMICRY. 401 


not live associated together, and even for the most part occur 
on separate islands. Hence, it is impossible in this case that 
the resemblance should have originated by selection through 
mimicry, since protection by imitation against pursuit is here 
out of the question. 

The question that then occurs is, how such a remarkable 
likeness in form and colour between two quite distinct crea- 
tures can have originated. I will investigate this problem by 
means of yet another example, because it is by this means that 
we may most easily succeed in ascribing to the influence of 
mimicry its due proportions, and in showing that this branch of 
natural selection, like every other, can do no more than avail 
itself of such characters as already exist for its own purposes— 
so to speak—but can never be in a position to act asa funda- 


bee — saa 


— SN a aad 


Fie. 106.—Myxricala infundibulum, copied from Claparéde. 


mental cause, originating differentiation in the form and colour 
of animals. 

During my last stay at Port Mahon in the Balearic Islands, 
I found among the polypes of Cladocora ccespitosa—a coral 
which is there very common—a species, as it seems to me new, 
of the genus Myxicola (Annelida). I have here given a repro- 
duction of Claparéde’s representation of another species of this 
genus (fig. 106). The species of this genus spread out the ten- 
tacles with which the head is furnished—and which are often 
regarded as branchize—in the form of a funnel ; the sides of this 
funnel are perfectly closed and are formed of the filaments of 
the branchize which lie in the closest contiguity ; the section of 
the funnel is circular. Each branchial filament has on its 
inner surface a multitude of fine and minute hairs, which, how- 
ever, are rendered rigid by having in their interior cartilaginous 
cells; these hairs radiate towards the centre of the funnel, so 


402 TIE INFLUENCE OF LIVING SURROUNDINGS. 


that the space enclosed within the funnel is divided perpendicu-. 
larly by a great number of septa into a corresponding number 
of chamhers. 

The new Myxicola cf Port Mahon I found, as I have said, 
among the polyps of Cladocora; they lived in long mucilaginous 
tubes which they had formed in the rifts in the coral, and in 
which they could move about freely. As long as no light was 
thrown upon them they protruded themselves just so far as that 
the top rim of the corona of tentacles was on a Jevel with the 
tentacles of the polyps, so that when the worm and the polyps 
were both extended the coral itself presented a perfectly level 
surface of cups. Moreover, the funnels of the Myxicola were of 
precisely the same chocolate-brown colour as the polyps; and 
when fully extended the interior of the funnel formed by 
the tentacles looked exactly like the oral disc of one of the 
neighbouring polyps, for the radial pinnules were in the same 
position as those lines which, on the oral disc of the polyp, 
radiate towards the narrow central oral slit; in the Myxicola 
also a small central slit was observable, and all the parts which 
corresponded so exactly in size and position also displayed 
exactly the same colouring of greenish-grey with radial lines of 
a lighter hue and a narrow white streak in the middle. In 
short, the resemblance, in size, position, and colouring, of every 
part of the two creatures was so perfect that for a long time I 
took the corona of the Annelid for a polyp, until by an acciden- 
tal blow I caused all the Myxicole of a large coral-stock to 
shrink suddenly into their tubes, though it was not severe 
enough to induce an equally rapid movement in the polyps of 
the apathetic Cladocora. At the first moment I must confess 
I felt an almost childish delight at having detected so flagrant 
an instance of protective mimicry: here was a defenceless tube- 
worm ovidently most effectually protected by its resemblance to 
a polyp well defended by powerful weapons. 

However, I soon found reason to doubt this interpretation 
of the facts ; why should the Annelid require any such protec- 
tion, since it could withdraw itself with the swiftness of light- 
ning into its tube imbedded in coral, where probably no enemy 
would be able to follow it? Still, the wonderfully complete 


POSSIBLY A SURVIVING RESEMBLANCE. 403 


resemblance between the two creatures could not be disputed, 
nor could the fact that this resemblance was perfectly normal. 
Among the hundreds of specimens of Myxicola which I found 
in various pieces of coral, procured from the most various 
localities, I never found one that had not these same points of 
resemblance to the polyps. One day, finally, I found a marine 
sponge in which hundreds of this same Myxicola were living, 
and in every portion of it their funnels of tentacles extended 
just to the level of the surface of the sponge; but the sponge 
was coloured very differently from the Annelida, so that these 
when protruded were very easy to distinguish from the sponge. 
I then sought for the Myxicola in other spots, and succeeded in 
finding it almost everywhere; in the rifts in rocks and in the 
sand, between marine plants or the tubes of other worms—in 
short everywhere—and wherever I examined it closely it was 
exactly of the size and colour of the polyps of Cladocora cces- 
pitosa. Mimiery, it is plain, is out of the question; the resem- 
blance between the two creatures is simply and who'ly acci- 
dental. 

It seems to me that the obvious conclusion from all this is 
that, under some circumstances, the most perfect and complete 
resemblance between two creatures not living associated, may 
originate without its being referable to the selective power of 
mimicry, 2.2. a protective resemblance. The possibility might 
certainly, however, be conceivable that that resemblance may 
originally have been acquired by such means, and subsequently 
retained after the Myxicola had been enabled, by the aid 
of some other means of protection, to establish itself in various 
other places. We have not, however, any single analogous 
instance to support this assumption ; besides, it should be 
observed that all the species, without exception, of the genus 
Myxicola have the same funnel-shaped arrangement of their 
tentacles, and the colouring of their head generally harmonises 
with that of many polyps; they are commonly brownish, 
greenish, or red. A general resemblance between the worm 
and the polyps is thus of common occurrence ; and as we are 
compelled to assume other causes than selection by protective 
resemblance in explanation of this likeness, it will be equally 


404 TUE INFLUENCE OF LIVING SURROUNDINGS. 


easy to deduce from analogous causes the somewhat more perfect 
likeness in form and colour between Cladocora and the un- 
described Myxicola. 

Still, true mimicry might, no doubt, be developed from such 
a case of extreme resemblance between two quite distinct crea- 
tures. Supposing that some animal not hitherto living in the 
sea at Port Mahon were to be introduced there which was able to 
capture the Annelida wherever their unlikeness to the objects 
around them—as the sand, sponge, &c.—rendered them con- 
spicuous, they would only find the protection they would need 
by living associated with the coral, because there only could 
they be effectually concealed. And if the enemy—pursuing 
them even among the polyps of the Cladocora—should have 
learned to regard the polyps as dangerous foes whose fine sting- 
ing threads were capable of inflicting much injury, a perfectly 
characterised case of true mimicry might be developed from this 
instance, originating in simple shelter. 

The theoretical possibility of the process thus indicated can- 
not, I think, be denied ; but then the question arises whether 
many cases of mode of colouring and resemblance which we 
have hitherto been disposed to regard as cases of exquisite 
mimicry may not have originated in the same way as the 
spurious mimicry of the Myxicola. And here we find ourselves 
brought face to face with the same conclusion that we have 
become familiar with in each separate chapter of this work, and 
which I must once more repeat: Namely, that no power which 
is able to act only as a selective, and not as a transforming 
influence can ever be exclusively put forward as the proper effi- 
cient cause—causa effictens—of any phenomenon. In all cases, 
including those of mimicry, the point finally must be to in- 
vestigate the causes which may have availed to produce, by 
their direct action, any advantageous and protective change of 
colouring; it was not until the change had actually taken place 
that selection between the better or worse endowed individuals 
could lead to the further development of the advantageous cha- 
racter. Itis extremely difficult to decide in particular cases—in 
most, indeed, it is impossible—the precise point where one ceases 
and the other begins to act. But it is precisely by reason of the 


_A FINAL WARNING. 405 


universal difficulty of deciding whether a certain modification 
which has taken place is to be ascribed to some direct determin- 
ing and transforming cause, or to the enhancing of a previously 
modified character, which is frequently connected with selection, 
that it becomes imperative that we should in the first place 
carry out the most exact research possible by means of experi- 
ment, and also wean ourselves of the convenient—but, as it 
seems to me, highly pernicious—habit of theoretical explana- 
tions from general propositions. Otherwise there is great danger 
that the bright expectation which Darwin has opened out to us 
by his theory may be bafiled—the prospect of gradually bring- 
ing even Organic Being within reach of that method of inquiry 
which seeks to discern mechanical efficient causes. 


NOTES. 


INTRODUCTION. 


Note 1, page 10. Professor Marsh has lately discovered a colossal fossil 
reptile, belonging to the family of the Atlantosauride, and which was 
of such enormous size—more than eighty feet long—that it could hardly 
have been able to drag itself along on the ground or even to raise itself, 
if its bones had been as heavy in proportion as those of the reptiles or 
mammals now extant. And in fact the bones of this fossil Saurian are 
remarkably light in comparison with their size, so that Professor 
Marsh’s view that the peculiar large cavities which occur in all the 
bones of the skeleton were air-cavities, and that they were thus 
actually pneumatic bones, seems highly plausible. 

Note 2, page 17. There must, of course, be a number of characters 
of adaptation, of which we cannot avail ourselves as hereditary cha- 
tacters of general importance, from their appeariug only in small grours, 
or merely in genera and species, or even in single individuals; these 
lose all diagnostic value in an inquiry as to the relations of affinity 
among the larger categories to which the animals under comparison 
may belong. Hence arises the necessity, insisted on in the text, for 
inquiring how far hereditary characters of general value are to be dis- 
tinguished from characters of adaptation. But every character which 
can be regarded as a true sign of the common descent of large groups of 
animal forms, may be ultimately traced to the stage at which it first 
appeared, and where it was a character of adaptation. An example 
will help to explain this. We know that in all the Vertebrata, without 
exception, the first appearance of the skeleton, which is the most im- 
portant organic system of the vertebrata, is inseparable from the 
presence of an axial cord,.known as the notochord, or chorda dorsalis. 
This does not become merged in the vertebral column, but is displaced 
by it; it is perfectly inarticulated—a simple string of cells. The great 
uniformity of the conditions of its first appearance, structure, and pcsition 


408 NOTES. 


in the embryo of every vertebrate animal, show that we here have to 
deal with an hereditary character, and its remarkable persistence and 
constancy are evidently due to the fact that its presence is essentially 
bound up with the development of an organ so wonderfully fitted for 
the most various morphological and physiological differentiation as 
the vertebral column (and skeleton) of the Vertebrata. But, working 
backwards, in the series of invertebrate animals we first lose sight of 
the skeleton, and presently even the chorda dorsalis disappears. 

From this it would seem that such an axial cord must bave appeared 
at first, once or even more than once, in some group of invertebrate 
animals, i.¢., was elaborated from cells already existing in other forms. 
This primitive chorda must from our point of view have had some deti- 
nite function, and from everything that we know of the histology of 
the cells of the chorda, we must regard it, even in its simplest form, 
as an elastic prop or fulcrum for the movements of the animal; conse- 
quently the primitive chorda was an organ that derived its fitness to 
exist from its adaptation (and consequent modification) to the function 
of affording support to the whole animal structure. Thus in the first 
instance it can only have had the value of a character of adaptation. 
Moreover, it must inevitably have preserved this value only, if it had 
not contained, in itself, and through its influence on the other organs 
connected with it, the elements of the most varied differentiation into 
numerous dissimilar forms, It is only by the fact that it and the 
tissues immediately surrounding it were in the highest degree plastic, 
that it acquired its value as an hereditary character. An accurate 
analysis of each separate organ will bring us ultimately to a stage in 
which its existence seems to be wholly dependent on its special adapta- 
tion to some definite purpose, or to some condition of existence. 

Note 3, page 17. Rudimentary organs are extraordinarily numerous, 
and occur in amore or less significant form in most animals. Their 
high theoretical importance has been sufficiently indicated by Darwin, 
to whose works the reader is referred. Their most essential peculiarity 
is their incapacity for fulfilling the functions for which, by their 
structure and position, they would seem exclusively intended. The 
question, however, still remains, as Leuckart has pointed out, as to 
whether we are justified in saying that such rudimentary organs are in 
fact wholly useless. The teeth of the Vertebrata are, as we know, used 
only for biting and masticating, or as weapons. The male dugong does, 
in fact, so use his tusks, as is shown by the invariably worn condition of 
the point, on the external surface of each. The female has equally large, 
nay, even larger tusks; but they are not used—at any rate, not in the 
same way. Butitseems not improbable that, merely by theirgreat weight, 
they may assist incertain movements of the head, for instance, in grazing 
on sea-weeds; and in this respect they may actually have acquired a 
physiological significance without losing their character as rudimentary 


NOTES. 409 


teeth. The most extreme case in the whole series of rudimentary organs 
which have lost their original use is offered by the roots of the Rhizo- 
cephala (Peltogaster), (fig. 12, a, p. 47); the parasite plunges them into the 
body cavity of the host it lives upon, and absorbs its nourishment 
through them. When the parasite has reached a certain age, it falls 
off, leaving its roots in the body of its host ; they live on, though their 
purpose as organs of nutrition for the Peltugaster is, of course, entirely 
lost. : 

Note 4, page 22. This use of the clinging foot of the Geckotide is 
well known. In handbooks of Zoology—even in Claus—we read that 
these clinging hairs or suckers are formed by the secretion of a glu- 
tinous matter from the glands of the toes. Ido not know on what this 
assertion rests, but there is, in fact, no truth in it. In the first place, 
there certainly are no glands present on the feet and toes; the clinging 
power is, on the contrary, effected in a wholly mechanical way, and 
derived from the rows of bristles beneath the toes, resembling the 
sucking discs of flies or of leeches. By the pressure of the foot on a 
smooth wall, the air between it and the wall is quite driven out; when 
the pressure is removed, the inner surface of the foot is raised by the 
elasticity and rigidity of the hairs, and this effect is increased by the 
special muscles which move the lobes, each of which bears bristles ; 
thus a vacuum is formed between the wall and the sole of the foot, and 
atmospheric pressure secures the foothold. 


CHAPTER I. 


Note 5, page 27. Thacker has lately attempted to solve this problem 
on morphological grounds (Connecticut Academy, 1877, vol. iil, On 
Median and Paired Fins, a Contribution to the History of Vertebrate 
Limbs). He is of opinion that the four extremities of the vertebrata 
are to be considered as the survivals of two longitudinal folds of the 
skin, such as are to be seen in the living Amphioxus. If this attempt 
could be successfully followed up, we should no longer have to seek the 
analogues of the extremities of the Vertebrata among the Invertebrata, 
as Dohrn has lately done. But the physiological side of the question is 
not touched by it; it is this: Why could only exactly four extremities 
be developed, and not six, eight, or more out of this skin-fold ? 
hacker does not go into this question. Dohrn certainly thinks he 
has found the use of the number four as applying to the limbs ; accord- 
ing to him a long narrow fish swims best when it possesses two pairs of 
fins as far as possible from each other, one pair in front and one behind, 


410 NOTES. 


This idea at first seems strikinz, but it is opposed alike to the facts, and 
to the laws of mechanics. 

Note 6, page 37. With reference to the constancy of such characters 
as have arisen under external influences, Sachs says, in his 7eaxt-book of 
Botany : ‘We may infer very decidedly that hereditary characters, or 
such as might become hereditary, are not produced by external 
elementary influences, from the fact that seeds from the same fruit 
produce several varieties, or one variety side by side with the here- 
ditary parent-form ;’ and further on: ‘ We come to the conclusion that 
hereditary varieties first arise independently of direct external in- 
fluences, but that the possibility of their continued existence depends 
on such influences.’ There can certainly be no doubt that a great 
number of modifications which are in a high degree hereditary were 
due to causes which—like sexual reproduction and hybridisation—we 
are not accustomed to designate as external influences, although I 
reckon them as such; and it is equally little doubiful that those con- 
ditions which are universally recognised as external—such as climate, 
nutrition, the nature of the soil, &c.—are able to acquire a modifying 
influence on living and growing animals; thus the modifications called 
forth by such causes must constantly recur as long as the causes them- 
selves remain constant. In the course of our investigations I shall have 
occasion to discuss a few examples which may be regarded as having a 
direct bearing on this. So far as I know, it was Helmholtz who first 
pointed out that the constancy of an altered condition of life must 
resuit in the permanence of any deviation from the parent form of the 
species—or of the organ—to which such an alteration had given rise. 

Note 7, page 38. I need here adduce only a few quotations, ‘There 
can be no doubt that changed conditions induce an almost indefinite 
amount of fluctuating variability by which the whole organisation is 
rendered in some degree plastic’ (Darwin, Descent of Man, i. 114). 
‘ Such changes are manifestly due, not to any one pair, but all the indi- 
viduals having been subjected to the same conditions’ (Darwin, 7did., 
in speaking of horses, page 236); and finally, ‘We do not know what 
produces the numberless slight differences between the individuals of 
each species, for reversion only carries the problem a few steps back- 
wards ; but each peculiarity must have had its own efficient cause. If 
these causes, whatever they may be, were to act more uniformly and 
energetically during a lengthened period (and no reason can be 
assigued why this should not sometimes occur), the result would pro- 
bably be not mere slight individual differences, but well-marked, con- 
stant modifications (ibid. p. 153). And in other places in the same 
book, as well as in his other works, Darwin expresses himself in a way 
that proves that he no longer rates the influence of the conditions of 
life (external causes) on the transformation of forms (in species as 
jn organs) so low as he seemed to do in the first edition of his Ovigin 
of Spectes 


NOTES. All 


CHAPTER II. 


Note 8, page 40. Hitherto only one single genus of animals is known 
of which the species are occasionally able to go through their whole 
cycle of development without their requiring to take nourishment from 
the time they escape from the egg till their death. The male indi- 
viduals of some species of Jxodcs never take any food after they have 
left the egg, and they perish after fulfilling their sexual function. 
Hence the amount of food contained in the egg must have been suffi- 
cient for the requirements of the creature throughout life. 

Note 9, page 41. Numerous instances are known of such long- 
enduring resistance to want of food in the Invertebrata as well as in the 
Vertebrata. In this respect those snails must be mentioned which were 
kept for years with their shells glued down, in the collection in the 
British Museum, and at last, under specially favourable meteorological 
conditions, were enabled to creep away, At all times those cases have 
excited much attention which have seemed to prove that Amphibia, 
such as toads, salamanders, &c., have been preserved from the remotest 
times, though shut up in a perfectly hermetically closed stone. But 
the data in these individual cases are always so far from exhaustive that 
it is impossible to found any explanation on them; it therefore seems 
to me superfluous to give here any special cases, and I refer the reader 
to the literature of the subject given by Schmarda (Geographic der 
Thiere, vol. i. p. 101). 

But it is highly probable that they all, without exception, might be 
explained in the same way asa case described by Frauenfeld in the 
year 1867. A stone of about the size of the palm of the hand had in it 
two cavities which communicated with each other, and in one of these 
an Amphibian about two inches long—probably Yriton cristatus--was 
living curled up when the stone was split open. The cavity communi- 
cated with the outer air by a small hole, which was 1 millimétre (about 
3s of an inch) in depth, and about 3 millimétres wide. Frauenfeld 
assumes that the larva found its way into the inner cavity, there grew, 
and at last became too big to get out again. It had, moreover, not been 
able to obtain enough food (it was apparently at least a year or two 
years old) to grow beyond two inches in length. According to this 
the Yriton had not, however, been absolutely without food; but we 
may regard it as quite certain that it had obtained a very insignificant 
amount of nourishment, since through so narrow an opening it was 
impossible that it should have been supplied in sufficient quantity. Ex- 
periments have even been made on this matter, for living amphibia have 
been enclosed in masses of gypsum, and left thus enclosed, and they have 


412 NOTES. 


lived more than a year. However, so far as I know, no satisfactory 
series of experiments have been carried out to the end. 

Note 10, page 43, Even the statements in handbooks (see Milne- 
Edwards, Zecons d’Anatomie comparée, viii. 169, 184) or in special 
treatises, as to the requisite maximum (or optimum) of the amount of 
food, show great errors, because itis impossible to obtain by experiment 
any perfectly satisfactory information as to the relations of consumption 
and ejection. Thus Hermann says: ‘Even far more uncertain are the 
statements as to the absolute amount of the minimum of excretion or 
the minimum of food required to cover it, in consequence particularly 
of the uncertainty of the methods of inquiry formeily pursued ;’ and 
he therefore forbears mentioning any absolute figure for the minimum 
of assimilated mutter, as ascertained by these methods. Moreover, the 
subject has never been, to my knowledge, treated from any more general 
point of view than a purely medical one. Thus, for instance, the fact 
that a kilogramme weight in a pigeon needs more nutrition than the 
same weight in a dog, and this, again, more than a kilogramme ina 
man, is explained by a reference to the greater energy of the vital 
processes in the smaller organisms. This argument is just, if it is con- 
fined to the warm-blooded animals, but it is false with regard to the cold- 
blooded animals ; for as, in these, the variations in the temperature of 
the body agree perfectly, or very nearly, with those of the surrounding 
medium, water or air, no more warmth need be produced in the small 
animals than in the large, nor need a correspondingly larger amount 
of nourishment be consumed. Beyond this we know absolutely nothing 
of their relations in cold-blooded animals. 

Note 11, page 50. At p. 46 a similar example was mentioned, where 1 
pointed out that the assimilation of food in Lymnea stagnalis did not 
depend merely on the circumstance of the food being obtainable in suffi- 
cient quantity and of suitable composition, but also on the osmotic 
action of the skin itself. If the temperature is too low, below 12° C. for 
instance, assimilation ceases in these snails although they continue to 
feed; at 20° C. the greatest proportion of food is digested and the 
most rapid growth attained. But even then, this only takes place 
when the influence of the volume of water, which can only be exer- 
cised by the osmotic action of the skin, is exercised in the most 
favourable manner. For more details on this point see Chapter V. and 
the notes. 

Nole 12, page 60. For all who think that the number of cases men- 
tioned in the text are insufficient evidence for the position here advanced, 
J here briefly give a number of other similar ones. Hels, which usually 
live on animal food, will also eat bread. Spiders feed almost exclusively 
on Articulata; a few species, as our European Atypus Sulzeri, feed on 
snails ; the tropical species of Afygale are said to eat small birds; here 
in Wiirzburg, I and Menge fed them with young mice. The Loach, 


NOTES. 413 


Cobitis fossilis, which is specially adapted to animal feeding, frequently 
eats species of Lemna. Many caterpillars, among the Noctuide—for 
instance, species of Agrotis—eat each other if they are shut up together 
in a box, though in freedom they feed only on roots and leaves. The 
larvee of the common frog eat plants, the full-grown frogs only insects, 
worms, or even amphibia. All apes, although their teeth are apparently 
adapted to a fruit or vegetable diet, are passionately fond of animal 
food, as birds, eggs, insects, &c.; they will even gnaw bones. Many 
parrots eat butter, bacon, lard, snails, raw eggs, beetles, the brains of 
small birds, and marrow. Most Nematoda live as parasites in animals, 
but a few live in plants—Tylenchus tritici, for instance, which lives in 
the flower of wheat, and Lylenchus dipsaci, 

Most Holothuriz shovel sea-sand into their mouths with their 
tentacles, and leave it to the intestine to select the organic particles 
of nutriment that are mixed with it. Thyonidium molic, on the Peruvian 
coast, feeds, on the contrary, on sea-weeds. Almost all Hymenoptera 
are phytophagous, excepting only a few wasps and hornets, and ants 
which feed on dead flesh. Certain snakes—Leptognathus and Ambly- 
cephalus (see Giinther, Ann. Mag. N. Hist. 1872, ix. 29)—feed on snails, 
while all other species eat vertebrata or sometimes minute insects, 
Cyclura lophoura, a species of iguana-like lizard in Jamaica, is herbi- 
vorous, although it belongs to a carnivorous group. Most tortoises live 
on animals; only afew land tortoises eat vegetables. All birds of prey 
feed on mammals, birds, or reptiles; but the secretary-bird, that stalks 
about on long stilt-like legs like a heron, rammages about in the mud, 
like a duck, for aquatic creatures of all kinds. One of the most interest- 
ing examples is afforded by the genus Onchidium among the pulmonate 
mollusca. The lingual tongue of those mollusca which live exclusively 
on animal food is very sharply distinguished from that of the herbi- 
vorous species ; a few of these last, as Lymnea stagnalis, are, no doubt, 
carnivorous also (see p. 59 of the text) ; but in general we may consider 
ourselves justified in determining those molluscs as herbivorous of 
which the rachis has the same structure as those of Helix or Lymnaea. 
All the species of Onchidium which I have hitherto been able to examine, 
about four-and-twenty, have exquisite herbivorous teeth, and, neverthe- 
‘less, do not use them for eating off plants, but exclusively for shovelling 
in sea-sand or mud. Hence we see that even the organs of mastication, 
which yet must be quite specially adapted to the nutriment obtainable 
at the time and to the mode of obtaining it, may under some circum- 
stances be used in very various ways; and we must therefore conclude 
that in comparing living creatures with fossil ones these organs can 
afford no absolutely reliable means for determining the food and mode 
of life of these primeval creatures. These exceptions, moreover, afford 
us a further example of the proposition stated in the text, that even 
organs which appear to us to be adapted to a special office are never- 


414 NOTES. 


theless able, and by their internal nature must be able, to accommodate 
themselves to others. 

Note 13, page 64. It seems to me desirable to add a few more in- 
stances of voluntary change of food by monophagous animals to those 
given in the text. The palm-crab, Birgus latro, in a free state feeds on 
fruits, namely cocoa-nuts, but in confinement eats its fellows (as I know 
by experience). Canary birds, and fowls, often and readily eat lard. 
The American prairie dogs Hans and Gretel, which I have already 
mentioned in the text, quickly accustomed themselves to a diet of fish, 
which was wholly unknown to them, with mollusca and meat. I owe 
to my friend Professor Hagen (of Cambridge, Mass. U.S.) the following 
interesting notes. At Cape Cod, the cows are regularly fed on herrings’ 
heads; in Norway a mash is prepared for the cows by mixing and 
stirring horse-dung with the heads of dorse boiled down; this serves 
them for fodder only in the winter ; in the summer they eat grass, as they 
do everywhere else. The reindeer, according to Brehm, sometimes eat 
lemmings. 

Note 14, page 68. Tie change of structure which takes place in the 
stomach of the pigeon and the gull in consequence of the change of 
function is as follows. The stomach of a bird subsisting on flesh has a 
comparatively feebly developed muscular layer and a soft mucous meme 
brane, which penetrates the coats of the stomach, forming long tubules ; 
these tubules are the glands which secrete the gastric juice. In the 
grain-eating birds the muscles of the stomach are particularly strong; 
instead of the soft mucous membrane a thick brown membrane covers 
the inner surface of the larger part of the stomach, while the small 
anterior portion exhibits the same soft skin and glandular Jayer as are 
everywhere distributed in the stomach of birds of prey. This brown 
skin in the gizzard of the pigeon (see fig. 13) is very strong ; it has long 
fine filaments which penetrate the cavities of the tubules which extend 
perpendicularly into the muscular layer of the stomach. Now, if the 
stomach of the pigeon is acted on for a sufficiently long period by feeding 
on flesh, this brown skin (called a cuticula) withdraws entirely from 
the tubules and is ejected ; the tubules now no longer secrete any 
solid matter, but only a fluid, and so become true glands. It would be 
interesting to ascertain whether the secretion now produced by these in 
the gizzard is to be compared, chemically and with respect to its diges- 
tive qualities, to the gastric juice in the stomach of birds of prey. In 
gulls, on the other hand, which have become accustomed to a grain diet, 
the hitherto fluid secretion from the glands opening into the stomach 
becomes rigid, and a more or Jess firm thick skin is formed in the 
interior of the stomach. 

From the text it might perhaps be inferred that the stomachs of 
graminivorous and of carnivorous birds were two distinct forms of 
stomach corresponding uniformly and exactly to these mocles of feed- 


NOTES. 415 


ing, so that the former occurred only in birds that feed on grain, 
and the latter in those that feed on animal food. This, however, 
would be an error, since there is a considerable number of flesh-eating 
birds of which the stomach (the muscular stomach, as it is called) is 
constructed exactly like that of a pigeon or a hen. Podiceps minor 
(thé little Grebe) lives on fishes, worms, and soft-bodied aquatic 
creatures. Corvus Cornia (the hooded crow) and C. Corax eat insects, 
birds, and small quadrupeds. The lapwing liveson soft aquatic animals, 
and the kingfisher on fish. In all these the muscular stomach has quite 
as thick a muscular layer as in the pigeon, and the internal coat is a 
hard, brown pseudo-cuticle, such as is always present in the graminivorous 
birds. Even among true birds of prey some have the pseudo-cuticle, at 
any rate for a time—as the recently fledged kestrel—though not, it is 
true very strongly developed. In these birds a meat diet does not seem 
to effect so rapid a change in the stomach-- from a graminivorous to a 
carnivorous type—as in the gull and the pigeon, nor, indeed, to affect 
them in any way. 

Note 15, page 69. In the text I have not mentioned a number of 
eects of food which in part are not suited for discussion in a popular 
lecture, but in part too have no bearing on the question we must steadily 
keep before us, i.e. how far the maintenance of the species or the origin 
of new forms may be induced by them. Thus, for instance, Voit's ex- 
periments on the assimilation of fresh-water mussels seem to have 
established beyond a doubt that the greater part of the ashy consti- 
tuents of their bodies, which they deposit almost exclusively in 
their shel's, are derived from the water which, according to him, is 
taken up by the kidneys. But then the question as to how the water 
penetrates the body of the mussel is still under controversy. Some 
authors entirely set, aside the old view that the kidneys, wholly or in 
part, have the function of absorbing water: according to them it takes 
place through rfores in the skin; but this, notwithstanding all that 
has been said on the subject, is not absolutely beyond dispute. In 
fact, so long as the morphological bases of physiological speculations 
are as little assured as in this case, any discussion must be regarded as 
premature. 

The second point, to be here only lightly touched on, regards the 
influence of food on the sexual functions and in the external distinc- 
tions of sex which partly depend on it. It is known that certain foods 
or stimulants at the approach of sexual maturity have a stimulating 
effect on the secretion of the semen. Too small a supply of food is as 
injurious to the germinal glands as too large a one; for this predomi- 
nantly important side of animal life, also, there is an optimum of nutri- 
tion, and any excess towards the maximum, or deficiency towards the 
minimum, must exert a proportionally injurious influence. Unfortu- 
nately we know of hardly any serviceable experiments on this subject, 


416 NOTES. 


for those made on domestic animals can scarcely be regarded as such, 
since their results cannot be considered applicable to other animals 
living in a state of nature. Sanson has lately made some very interest- 
irg experiments on domestic animals, which seem to prove that sexual 
maturity can be very much hastened by a careful and special hygiene, 
by an increased amount of nourishment and by the addition of certain 
substances to the diet. (Sanson, Comptes Rendus, 1874, lxxix. 1768; 
and Journal de V Anatomie et de Physiologie, 1872, p. 113.) The animals 
thus brought up are said to assume a quite special development—to 
become races ; the signs of their earlier development and precocious 
maturity are the cutting of the permanent teeth and the growing 
together of the epipbyses (the osseous portions of the hollow bones) at 
an age when, in animals fed in the usual manner, these tokens of 
approaching maturity are not yet visible. As we shall presently see, 
precisely similar effects follow from raising the temperature. In con- 
clusion I will only mention that Von Willich states that in frogs a defi- 
ciency of food causes darker colouring of the skin. 


CHAPTER III. 


Note 16, page 70. A comparison of the eye of the animal and the 
chlorophyll bodies of plants as standards of equal value for estimating 
the intensity of light was, in fact, attempted by Prillieux. Sachs has 
controverted this attempt in his usual brilliant and thorough way, and 
has set it aside, let us hope, once for all. ‘All comparisons as to the 
intensity of differently composed light, made by means of the eye, 
have, from the nature of the eye, no independent value.’ Hence the 
intensity of the different colours of the spectrum as thus estimated can- 
not be made use of to measure the gas disengaged by plants by means 
of the chlorophyll bodies, as Prillieux has done. 

Note 17, page 72. Ihere give a complete list of those species of 
animals in which chlorophyll or similar bodies—as xanthophyll, &e.— 
are said to occur. 

otozoa: 
Euglena viridis. 
Stentor viridis. 


Almost all Radiolaria. But among the marine Radiolaria most of 
the Acantho-metride are excepted (Haeckel, Monograph of the Radio- 
aria); and among the fresh-water Radiolaria, Actinophrys, Actino- 
spherium, &c. 


NOTES. 417 


Not to be confounded with these chlorophyll-bodies of the Protozoa 
are certain peculiar green or olive-green discs of colouring matter 
which occur in many Monadinz, as for example in Chromulina, Chilo- 
monas, Paramecium, Uvella virescens, Mallomonas Ploeslii, &c. 

Spongilla viridis (Sorby, Quart. Journ. Mie. Sct. 1875, vol. xv. 
p. 47.) 

Ceelenterata: Hydra viridis. 

Platyelmia: Vortex viridis. 

An undescribed marine Planarian, according to Geddes. It is 
stated that this creature decomposes carbonic acid; no exact report, 
however, has as yet been given, so that it is not possible to decide 
whether this statement rests on experiment or is only inferred from 
observatiozs, which alone prove nothing. 

Gephyrea: Bonellia viridis (Rolando). Schenk says that the green 
colouring matter of this worm is true chlorophyll. Sorby, however, 
(Quart. Journ. Mie. Sci. 1875, lviii. p. 166), after a very careful in- 
vestigation, proves that this matter is quite distinct from chlorophyll. 

Leydig has lately propounded the hypothesis that chlorophyll may 
occur even in insects. He founds it on the observation that the vivid 
green of various beetles, orthoptera, &c., changes towards autumn in 
the same way as the leaves of trees do. I regret that I cannot agree 
with him on this point; no such conclusion can be derived, in my 
opinion, from the observed phenomena. The view that insects can 
themselves elaborate chlorophyll cannot be supported by any argument, 
and it is equally improbable that chlorophyll, as such, can be stored up 
from the food in the circulating system of an insect, and then deposited 
in the skin without being wholly altered ; but even if after this process 
it could preserve its green colour and other original properties it must 
certainly have lost the characteristic property of chlorophyll, namely, 
that of decomposing carbonic acid under the influence of light. 

Note 18, page 76. Foreign bodies occur by no means rarely as inte- 
gral constituents of the tissue of animal organisms. Irrespective even 
of those cases in which parasites become normal (¢.e. constantly present), 
constituents of certain portions of the body in every individual of a 
species—as for instance the Nematoda living in the foot of certain naked 
mollusca—there are numerous other examples. Among the Ceelenterata 
there are, besides Sphenopus, various other genera which take up grains 
of sand directly into their skin—for instance, all the Zoanthinz (Zvan- 
thus, Palythoa, &c.)--whereby it acquires the firmness it would otherwise 
lack. The sponges often have the habit of including foreign siliceous 
particles, the calcareous shells of Polythalamia, fragments of corals and 
of the shells of molluscs, or merely sand, in their horny fibres; such 
foreign bodies occur in by far the larger number of keratose sponges. 
That the siliceous spicule are in these cases very often merely foreign 
bodies is proved by the circumstance that they are found withcut excep- 


19 


418 NOTES. 


tion with broken points, and this could not be the case if they were 
formed in the horny fibres by the creature itself. Since the horny 
tissue, even in living sponges, is much too tough passively to allow of 
the intrusion of foreign bodies during the life of the sponge, their being 
imbedded in the fibre can only result from the sponge drawing them to 
itself by a voluntary act and including them between the layers at the 
growing apex of the horny fibres. One of the most remarkable 
instances of the utilisation of foreign bodies as an integral constituent 
of an animal structure is offered by the species of Xenophora among 
the mollusca; they voluntarily incorporate into their shells, in regular 
order, other shells and fragments of stone or of coral. Their intimate 
adhesion shows that this combination must be effected before the shell 
has hardened, and each particular species of Xenophora seems also to 
have more or less choice among the materials at its disposal. Bergh 
has recently shown that Staurodoris Januarti, a naked marine mollusca, 
eats the spiculz of sponges and deposits them in its skin. 

We may here allude to the use made by the larve of the Phry- 
ganeid@ of leaves, snail-shells, roots, &c., in building their cases 
(caddis-worms). 

The above-mentioned regular association of two kinds of animals, 
one of which lives in or on the other as a constant parasite, offers many 
remarkable phenomena. It is known, for instance, that the hermit crab 
(Pagurus) is sometimes infested by dark-brown Crustacea of the group 
of the Rhizocephala ; when these are present the female germ-glands 
never develope in the host, and eggs and parasites are never found 
together in the same individual. In certain localities the parasites are 
so common that out of hundreds of hermit crabs scarcely a single 
individual will be found without one, although their numbers are no 
less than in other spots in the vicinity where there are no parasites, or 
hardly any. This proves that the growth of the Pagurus is not in the 
least hindered by the foreign parasite, while the development of sexual 
maturity is wholly arrested ; and we see, moreover, that the conditions 
for the reception of the parasite or for its avoidance may be quite dif- 
ferent in two contiguous spots. 

I myself have made similar observations with regard to Lymnea 
stagnalis. The larve of Trematoda which infest these water-snails, in 
the first place at any rate, and almost exclusively, as it would seem, 
destroy the germ-glands, but they do not check the creature’s growth. 
In however great a number the parasites may be present, the mollusc 
grows all the same, but propagation is completely prevented. How far 
the sterility thus induced in certain individuals may possibly give rise 
to other changes in their external or internal conditions is wholly un- 
known, and has not been investigated. The larva of a fly, Cuterebra 
emasculator, destroys the testes of various American species of squirrel 
without affecting the other vital functions; and the number of such 


NOTES. 419 


animals, healthy in all other respects, annually shot by hunters seems to 
be very considerable. (Dr. Hagen.) 

Note 19, page 78. There are but few totally blind vertebrata—abso- 
lutely deprived, that is to say, of eyes. All the species of mole have 
rudimentary eyes, as have the Proteus and the blind-fish, as they are 
called, of the American caves; Amblyopsis speleus, Typhlichthys 
subterraneus, Stygicola dentatus and subterraneus (from caves in Cuba), 
Gronias nigrilabris, Stygogenes cyclopwm—or of the caves of Asia, Ailia, 
Shilbichthys, Bagroides, kc. Actually eyeless fish have hitherto been 
found only at great ocean depths, and we owe our knowledge of them 
to the ‘Challenger’ expedition. They are Scopelide or Lophivide. 
What makes them especially interesting is the occurrence of the peculiar 
organs on the head, first observed by Von Willemoes-Suhm, and subse- 
quently accurately described by Giinther, who regards them as organs 
of phosphorescence (see note 22), Truly blind invertebrate animals 
are far more numerous. Most ento-parasites are perfectly eyeless, 
The number of species of blind cave-insects, which is being added to 
every day, already amounts to hundreds. The reader who is specially 
interested in these creatures will find a very complete review of the 
literature of the subject in an admirable paper by Simon and Bedell in 
the Revue Zovlogique. Associated with the blind cave insects we 
find blind spiders, Crustacea, and Myriapoda; the blind crab of the 
Kentucky caves has, according to Hagen (Monograph of the North Ameri- 
can Astacide), certainly only rudimentary eyes ; while other Crustacea, 
as Cecidotea, Stygia, Titancthes albus, and others, seem to be totally 
blind. In the work of Putnam and Packard on the Mammoth Cave of 
Kentucky there is a list of these forms with excellent illustrations. 
Various Crustaceans which are called blind are known from the caverns 
and subterranean waters of Europe ; to these belong Miphargus puteanus, 
Titanethes albus, Crangonyx, Asellus Sieboldiit. Univalves seem always 
to have eyes, with the exception of a few which live as parasites, but a 
Aydrobia found living in Munich by Rougemont, and which inhabits 
deep springs, seems to have no eyes. Wiedersheim found rudimentary 
eyes in the Hydrobia of the Falkenstein cavern. 

The ‘ Challenger ’ expedition also has furnished us with rich materials 
on this subject. Willemoes-Suhm, whose premature death we must 
deeply deplore, made us acquainted with a large number of peculiar 
blind Crustaceans, some of which live at a depth of more than 2,000 
fathoms ; for instance, Petalophthalmus of various species, all the Aun- 
opsida, several Myside@, several blind larve belonging to the Zvea and 
Megalopsis forms, Astacus zaleucus, Aspeudes ceca, Deidamia, &c. 
Notices of these occur in the narratives of the voyage communicated 
to Nature ; Ann. and Mag. Nat. Hist.; Proc. R. 8.; Linn. Soc. Trans. 
And a tolerably complete guide to the literature of the subject is to be 
found in Siebold’s supplement to Willemoes-Suhm’s Challenger-Briefen 


420 NOTES. 


(Zeitschrift fiir nriss. Zoulogie, 1877, v. 27). The recent treatise by 
Pagenstecher, Ucher die Thiere der Tirfen, also contains a list of blind 
as well as other deep-sea animals, though in w somewhat different 
arrangement from tliat which I have given here. 

Note 20, page 82. The following animals, furnished with well-deve- 
loped eves, live in caves: Macherites, 7 species (Coleoptera) ; Antho- 
myia, Phora (Diptera) ; Hadenwcus, 2 species (Orthoptera) ; Spirostrep- 
tun, several species in caves (Myriapoda); Nestirus, 2 species; Liny- 
phia, 3 species (Spiders in the Kentucky caves). Animals having only 
rudimentary eyes must be partly included here. A AMelania having eyes 
I myself found in a cave in the Pelew Isles, and in the same spot was 
a grasshopper that could see. Also among fishes, Chologaster Agassizit 
Gn Kentucky), Ombre Crameri (in subterranean lakes in Austria, 
according to Schmarda, Gvaq. der Thiere, i. 13). In the caves of Utah 
(according to Packard, Bulletin N.S. Geol. and Geog. Survey, iii. 
1877), a Phalangium -- Nemastoma troglodytes—with eycs; a Univalve, 
Hyatlina subrupicula ; and a Podurida, Tomocerus plumbcus, equally with 
eyes, are associated with a blind Myriapod, Polydesmus varicula. Fries 
states that the blind Gammarus puteanus of the Falkenstein caves 
sometimes quits the regions of absolute darkness. 

Note 21, page 84. Many creatures furnished with well-constructed 
eyes live associated with the actually blind species which have been 
partly enumerated above. An attempt to account for this apparent 
contradiction is mentioned in the text. Of the very considerable num- 
ber of such denizens of the darkness which nevertheless can see, I will 
particularly mention the following: Bathytroctes, a new genus named 
by Giinther, 675 to 1,090 fathoms; Bathylagus, 1,950-2,040 fathoms ; 
Platytroctes, 1,500 fathoms; Chlorophthalmus gracilis, 1,100-1,450 
fathoms—all forms of I'ishes discovered during the ‘ Challenger’ expe- 
dition. Besides these, among Fishes, Mucrurus and Halosaurus, 1,375- 
1,600 (Willemoes-Suhm, ¢%allenger-Briefe) ; among mollusca, Chiton 
and Patella, 1,075 fathoms (Willemoes), Pleurotoma, n. species, 2,090 
fathoms, and Fusus sp., 1,207 fathoms (Thomson, Depths of the Sea, 
p. 465); Crustacea, a Palinurus in 700 fathoms; a Nephrops and an 
Amphion, between 1,875 and 3,125 fathoms. Various crabs, Galathea, 
Caluppa; Isopoda, Serolis; Macroura, Penwide, Curidide. Bathy- 
nomus giganteus (M. Edw.), a gigantic Isopod 23 centimétres long, 
having large eyes, each with 4,000 facets, and others. It is impossible 
here to give a complete list, nor is it within the purpose of this work. 
Other deep-sea forms have rudimentary eyes, as, for instance, Aphyonus 
gelatinosus, 1,500 fathoms ; yphlonus nasus, 2,150 fathoms, from N.E. 
Australia, &c. 

Note 22, page 85. Phosphorescent creatures are extremely common on 
the surface of the sea, as is well known. They belong to the most 
various classes, but are for the most part invertebrate animals, Infusoria, 


NOTES. 421 


Medusz, Polyps, Worms, Tunicata, &c. Phosphorescent fishes have 
been brought to our knowledge by the ‘Challenger’ expedition. Wille- 
moes was able to observe directly the phosphorescence in Sternoptys ; 
and according to Giinther (see below) it is highly probable that the 
peculiar organs occurring in blind fishes are phosphorescent. The fisher- 
men of Nice assert that the moon-fish, Orthagoriscus mola, is luminous. 
But few phosphorescent animals live on land; only afew Myriapoda and 
Annelida, besides the well-known Lampyris, Elateridz, &c. The litera- 
ture of the subject is wonderfully extensive; Ehrenberg, who gave more 
attention to it than almost anyone else, in the last year of his life gave 
us a work with the following title: Die das Funkeln und Axfblitzen des 
Mittelmecres hewirkenden wnsichtbaren Lebensformen (Berlin, 1873). 

Note 23, page 86. The anatomy of the fishes in which Giinther found 
these phosphorescent organs is not yet described, and I owe the notice I 
am enabled to give to «a verbal communication from my esteemed 
friend in London. 

Note 24, page 89. See on this subject the brief remarks in Sir 
Wyville Thomson's The Depths of the Sea (ed. 2, p. 465). Many 
observations on the brilliant colouring of Hoiothuride and Crustacea 
occur in Willemoes’ reports. This proves that the lack of light cannot 
directly hinder the development of pigment. But it is quite possible 
that it may have an indirect influence through modifications in the 
processes, at present unknown, which lead to the formation of the 
pigment. 

Note 25, page 89. Higginbottom, ‘Influence of Physical Agents in 
the Development of the Tadpole of the Triton and the Frog,’ Phil. 
Trans. 1850, p. 431. He reared larvz in dark cellars and in com- 
plete darkness without discovering any difference in their development 
beyond its retardation by the diminished warmth. 

Note 26, page 91. Within the last few years I have repeated a series 
of experiments with a view to investigating the effects of different 
light on the formation of pigment in animals ; the creature selected for 
the purpose was the Axolotl. The more general results attained by 
these experiments are given in the text. The origin of the pigment 
does not depend directly on light, as Bert states, nor do albinos or white 
Axolotl occur in the dark. It remains a mystery to me how Bert could 
have ascribed the occurrence of white Axolotl to the influence of a 
deficiency of light, and I am equally ignorant as to the causes which 
led to the production of albinos among the Axolotl kept by Kélliker at 
Wirzburg. 

I found, precisely on the contrary, that when the light was ex- 
cluded, or ina dark red light, the young animals were always dark- 
coloured; in a yellow light the pigment was abundant though less 
dark; in a white diffused light, but to the exclusion of the direct rays 
of the sun, they were of « much lighter hue, but still not white. 


422 NOTES. 


Specimens which I kept for a year and a half in white vess ls, ard as 
close as possible to a window with white blinds, assumed a very light 
yellow-green colour in which a very few scattered specks of black 
pigment could be detected ; but the small silvery spots which sometimes 
appear on these creatures when about six months old attained so large 
a size, particularly on the tail, that the animal might be described as 
yellow-green with silver spots; in some cases the silver spots predo- 
minated. In young specimens of this year’s brood, which were also 
from the first reared in a diffused white light, the green colour is already 
beginning to show, and the silver spots also on the larger specimens on 
the face of the external branchie. (This is in August; the young 
Axolotl are about 50 days old, and the larger-sized ones about 7 cent. 
long.) These results of my experiments agree tolerably well with 
those of Pouchet and Lister if we assume that the colouring of the 
Axolotl is produced entirely by the action of chromatophores. The 
black pigment and the silvery hue, at any rate, appear to reside in such 
cells, but the pale yellow-green ground hue of the specimens kept in a 
white light appears to result from a pigment distributed and diffused 
throughout all the organs. Hence any pronounced adaptation of 
colouring to the surrounding objects such as occurs among fishes and 
crustaceans is certainly not in question, and nevertheless the whiter 
light induces paler colouring. 

The pigment of butterflies of which the pupz lie buried in the 
earth is developed in total or almost total darkness before they escape ; 
ihe chitinous skin of many pup (of moths, &c.) is so dark as to be 
almost perfectly impenetrable to light ; nevertheless pigment of very 
various characters developes in them. We may, indeed, presume that in 
most viviparous animals the development of the embryo takes place in 
total darkness ; nevertheless they are all born with bright colours. Here 
too must be mentioned the observation made by Kerbert that in the 
embryo of the chick certain pigment cells which appear in the cutis 
about the fifteenth day have wholly disappeared by the twenty-third. 
It may be supposed that not much light, or none indeed, can penetrate 
to the embryo through the egg-shell and membranes; nevertheless, 
pigment cells are formed and disappear again during the course of 
the embryo stage. No explanation of this remarkable circumstance 
las as yet been given, It is not uncommonly supposed that the pre- 
sence of a dark pigment in the skin of human beings is due to the 
greater intensity of light, as proved by the predominance of dark 
races of men towards the equator and by the darkening of the skin 
in summer. As, however, no experiments are before us by which tle 
chemical or heat rays have been excluded from acting on the skin at 
the same time as the light rays, while the action of the air and that 
of difference of nourishment have remained disregarded, we cannot 
consider the conclusion as proved, or even admissible, which asserts that 


NOTES. 423 


the development of the skin-pigment is influenced by light, even with 
regard to man: 

Note 27, page 12. Heincke describes the colours of Gubius Ruthens- 
parri as follows : ‘ Inthe first instance a black velvety spot is conspicuous 
lying at the base of the caudal fin and surrounded by a beautiful golden- 
yellow margin. A similar black spot, but without any yellow rim, is 
found in the malJe, on each side, at the base of the pectoral fin; this is 
absent in the female. The ground-colour of the back of the male 
is, in the breeding season, of a dark brownish black, cutting in on the 
green; on the head it is lighter with a reddish tinge; there are five 
light-coloured saddle-shaped spots with a metallic sheen, and the under- 
surface of the head is of a vivid copper-colour with a golden gleam. 
On each side, between the light and dark colour of the body, and rather 
below the lateral line, there is a row of vivid spots as bright as jewels, 
glittering now blue, now green, and there is a similar spot on each gill- 
covering, which is otherwise red. The two dorsal, the caudal and the 
anal fins are cherry colour and yellow, or show green stripes and bands 
ona dark ground. The pupil of the eye shines a deep blue.’ To this 
somewhat abridged description he adds: ‘I have given these elaborate 
details that the reader may form some idea of the beauty of the Gubius 
at certain moments, for all this gorgeous colouring may vanish within a 
short time and not return in its pristine splendourfor along time.’ He 
then fully describes the chromatophores and their function. 

Note 28, page 97. Dewar, ‘The Physiological Action of Light, 
Nature, 1877, p. 433. 

Note 29, page 99. Darwin himself frequently uses the word ‘colour’ 
where ‘ distribution of colours ’ or ‘ mode of colouring’ would be better, 
Still it is always clear from the context that he attributes to origin 
through Sexual Selection only such variations in colour as occur for in- 
stance in the male and female of the same species, and to Natural 
Selection such as have proved a protection, and therefore advantageous 
to animals. To assume colouring, é.c. the determined mode and arrange- 
ment of colour, is to assume the pre-existence of colour; the question as to 
how this originated has never, to my knowledge, been inquired into in 
these later times, and perhaps it is for this very reason that it has fre- 
quently been confounded with the origin of the mode of its distribution. 
(Compare Darwin, Descent of Man, chap. on Sexual Selection.) Other 
naturalists, at any rate, have certainly made this confusion. Thus, for 
instance, the variation of green to brown which is exhibited by many 
Sphinx-caterpillars has been attributed to natural selection (see Weis- 
mann, Studien zur Descendenz-Theorie, Die Entstehung der Zeichnung 
bet den Schmetterlingsraupen, p. 80). Selection, however, could not 
possibly effect any alteration in the pigment, but could only operate 
after such a change had actually occurred. 

Note 30, page 100. There are many works, not here referred to, on 


424 NOTES. 


tke effects of light on the animal organism. Some are of too special a 
nature and too exclusively addressed to medical physiology—as, for 
instance, the observation that frogs disengage more carbonic acid ina 
green light than in a red one; those on the dependence of the colour 
cf the skin on certain constitutional diseases, &c., while some are not 
sufficiently detailed to allow of their being used here. To these belongs, 
for instance, Strethill Wright’s observation that polyps of the higher 
Acalephee are said to multiply abundantly in the dark by buds, while in 
the light, and with insuflficient supplies of food, they bring forth Meduse. 
To these also belongs all that has been said of the dependence of lighter 
or darker tones of colour on the various intensity of light. Thury’s obser- 
vation that, under a grecn light, tadpoles retain their gill-respiration, 
while their legs are not formed, and that finally they die, comes under 
the same category. Likewise the phenomena of melanism and hyper- 
chronism (see Ridgway, ‘ On the Relation between Colour and Geogra- 
phical Distribution in Birds ;’ Silliman’s Amer. Sourn., sor. 3, vol. iv., 
1872, p. 454), which are attributed sometimes to the influence of heat 
or of light, and sometimes to the general climate. The statement, too, 
that white rabbits are most easily and certainly reared in a white re- 
flected light, is worthy of attention. I owe this remark to Dr. Braun of 
Wiirzburg, who met with it recently in an agricultural journal. In this 
case, as in all cases of experiment, it is requisite to distinguish between 
the different causes, and to investigate separately the effects of each. 
Hitherto we, the zoologists, have made very light of these physiological 
labours. Hartmann, and a certain Herr Hesse, declare that a combina- 
tion of absence of sunlight, cold, and damp, is the cause of the occur- 
rence of albinos (!) among snails; if it isnot the one it may be the 
other, 


CHAPTER IV. 


- Note 31, page 104. The metked of meteorological diagrams could 
at’ most be applicable to cases uninfluenced by annual and diurnal 
variations of temperature ; as those of creatures living in the depths of 
the sea or of fresh-water lakes, in deep springs, or in the intestines of 
warm-blooded animals. And even in these cases such curves would be 
of no practical application, since we cannot transfer them from the 
animals for which they hold good at the present time, so as to draw 
any conclusion with regard to unknown, i.e. fossil forms ; nor can we form 
any opinion as to how the creatures living under such equable tem- 
peratures would behave if they were suddenly or gradually exposed to 
the effects of a different degree of heat. In point of fact, the applica- 
tion of climatic curves—even in its more limited form—has only hin- 


NOTES. 425 


dered the progress of our knowledge of the real effects of temperature 
on the life of animals. 

Note 32, page 165. ‘In the shallow seas of temperate latitudes, only 
such animals can exist and preserve their reproductive powers as are 
qualified to endure every variation of temperature which may occur in 
the course of the seasons—Eurythermal animals, as they may be desig- 
nated in one word. The number of eurythermal marine animals is very 
much smaller than the amount of species which can only live in such 
provinces (marine) as exhibit an equable or slightly variable tempera- 
ture—Stenothermal animals, as they have been called.’ 

Note 33, page 109. I must, however, warn the reader against the 
assumption that every such dwarfed race was produced by the influences 
here described. In many places, for instance, where formerly really 
gigantic pond-mussels were found, now only quite small ones occur; 
and it is well known that the European oysters are gradually becoming 
smaller. This results from the circumstance that both these mollusca 
are capable of reproduction while they are still quite small, and now 
never grow to their full size, because they are destroyed before they 
have accomplished their full growth. The dwarf races of certain Libel- 
lulz in the south of Europe appear again to depend on other causes. 
The multiplicity of circumstances by whose co-operation dwarf races 
are produced appears to be very considerable ; we shall have occasion to 
examine them somewhat more closely in another chapter. Unfor- 
tunately no satisfactory experiments have ever been made. 

Note 34, page 110. Winter-sleepers—i.c. animals which during the 
winter fall into a dormant state, and remain in it for weeks, or even 
months, without dying—are to be found in almost every group. I may 
refer the reader to the enumeration given by Schmarda (Thiergeogra- 
phie, i. 9-11). It might be well to distinguish two groups of such 
animals, according to whether they are warm-blooded or cold-blooded. 
These last—as we infer from some observations merely, it is true, and 
not from experiment—appear to possess the faculty of living a latent 
life at a very low temperature, i.c. to sleep; and if we suppose that no 
alteration takes place in the processes of assimilation, but only a retard- 
ation, every cold-blooded animal might fall into winter-sleep. With 
warm-blooded animals it is otherwise. These, as is well known, are 
easily frozen; according to Horvath’s experiments—to be more 
exactly described presently—it is extremely probable that no warm- 
blooded animals can become winter-sleepers but those which are able 
to become actually cold-blooded at a sufficiently low temperature. 
Even young animals of other species which have at birth a remark- 
ably low temperature, almost as low as that of cold-blooded animals, 
are incapable of enduring low temperatures for any length of time ; they 
fall asleep, it is true, but at the same time they die. Of course, even 
the winter-sleepers among mammals cannot bear to be actually frozen. 


426 NOTES. 


Note 35, page 111, The high body-temperature of the true warm- 
blooded creatures, birds and mammals, oscillates within very narrow 
limits. In man it rises to 36-38° C.; in dogs to about 39°; in the sheep 
to 40° or rather more; in birds it is higher, rarely under 40° and 
usually as high as 42° to 43° C. (For very ample data see M. Ed. Anatomie 
ct Physiolegie Comp., viii. pp. 16-18.) In cold-blooded animals even it is 
always a little higher than that of the surrounding medium, but it 
rises and falls with this, while the true warm-blooded animals maintain 
the same, or nearly the same, temperature in spite of the variations in 
the air or water. How far this may also prove to be the case with 
such cold-blooded animals as have a temperature considerably higher 
than that of the surrounding medium has not yet been investigated ; 
we know, for instance, from Davy, that in Bonitos the temperature 
is 10° C. above that of the water; in Pelamys sarda, 5°; according to 
Czermak, the Proteus of the Adelsberger grotto has an internal heat 
of sometimes 5-6° C. above that of the water. It may also be men- 
tioned that some species of Python, when depositing eggs, have a body- 
warmth of 6° C., and that sometimes a very considerable degree of heat 
prevails in a beehive. 

Note 36, page 112. The facts given by Horvath are of the greatest 
interest. ‘The following is perhaps of the highest physiological impor- 
tance. It is usually supposed that the awakening of winter-sleepers 
is occasioned by a rising temperature; but in Horvath’s investigations 
this was never the case ; during two hours and forty-five minutes, which, 
in the one experiment communicaved, were needed for complete awaken- 
ing, the temperature of the room remained exactly the same—10° C.—as 
during the three previous days when the animal was still asleep. This 
proves that the waking up of the weasel must be caused by some internal 
cause which we do not as yet know. But his other observation is far 
more remarkable; namely, that during the awakening, the body tem- 
perature of the weasel rises rapidly, and more rapidly during the second 
half of the process than at the beginning; for instance, in the experi- 
ment which is given in detail it rose in the first hour and fifty-five 
minutes only about 6°6° C., and in the following fifty minutes about 
17°. This remarkably rapid increase of body-heat took place, moreover, 
without any vigorous movements, which might otherwise have been sup- 
posed to cause it—even the rapidity of breathing showed no increase 
corresponding to the rise of temperature. 

I must not here pass over in silence the view lately expressed to me 
by Dr. August Forel, of Munich (well known by his admirable researches 
on Ants); a view founded on certain observations, hitherto unpublished, 
that winter-sleep does not depend at all on the diminished temperature 
in winter, but rather on influences determined by food. A dormouse 
that he kept went to sleep even at a high temperature of the air, in 
August and September, and slept as soundly as in a true winter-sleep, 


NOTES. 427 


while its body temperature, according to the figures he was so good as 
to communicate to me, was never more than a few degrees higher than 
the air. 

Note 37, page 112. It is a fact that frogs often deposit their spawn in 
water hardly above the freezing-point, and a vast number of inverte- 
brate creatures live in equally cold water. These same individuals, 
however, do not perish when even their body temperature is raised in 
the summer to more than 30° or even 40°. How far they suffer from 
it is, however, not established. 

Note 38, page 114. An enumeration of the observed or reported cases 
of the resuscitation of wholly frozen animals is to be found in Schmarda’s 
Thiergeographic, i. pp. 8 and 98. Insects, fishes, toads, Actinia, 
Crustacea, Mollusca, and Nematoda figure in this list. It is well known 
that fish can be conveyed in ice, or even quite frozen up, and revive on 
being thawed again; but in all these cases the thawing must be very 
slow and gradual ; if it is too rapid the creature dies. Plants exhibit the 
same characters. 

Note 39, page 116. It is difficult—in some cases quite impossible—to 
decide whether the data given are to be depended upon or not; isolated 
and incidentally made observations are often made to serve as evidence 
for general statements. Thus it has been said that the Arctic fox is white 
in winter, and in summer of various colours; but Payne says this is in- 
accurate, and that Canis lagopus may be found white, blue, or grey at 
all seasons of the year. 

Note 40, page 117. Weissmann succeeded in transforming all the 
individuals of a summer brood of Pieris Napi to the winter-form by 
maintaining a low temperature. With reference to Weissmann’s estimate 
of the facts communicated by him as to the rearing of Vanessa levana- 
prorsa, I must observe that I cannot agree with him ; interesting as his 
researches are, they do not seem to me either to have been carried out 
systematically enough to allow of the deduction of any definite con- 
clusion, nor to prove that the speculations propounded by him are 
thoroughly well founded. 

Note 41, page 119. ‘ Assimilation in plants is only possible between 
a specific minimum and maximum of temperature; between these two 
extremes lies an optimum for the development of the species.’ So 
says Pfeffer, and this shows that the same law obtains for plants as for 
animals. Unfortunately we cannot assert that we have ascertained the 
curves of temperature for animals, as botanists have already done for 
many plants. This, it is true, results in great measure from the difii- 
culties offered in experimenting on animals; the phenomena of life 
are far more complicated in them than in plants, they grow much more 
slowly and at the same time are far less easy to measure and to confine. 
However, there are a number of rapidly-growing cold-blooded aquatic 
animals (water-snails, Naidie, Branchipus, Apus, &c.) which are not 


428 NOTES. 


particularly difficult to rear and which can even easily be measured ; 
systematically conducted experiments on them would undoubtedly 
yield very pleasing and valuable results. Brauer’s researches on the 
Phyllcpoda afford a striking proof of this. He found that individuals 
of a species of Chirocephalus lived for weeks in a temperature of 19° C., 
but never attained sexual maturity ; but they acquired sexual functions 
within two days when exposed to a temperature below 11° C. The 
eggs of a species of Branchipus could not be induced to develope 
until after he had strewn them on slowly melting ice broken into 
small fragments. It is much to be regretted that Brauer has given no 
exact curves of temperature for the different species he reared; but 
even as it is, it would seem that we may infer from his remarks that the 
optima of temperature are often very different for quite nearly related 
forms. 

Note 42, page 120. With reference to these animals I must refer 
the reader to Schmarda’s list (Thiergeographic). The highest degree 
of temperature hitherto observed as endurable by any fully grown 
animal without inconvenience of any kind is 75° C.; in Sparus Des- 
Sontainesit in the hot springs of Tozer and Cafrain Tunis. Plateau’s 
observations (Recherches physico-chimiques sur les Articulés aquatiques, 
2me partie, Bruxelles, 1872) only appear to contradict this; for he 
only experimented on such animals as live in cold water or thermal 
springs of moderate heat. There is even a certain contradiction 
between the results of his experiments and the facts he himself records 
as to the existence of animals in these thermal springs. These springs 
have, according to Plateau’s list, « temperature corresponding exactly 
to the maximum which can be generally endured by such animals; 
thus, if we regard his experiments as conclusive, they would live and 
propagate in a temperature which, being indicated by the extreme limit 
of the curve constructed to show their power of living, could not 
therefore be regarded as actually favourable to them. However, the 
experiments themselves were not conducted on a conclusive plan, for 
the gradual cooling of the water was not prevented, nor were the 
investigations carried on for a sufficiently prolonged period. But we 
must not be led to confound with these certain well-known cases of 
great resistance to extreme heat exhibited by the eggs of the lower 
animals, Insects, Rotatoria, Nematoda, &c., or by the capsuled Infusoria, 
or by certain larve. It would seem as though these were thrown into 
a state of latent vitality—exactly as under conditions of extreme cold 
—during which their vital functions remained inactive; while in the 
animals living in hot springs exactly the same processes of assimilation 
must take place as in those living in colder water. An experimental 
investigation on an extensive scale of the power of resistance to high 
temperature in the eggs or germs of the lower animals would certainly 
amply repay the trouble. Brauer states that the eggs of Apus caneri- 


NOTES, 429 


formis and Branchipus stagnalis and torvicornis may be exposed to 
the utmost heat of the sun without perishing. 

Note 43, page 121. Summer-sleepers are found among the most dis- 
similar groups of animals. The Tenrec of Madagascar (Centetes) is 
well known. Darwin found insects, spiders, snails, toads, and lizards, in 
a summer-sleep, in Brazil. Most of the land mollusca of the Mediter- 
ranean province pass the summer in a dormant condition, and the same 
is true in the tropics. (Comp. Schmarda, Zhiergeog.i. 12.) As I have 
said in the text, in most cases, perhaps in all, the true cause is the 
dryness of the air which is usually associated with a high tempera- 
ture. No experiments have been made that can prove that during the 
summer-sleep the vital processes are not merely reduced to a minimum 
of energy, but also altered as to their nature—as they are in winter- 
sleep. 

‘Note 44, page 122. Jt is known that in man for instance, sexual 
maturity is attained at a much earlier age between the tropics than in 
northern climates; girls of twelve are in Cuba regarded as fully grown 
and marriageable. This phenomenon is far more striking in swine. I 
myself have seen pigs in Manila of which the males at three weeks old 
were ready and fit to be put with fully grown females. But the great 
variety of circumstances which co-operate to produce such early maturity 
leave it doubtful whether it is solely due, as is assumed, to the high 
temperature of tropicai climates. We have seen, in note 15 to Chapter 
II., an instance of early maturity induced by suitable nutrition. 

Note 45, page 125. It is known that during the winter frogs eat 
little and hardly grow at all, nevertheless their eggs are formed during 
that season. Precisely the same obtains with regard to Lymnea; I 
have proved that the minimum of temperature which allows them to 
assimilate food and so to grow is much above ihe winter-temperature 
at which they deposit their eggs. It is known, moreover, that the larvae 
of frogs which have not grown fast enough in the first year to allow of 
their transformation taking place in due time, live through the winter 
as tadpoles and do not begin to grow again til) the following summer. 
Now, it would be interesting to investigate whether in larve thus 
retarded by cold the germ-glands are any further developed than is 
normally the case in the larva stage. 

Note 46, page 125. According to the result of researches conducted 
by me during many years, in large individuals of the Axolotl about 
48 hours are sufficient to allow of a large number of unfertilised ova to 
pass from the ovary into the oviduct, to become surrounded with an 
albuminous envelope, to be fertilised and deposited. I have repeated 
the experiments which prove this many successive years with the same 
results. If the Axolotl is kept in small aquaria without plants or 
sand, individuals that are sexually mature will deposit no ova, even 
thongh the water is changed daily and they are well supplied with food. 


430 NOTES. 


I have by this process reduced to sterility for two whole years eight 
oid Axolotls, which had previously produced seven generations of young 
ones; when I replaced them in a large aquarium with running water, 
sand, pebbles, and plants, in fifty hours they began to deposit eggs again 
and produced from 900 to 1,000, of which at least 700 developed. This 
in no way depends on the season of the year; at least three broods of 
eggs can be obtained from the same female inone year. But the experi- 
ment is apt to be rather a dangerous one, for the males not unfrequently 
perish if the sexual processes are interrupted too soon. 

Note 47, page 126. To these larva-forms belongs the much-talked- 
of Axolotl, whose capability of becoming under certain circumstances 
a gill-less land-animal—Amblystoma—has been most undeservedly 
celebrated as a perfectly marvellous phenomenon. It was assumed 
that in Mexico, its native home, it never underwent. any such transfor- 
mation. But this is incorrect, for in the museum at Vienna there are 
specimens of Amblystoma and of Axvlotl which were collected at the 
same time in the lake of Mexico. I owe this observation to my friend 
Steindachner. The Axolotl of Lake Como, by the Central Pacific Rail- 
way on the summit of the Rocky Mountains, according to Mr. Carlin, 
always is transformed into Amblystoma—it is Amblystoma mavortium. 
But the effects of an insufficient body of water, which is said by Weiss- 
mann to cause the transformation of the Mexican Axolotl, cannot occa- 
sion it in that of the Rocky Mountains, for it takes place in the water ; 
and the Amblystoma, so long as they are little, actually live exclusively 
in the water, asI know by my own experience. A young Amblystoma, 
which I kept alive for a long time, never went out of the water of its 
own free will, while one nearly twice as large lives entirely on Jand 
and only takes a bath now and then. It always goes into the wate. 
when the temperature of the air in the cellar, in which my aquaria 
stand, falls below that of the water—down to about 6° or 8° C. 

Note 48, page 129. Brauer’s researches on the Phyllopoda contain a 
mass of valuable observations on this point, which I will here collect 
and reproduce for the sake of their wide general interest ; unfortunately 
they cannot be tabulated. 

In all the species of Branchipus, e.g. Chirocephalus Braueri, which 
first appear in early spring in pools of snow-water, a rapid rise of tem- 
perature from the freezing-point is the first and chief condition of de- 
velopment. These species perish at a temperature of 19°C. Atasuitable 
temperature—about 10°C. ?—the development of Chirocep!alus from the 
egg to sexual maturity takes only twelve days. , 

Freezing the soil acts upon Apus caneriformis, Branchipus stagnalis, 
and B. torvicornis, in the same way as desiccation, and in warm spring 
days they develope in snow-water pools just as quickly as at midsummer 
in warm rain-water pocls. 

Eges of Lepidurus productus kept in damp earth from Aoril till 


NOTES. * 431 


December, and then exposed to freezing for 14 days, at 6° C., yielded a 
large number of larve. The fully developed Lepidurus productus can 
bear a temperature only of from 0° to 18°C. 

The eggs of a species of Apus from Khartoum, on the other hand, 
developed in great numbers at 25°C. 

Compare on this subject the data given in ‘ Saison-Dimorphismus’ by 
Weissmann, as to the accelerated development of different individuals 
of the same species of caterpillar under a raised temperature. 

Note 49, page 132. Mobius says: ‘ Mollusca, Crustacea, and worms 
which occur in the deepest parts of the Arctic Ocean, are also found in 
the shallow portions of the Baltic ; but they are much larger than those 
in our milder latitudes, because no extreme changes of temperature there 
interrupt the quiet order of the vital processes, as they do in our more 
variable seas.’ 

Note 50, page 133. Mr. Buxton himself has not given any account 
of the matter. I have taken my information from an interesting paper 
by Herr E. Friedel, in the Zvologischen Garten, 1871, p. 65. In the 
winter of 1867-68, the co!d in Mr. Buxton’s wood marked —7° C., and 
yet not one cockatoo perished. Strangely enough, the Carolina parrot 
(Psittacus carolinensis) suffered most, though in America it is distributed 
as far as Canada; while the true tropical Cockatoos of the Moluccas 
throve extremely well. It is to be regretted that this experiment of 
Mr. Buxton, who is now dead, should not have been still further carried 
out by his brother. : 

Note 51, page 136. Mobius says: ‘We ought not to be surprised at 
finding eggs of mollusca and of worms in the depths of the Gulf of Kiel 
at every season of the year, even when it is covered with ice. On 
January 26, 1862, a stake was pulled out of ice which had been formed 
eight days, to which clung clusters of the eggs of Dendronotus arborescens 
and Aolid@. Nevertheless, most of our Opisthobranchiata spawn most 
freely from May to July.’ Hence a certain periodicity is displayed in 
the Gulf of Kiel ; and it would seem, according to Mobius, that the young 
brood is caught in the greatest abundance soon after the time when the 
deposition of eggs has reached its greatest height, while large specimens 
are found all the year round. 

Note 52, page 136. In the Gulf of Kiel the mean monthly tempera- 
ture varies, at the depth of 16 fathoms, between a maximum of 14° and 
a minimum of 15° C. In the Philippines the difference between the 
extreme monthly mean of the temperature of the air reaches at the 
utmost 7°. In the Baltic and in England, as remarked by Meyer and 
Mobius, at the same depth, the water is much less strongly affected by 
variations in the temperature of the air. To what depth these variations 
do in general affect our seas is not known, and it must at any rate be 
greatly influenced by local conditions. 

Note 53, page 136. The largest number of Aspidochirote in the 


432 ‘NOTES. 


Philippines live in0° F.; in the Mediterranean, in 20-30° F.; in the 
North Atlantic, in 30-60° F. 

The largest number of Dendrochirote in the Philippines live in 1- 
10° F.; in the Mediterranean, in 10° F.; in the North Atlantic, in 
20-60° F. 

The largest nuwber of Synaptid@ in the Philippines live in 0° F.; in 
the Mediterranean, in 1-10° F.; in the North Atlantic, in 1-10° F. 

Brachiopoda—Lingula—which occur only at great depths in the 
North Sea, in tropical seas are found near the surface, and even some- 
times exposed to the ebb and flow of the tide. 

Note 54, page 137. If we assume that the place where we find the 
greatest number of individual species and genera living together is to be 
regarded as their primary habitat—or centre of distribution—then the 
Crinoids, Sponges, and many other remarkable forms now living at the 
bottom of the sea must decidedly be designated as cold-water animals. 
For by far the larger number of them live at depths where the tempera- 
ture remains without any conspicuous variation throughout the year at 
the low point of from 1-2°C. 

Note 55, page 139. Schmankewitsch’s observations on Artemia and 
Branchipus promise to be of the highest interest in this respect. He 
found that in individuals which had their assimilation interfered with 
by a too considerable increase or diminution of the saline components 
of the water, the injurious effects of this saltness could be entirely neu- 
tralised by adiminution or, on the other hand, by an increase of tempera- 
ture. He furthermore observed that the size of the gill-sacs of Artemia 
was directly dependent on the temperature of the water, increasing in 
size with a higher degree of warmth. However, he detracted from the 
value of his observations by introducing into his estimates one wholly 
unknown quantity, namely, the amount of air contained in the water 
(in his experiments), and by attempting to explain everything by the 
variable proportion of air contained in water of different salinity at 
different temperatures. General propositions, such as he puts forward 
hypothetically as to the part played by the air contained in the water, 
are in such a case of no use, or even misleading. 


CHAPTER V. 


Note 56, page 146. It was impossible to enumerate in the text all 
the fresh-water animals that live in salt water; I heresubjoina tolerably 
complete list, which, however, makes no pretension to being absolutely 
exhaustive. 


NOTES. 433 


Turbellaria. 


Microstomum lineare, at Greifswald, in the Baltic (M. S. Schultze, 
Arch. f. Naturges., 1849, v. 15). 


Annelida. 


Enchytreus spiculus \ Frey and Leuckart, at Heligoland, on the 

Senwrus neurosoma f seashore in mul (Frey and Leuckart, Zur 
Kenntniss virbelloser Thiere). 

Tubifex papitlosus, Clap. 

Heterochata costata, cian | In the A‘lantic (Claparéde). 

Ctenodilus pardalis, Clap. ie 

Pachydrilus, Clap.—All the species live in brine-pools, as Kissingen, 
Kreuznach, &c. ; 

Pontodrilus Marioni.—Sea-coast near Marseilles, in pure sea- water. 

Cystobranchus viridis, Verrill.—A leech, living equally in fresh and 
salt; water (Report of Prof. Baird on Fisheries for 1872-73, p. 686). 


Arthropoda. 
1. Crustacea. 
( Found by me in estuaries which occasion- 
ally contain strongly salt water, at Zamboanga, 
S.W. point of Mindanao, Phiiippine Islands— 


Gammarus, species October 15, 1859, by my diary. The typical 
Cyclops, species genus Palemon is a true fresh-water form; 
Cypris 4 almost all the species live in pure fresh water, 
Palemon Id@ (Heller) | and many occur high up in mountain streams 
Palemon, species n. as far as 6,000 feet above the sea. Only the 


two species here mentioned occur in brackish 
water or on the sea-shore. Palemon Ida I also 
(found in the harbour of Hong Kong. 

Astacus._Two species in the Caspian Sea; associated with marine 
species (Hichwald, Arch. fiir Naturg., iv. Jabr.). 

Branchipus stagnalis, a typical fresh-water form, is said by Braun to 
grow much larger in salt than in fresh water, but he does not mention 
whether the Crustacean remains otherwise unaltered. 

Daphnis rectirostris and other species live, according to Schmanke- 
witsch, equally well in salt and fresh water, but they exhibit certain 
differences depending on the medium. 

2. Avachnida.—Sea-mites are by no means rare. Gosse has described 
three English species (Ann. Wat. Hist., ser. 2, vol. xvi. pp. 27, 305). 

Pontarchus was found by Philippi on the shore at Naples. 

Thalassarachna Verrillit (Pack.) lives in deep water off the American 
coast. (Silliman’s Am. Journ., 1871, February). I myself have found 


434 NOTES. 


true spiders in fissures in blocks of coral which were under water at 
every high tide ; they were very common in Bohol and at Zamboanga 
in the Philippines, but as yet remain undescribed, for my collection of 
Arachnida is in the Hamburg Museum, and has not been worked 
upon. 

8. Insecta.— Darwin, in his well-known Naturalist’s Voyage, alludes 
in many places to marine insects, principally beetles and bugs. Many 
insects have lately been discovered on the coast of North America by 
Baird (Rep. on the Condition of the Sea-fisheries of the South Coast of New 
England, 1871-2, p.1) and Packard (Proc. Essex Inst. vol. vii. p.44, and 
Silliman’s Journal, 1874, p. 131). These are beetles, bugs, and flies. I 
found a few marine insects in the Philippine seas, but they unfor- 
tunately remain undescribed., Of older observations I may mention 
Slabber’s dipterous larva, probably the larva of a species of Chirononus ; 
and I found an abundance of a very similar species in the Philippine 
seas, where swarms of flies sometimes cover the surface in still bays; 
then Audouin, who observes that Blemus fulvescens surrounds itself, 
like the fresh-water Argyroneta, with a bubble of air. Among the 
Hemiptera—Salda, Coriva, Hygrotrechus, and. Halobates—the species of 
Halobates are most conspicuous, for they are found in every stage of 
development running about on the surface of the sea, often hundreds 
of miles from land. Hight species of the genus, as I am informed by 
my friend Dr. Hagen, have been described ; that described in the text 
and discovered by me is a new species and the largest of all. They are 
found in the Atlantic, Indian, and Pacific Oceans, as well as in the 
Chinese Sea, but only in tropical or subtropical regions. 

Insects are also found in salt-water lakes inland. Packard found 
eight different species in Clear Lake (Silliman’s Journal, 1871) and one in 
Lake Mono. Numerous insects exist in the brine lakes of Europe, but 
no collection or complete description of them is known to me. I experi- 
mented this year on some larva: of flies which I found in a basin in the 
courtyard of the Wiirzburg University; they lived in sea-water very 
happily for five orsix days, but then perished. I suspect, however—and 
shall test it more accurately next year—that they died for want of food. 
Compare with this Plateau’s experiment ; see below. 

Mollusea.— Cyclas, Unio, and Anodonta live in the Livonian Gulf 
associated with Yellina and Venus. In the Baltic we find Lymnea 
auricularia and orata, and Nevitina fluviatilis with marine mollusca. 

Paludina and Neritina are found in the Caspian with Mytilus and 
Cardium, according to Kichwald. 

Planorbis glaber (Jeffreys) is found in 1,415 fa’ homs north of Cape 
Tenez, Algiers. 

Unio sp., within reach of the salt-water flow in Brisbane river; 
(Voy. of Rattlesnake, vol. ii. p. 362). Baer found Unio at the mouth of 
the Dwina. 


NOTES. 435 


Nilsson found an Anodonta on the sea-shore in Sweden and Norway, 

Neritina viridis in the sea, in 3-10 fathoms, and in estuaries in the 
West Indies. 

Neritina matonia (Risso) at Nice. 

There are many brackish and salt water species among the Neritine, 
and several of them are highly characteristic of their habitat. I my- 
self found not less than 16 or 17 species in the Philippines. In pure 
salt water (3-4 per cent.) I found Meritina Mortoniana, pulchella, and 
punaensis new sp. all belonging to the same group. In brackish 
water, or in spots bathed by salt and fresh water alternately, were 
the following: W. Mortoniana, paradova (new sp., cassiculum ; then 
eubauriculata and four allied species; in the mangrove swamps, JV. 
communis, ziczac, and a few other species, and finally, in the same 
locality, but exclusively on the trees, V. dubia, cornea, and subsulcata. 

Melanopsis costata, in the Dead Sea (Schmarda, Geog. der Thiere, 
i. p. 53). 

Rissoa ulve, a Hydrobia, in slightly salt water or in very salt water. 


Vertebrata. 

Gastcrosteus aculcatus 

Anguilla fluviatilis 

In the brackish water of the Baltic Archipelago, according to 
Eckstrém, the following fresh-water fishes are found living: 

Cottus gobio, Lota vulgaris, Gasterosteus, Acerina, Lucioperca, and 
thirteen Cyprinidzx. 

Eichwald found the following fishes in the Caspian Sea: Cyprinus, 
Esox, Perca, Lucioperca, and Cobitis, associated with true marine species 
— Clupea, Syngnathus, Gobius. 

Jé we regard the Crocodile as a typical fresh-water animal, we must 
mention here that Crvcodilus biporcatus of the eastern hemisphere, 
and an American species, according to Humboldt, live in thesea. Ambly- 
rhynchus ater is also a marine reptile. 

The mammalia and birds.that live in the sea can scarcely be in- 
cluded under this head, and an enumeration of them would be super- 
fluous, as they are very generally known. 

Note 57, page 146. All sea-snakes are viviparous. The females retire 
to hollows in the rocks in low islands where the young are born, and 
they do not immediately abandon them, though it is not known how long 
they remain with them. I found once on the east coast of Mindanao 
an enormous female, apparently of Platurus fasciatus, lying quietly 
curled up between limestone cliffs, and among its rings and partly on 
its body lay at least twenty young ones which already measured, as I 
should estimate, more than two feet in length. It was by the narrowest 
chance that in climbing over the ctiffs I did not walk into this nest of 


\ Gulf of Riel. 


436 NOTES. 


snakes. My foot was raised, and not more than two feet from the spot, 
when I discovered the venomous brood just below me. 

Note 58, page 148. The following list of marine creatures living in 
fresh water is not complete. 


Colenterata. 


Cordyluphora lacustris (see text). 


Bryvzoa. 


Menbranipora bengalensis (Stoliczka, Proc. Asiat. Soc. Beng. July 
1878). 

Victorella pavida (Kent, Quart. Journ. Micro. Soc. 1870). 

A Flustra of a closely allied species in a fresh-water tank at Nagpoor, 
on the shells of Paludina bengalensis and on water plants (Ann. Mag. 
Nat. Hist., ser. 3, vol. i. p. 168). 


Annelida. 


Nercis and Nemertes of various kinds were found by Tscherniawsky 
in a lake in Mingrelia of which the waters are drinkable. A/anayunkya, 
a Cephalobranchiate discovered by Leidy in fresh water at Philadelphia. 


Arthropoda, 


The number living in fresh water is extraordinarily great; I will 
here mention only the most important, and refer the reader to the great 
work of Ed. von Martens. 

Species of Balanusin Lake Paleotoma, according to Tscherniawsky. 

Cypris salina and Cypridopsis aculeata in quite fresh water, accord- 
ing to Brady (Aut. Hist. Trans. Northumberland and Durham, vol. ii. 
part i. p. 121). 

Bopyrus, sp. div.? Besides those in the Philippines (see text) 
species occur in India; and in various museums, as in Munich, I have 
seen them with a fresh-water Palamon, P.indicus. To my knowledge 
they have not yet been described. 

A species of Peneila, according to Peters, lives ona Gobius in pure 
fresh water in the Laguna de Bay at Manila. 

The marine forms which now are found at the bottom of the 
Swedish and Norwegian lakes are of the highest interest: Mysts velicta 
(Lovén) is still living as Mysis oculata in the sea near Greenland; 
Pontoporcia affinis lives in the Baltic and in northern fresh-water lakes. 
Both species have lately been found by Alleyne, Nicholson, and Smitb, 
in the great North American Lakes Ontario, Superior, and Michigan (see 
Silliman’s Journ, ser. 3, vol. v. 1873, p. 387; vol. ii. 1871, pp. 373, 448 ; 
vol. vii. 1874, p. 161. 

I may add to the list of fresh-water crabs given by Martens Varuna 


NOTES. 437 


litevata, of which I have found identical specimens on Fucus, on the 
high seas, in brackish water in the estuaries of tne Philippines and in 
pure fresh water high up the country in Luzon; it also livesin Lake 
Taal. Three or four still undescrited species of the marine form 
Hymenosoma | discovered in the bogs and rivers of the Philippines and 
in the river near Canton. Birgus, Cenobita, many Grapsvida, Gecarcinus 
and others—Crustaceans living on land—properly speaking do not 
belong here, but they may be mentioned, for at least they do not live in 
sea-water and are certainly often enough exposed to rain. 

A species of Penwus lives in a tributary of the Sutlej at the foot of 
the Himalayas (Huxley, Proc. Zool. Soc.1878, p. 787). Peneus brasiliensis 
goes high up the rivers of North America. (Baird. Rep. on Fisheries, 
1872.) 


Mollusca. 


No marine mollusca living in fresh water, besides those named in 
the text, are known to me; perhaps, however, Yeredo senegalensis 
(Blainv.) may be added. Aucapitaine’s statement, that Cyprea moncta 
is found in the waters of the interior of Sudan and caught by the 
natives with calf’s hides, has been disputed, but Aucapitaine repeatedly 
maintained the truth of his assertion. It does not appear, however, to 
have been confirmed by later travellers. 


Vertebrata. 


‘Many migratory marine fishes might be added to the list given in 
the text; for instance, the species of Shad, many Pleuronectes and 
allied forms—a flounder occurs as high up as Metz and Tréves (Trier) 
according to Leuthner. Cat-fish and the well-known Manatus live 
in the rivers of S. America (the Indian sea-cow-—Halicore—the nearest 
ally of the Manatus—is found only in the sea) with numerous other 
fishes whose nearest congeners are typically marine, such as a species 
of Diodon. In the eastern hemisphere several species of the genus 
Hemirhamphus live in fresh water which are only specifically distinct 
from their marine allies. 

Note 59, page 151. According to Bernard’s researches on the frog 
and Plateau’s on Crustacea, we might almost be tempted to suppose 
that in all animals that migrate from the sea to rivers, and vice versd, 
the different degree of saltness between their tissues and the surround- 
ing water would be rapidly equalised by the osmotic action of the skin. 
In many creatures, as, ¢.g., the Stickleback, this is no doubt the case— 
though no conclusive experiments have been made even on this fish; 
in others, as the Crocodile, it may be doubted whether even in indi- 
viduals actually living in the sea the flesh would be salt. No exact 
investigations exist. From the easily observed fact that a fresh-water 
stickleback when suddenly transferred to salt water cannot at first 


438 NOTES. 


swim at the bottom of the aquarium, on account of its relative lightness, 
and that, by degrees, it acquires the power of doing so, it has been 
inferred that this acquired power is dependent on the impregnation of 
its tissues by the salt water. This, however, has not been proved, and 
there is another way by which the fish may be able to alter its specific 
gravity—by the reabsorption of the air contained in its air-bladder 
and constantly renewed in it. In highly aerated fresh water so much 
air is deposited in this air-bladder and in the vessels generally, that the 
fish is rendered lighter than the water, and cannot go to the bottom 
even in fresh water. In my aquaria I have seen Sticklebacks, Bleak, 
and Axolotl perish from the superabundance of air in their tissues in 
consequence of the constant addition of highly aerated water. On the 
other hand, a diminution of the air contained in the air-bladder might, 
of course, easily occur, and thus the specific gravity of the animal would 
be raised while the whole volume of its body remained the same. 

Note 60, page 154. As Beudant’s small work is not easily accessible, 
I shall present the reader with an epitome of his tables on the next 
page. 

It must, however, be observed that in this inquiry no regard is paid 
to temperature. Now, since, under variations of temperature, the 
respiratory requirements of the cold-blooded animals are extremely 
different, we may be allowed to assume that a due regard to this cireum- 
stance would have led to somewhat different results from the same 
experiments, 

Note 61, page 154. At four miles east of Kiel there is a fossil oyster- 
bed. ‘Thousands of years after the oyster-bed of Waterneversdorf had 
become dry land, oysters lived in such numbers on the coasts of the 
Danish islands that they were used for food by man of the “stone age” 
in this region.’ Mobius is inclined to attribute the failure of the oyster 
in the Baltic to its low degree of saltness, combined with the long 
duration of alow winter temperature, and the absence of any regular 
movement of the sea by the tide; he assigns the same reasons for the 
absence of the lobster, the large crab, Platycarcinus pagurus, and the 
edible sea-urchin, Hehinus esculentus. 

Note 62, page 155. Prof. Verrill, of Yale Coll., U.S., one of the most 
accurate students of the American Crustacea, in a conversation I had 
with him on this subject, disputed the accuracy of this estimate of 
Schmankewitsch. He said: ‘ The only characters which can be relied 
upon for distinguishing the genera Branchipus and Artemia are the 
male prehensile organs, and these have been entirely overlooked by 
Schmankewitsch.’ I have neither time nor materials at my disposal for 
a close investigation of this point, and I will only observe that in hig 
latest work he does take the prehensile antennz of the male into con- 
sideration. 


439 


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440 NOTES. 


Note 63, paye 159. It is quite evident that, in point of fact, a modifi- 
cation of the animal does not always result from a change from fresh to 
salt water, and vive versd, since migratory fishes exhibit no effects from 
the change of medium. Here it might, no doubt, be said that these 
changes have not time to take effect, being too rapid, Lut there are 
animals which occur simultaneously in fresh, brackish, and salt water, 
and yet exhibit no differences, while other species display widely diver- 
gent forms according to their habitat. To the former belong Crocodilus 
biporcatus, Varuna literata, and others ; to the latter Nevitina Mortoniana, 
which in the sea is smooth, but which in brackish or fresh water often 
developes spines, the distinguishing mark of the sub-genus Clitun, which 
is characteristic of fresh-water streams. 

Note 64, page 159. The remark that only small animals occur ina 
small area is an old one, Lut not altogether accurate. The saying is 
familiar that the largest mammals occur only on continents. Even man 
is to a certain extent subject to this law. Seafaring men, who pass the 
greatest part of their lives, from their youth up, confined in an extremely 
narrow space, are generally small, often below the middle height; but 
it may, at any rate, be questioned whether their small stature is a result 
of this mode of life, or not rather of the nutrition, the lack of air, hard 
labour, &c. In other cases, as those of land mollusca, insects, land- 
vertebrata, and others, of which the same observations have been made, 
it seems scarcely credible that their small size should be attributable to 
the direct influence of a narrow area and to nothing else. Thus, for 
instance, the fact that only sma}l Jand-animals occur on small islands 
of recent origin is easily explicable; for as each of these has received 
its fauna frora beyond seas, the smallest animals have most easily reached 
them, being the most easily transportable, while many large species 
must be wholly excluded. All investigations on this question of the 
influence of area ought at any rate to begin with fresh-water animals, 
since in these the combined causes exhibit the least diversity. 

Note 65, page 163. The fact is not new. Mr. Jabez Hogg observed 
it long ago, but he arrived at no general results from experiments, and 
even his incidental observations are not particularly satisfactorily set 
before us. (See Journal of the Microscopical Society, vol. ii. 1854, 
‘Transactions,’ p. 91.) Blanchard’s observations are neither useful nor of 
general interest. 

Note 66, page 166. In order not to oceupy too much space in my 
text, I have forborne from mentioning many details of my experiments ; 
but a full report of them will be found in my treatise, Veber die Wachs- 
thumsbedingunaen der Lymnaus stagnalis (in Arb, aus dem Zool.-Zoot. 
Inst., Wirzburg, 1874, vol. i.). So far as I can detect, every objection is 
met by the facts there detailed ; some even are fully discussed, particu- 
larly one not mentioned above—that the relative proportion of the 
surface of the water to its volume may affect the growth, because the 


NOTES. 44] 


amount of air absorbed depends upon it. This objection is positively 
refuted by experiment, and I have given the figures, which prove that it 
is quite immaterial whether the surface of the water exposed to the air 
is large or small; and in the same way the variety of forms and sizes in 
the vessels employed in the experiments sets aside the idea that the 
lateral pressure could have any appreciable effect. 

Note 67, page 167. This seems to have been the case with the 
Asellus reared by me in an hermetically closed aquarium (see p. 160). 

Note 68, page 167. In view of the obscurity which prevails on this 
point, I think it advisable to appeal to a physiologist of acknowledged 
repute. Paul Bert says in his Legons sux la Physivlogie comparée de la 
Respiration, word for word, as follows: ‘La question de savoir 4 quel 
organe il convient d’attribuer . . . la fonction respiratoire est souvent 
débattue avec unc insistance pour le moins inutile. Toute membrane 
animale étant susceptible de dissoudre l’oxygéne et, par suite, de se 
laisser traverser par lui, il est évident que la surface extérieure du corps 
est, tout entiére, une surface respiratoire, et que toute surface intérieure, 
comme le tube digestif, peut et doit étre elle-méme, si le milieu oxygéné 
s’y introduit, une surface respiratoire.’ (‘The question to which organ 
we should attribute the function of respiration is often discussed with a 
persistency which, to say the least, is useless. Every animal membrane 
is capable of dissolving oxygen, consequently of being penetrated by it ; 
so it is evident that the whole external surface of the body is a respira- 
tory surface, and that any internal surface, as, for instance, the digestive 
canal, can and must also be a respiratory surface, if the oxygenated 
medium can but reach it.) We naturally designate as an ‘organ of 
respiration ’ in the stricter sense, one which by its laminated or foliated 
structure and highly developed vascular tissue appears to be specially 
qualified for the function of respiration. 

Note 69, yage 167. All animals living in water are not characterised by 
a soft skin—for instance, crocodiles, turtles, many snakes, the whale, 
many insects, &c. In all these respiration is effected by vessels fitted 
for the passage of air, by lungs in the vertebrata, and by trachez in 
insects. But among these last, when gills do occur, as is the case with 
many larve, the membrane which covers them is extremely thin, and 
easily penetrable by the air. 

Note 70, page 171. These mantle-gills of Lucina philippinensis have 
not been hitherto described, and are figured for the first time in the 
text. They are large tufts which form two pairs situated on the sur- 
face of a membrane which begins at the anterior adductor and traverses 
the pallial cavity, and which has in its posterior part a narrow slit for 
the passage of the very long and slender foot. These tufts of mantle 
gills are during life very large; they contain, internally, an extremely 
developed vascular network ; the vessels unite at the root of the gills to 
form a large trunk, which passes, without becoming confounded with 


20 


442 NOTES. 


the gill-veins, directiy into the ventricle of the heart. The other 
species of this genus are not endowed with these mantle-gills. 

Note 71, page 171. Besides the twe modes of respiration proved 
by experiment to exist in water-animals (by the outer skin or by the in- 
testine), yet a third mode of respiration seems sometimes to occur. In 
all mollusca, beyond a doubt, a certain amount of water is taken up into 
the body and actually into the blood; this certainly serves to dilate 
the tissues—e.g. in the foot—but it is probably useful also for respira- 
tion. But this is at present merely an assumption, founded on no exact 
experiments. The ways in which water, whether fresh or salt, is said 
to penetrate to the blood are twofold. Many authors assert that it 
takes place through the pores and the margin of the mantle; others 
say that it must first pass through the renal organs, which are never 
absent from mollusca. After carefully weighing all the treatises on 
the subject, even the most recent labours of Griesbach, I must declare 
that neither the one nor the other is absolutely proved; the second 
hypothesis, however, seems to me, judging too from my own investiga- 
tions, to be the more probable. Both perhaps may be correct, but here, 
as in all physiological questions, experiment can alone supply the 
answer. 

Note 72, page 172. Comparing an eel with a gudgeon of equal 
weight, the cylindrical form of the eel giving it a much greater extent 
of surface, the gudgeon consumes in the same time—three hours—on an 
average 13:8, and the eel only 7°4 of oxygen (see Bert, Lecons sur la 
Physiologie comparée de la Respiration, 1870). His critical observations 
on the popular, but erroneous, hypothesis that fishes which are tenacious 
of life, as the eel, can live for a long time on land because their gills 
are kept free by means of the water contained in the gill-sac, are well 
worthy of attention. 

Note 73, page 172. Many important observations have been made 
as to the interesting phenomenon of intestinal respiration in Cobitis 
fossilis. This fish swallows the air, taking it in through its mouth, and 
it is deprived of a portion of its oxygen in the intestine. Many other 
fishes, however, Go the same, as species of Cyprinus (see Note 75 
below). Jobert has recently shown that various Brazilian fishes breathe 
in the same way as the Cobitis, and even have in the intestine certain 
processes or folds of the mucous membrane which seem especially 
adapted to that end; these are species of the genera Calichthys 
(Siluridx), Deras, and Hypostomus. We might almost venture to ask 
whether the Cyprinide of European waters, when they take in air 
through the mouth, do not send only a portion of it through the gills 
and truly swallow the remainder, so as to keep the mucous membrane 
of the intestine directly supplied with oxygen. If we prevent the 
species of Leuciscus from coming to the surface of an aquarium by 
jlacing a wire net just below the surface of the water, so that they 


NOTES, 443 


cannot gulp the air, they soon die, even when an ample supply of highly 
aerated water is constantly added ; frogs, on the contrary, it is almost 
impossible to kill in this way. This, however, depends to a great extent 
on the temperature. The lower the temperature the greater is the 
fish’s power of resistance. 

Note 74, page 173. Bert’s experiments were extremely interesting. 
He proved that at a temperature varying between 0° and 13° C. the 
oxygen held in the water sufficed frogs for a considerable time, for 
they need but little. At 19° C. (water temperature) a frog died in 36 
hours when enclosed in a bladder which contained almost five litres (less 
than five quarts) of water; the frog had absorbed all the oxygen con- 
tained in the water, as was proved by analysis. This shows that at 
19° C. the requirements of the frog are very high. The Axolotl 
(Siredon mexicanus) can endure not merely the excision of the gills 
but even the complete removal of its lungs, so that in this animal, as in 
the frog, respiration by lungs and gills can be perfectly replaced by 
respiration through the skin. I will also observe, incidentally, that it 
does not appear to me to be clearly proved that those Amphibia which 
are provided with both lungs and branchia—as Sircdun, Menubranchus, 
Menopoma, &c.—do actually breathe through their lungs ; i.e. that the 
air they gulp in through their mouth is distributed to the lungs. The 
anatomical structure of the glottis does not seem to me particularly to 
support this assumption. May not their lungs correspond physiologi- 
cally rather to the air-bladders of fishes? (See Note 75.) 

Note 75, page 173. Since the publication in 1857 of Milne- 
Edwards’s great work, which treats of the processes and organs of 
respiration in animals, some newer and not unimportant works have 
appeared. Emery suggests the question whether Amphibia may not store 
up oxygen in their lungs, as it has been demonstrated that fishes do in 
theirair-bladders. Gréhant shows that a fishabsorbs the oxygen normally 
existing in its air-bladder when it is kept in water of the temperature 
of the air. Moreau asserts that the amount of oxygen contained in the 
air-bladder increases with an increase of the action of the air-bladder ; a 
tench, in which he tied up the air-passage leading to the air-bladder, at 
the end of a fortnight had in it more than the normal proportion of 
oxygen. Dividing the sympathetic nerve causes the amount of oxygen 
deposited in the air-bladder to augment continually. Puncturing the 
air-bladder occasions at first an increased deposition of oxygen. The 
researches of Gouriet confirm these statements of Moreau; they were, 
however, ins:ituted rather with the object of detecting the value of the 
air-bladder as determining the swimming motions of the fish. In a 
few cases the air-bladder of fishes seems actually to exercise the 
function of lungs. Mr. Burt G. Wilder (Proc. Amer. Ass. Adv. Se. 
1875) showed that it is very probable that the spongy air-bladder of 
Amia calva and of Lepidosteus osseus acts as true lungs, and he has 


444 NOTES. 


yecently published a further treatise on the same subject. Jobert has 
lately shown that the spongy air-bladders of Sudis gigas, Erythinus 
teniatus, and EL. brasiliensis, actually take up air; and the distribution 
of the vessels in these fishes as well as in Amia and Lepidosteus is 
such as we generally find in true lungs. In these instances, according 
to Jobert’s experiments, tying up the air-passage by which the gullet 
communicates with the air-bladder, and by which apparently the air is 
introduced, was speedily followed by death. 

Note 76, page 173. Besides the Rotatoria, Tardigrada, the Anguil- 
lulida in mosses and a few little-studied Worms, the following Crusta- 
ceans have hitherto become known, of which the ova can endure 
desiccation without suffering the smallest injury: Apus, Branchipvus, 
Artemia, Cypris, Cypridina, Daphnia, Limnadia, Estheria, and many 
Copepoda. How long the eggs may generally lie dry without perish- 
ing is at present unknown. To the data given in the text I may here 
add the following which I owe to the kindness of Professor von Siebold 
of Munich. Mud containing Artemia, collected in 1872 by Professor 
Zittel in the oasis of Dahel (or Dahleh?), produced several broods in 
the beginning of May 1877, but none in the previous years. Mud out 
of a ditch at Ingolstadt, collected in 1871, produced a quantity of 
Estheria in the winter of 1876. Mud containing Branchipus produced 
a Nauplius in 1877 after lying dry for ten years. The ovaot Lepidurus 
productus, singularly enough, cannot endure desiccation. 

Note 77, page 175. Brauer has studied this subject. Eggs of 
Branchipus (Chirocephatus) diaphanus developed after a long time, four 
to seven months, even without having been kept dry ; and those of the 
marine species of Artemia also dispense with drying. But for other 
species of Branchipus and for many species of Apus, according to 
Brauer, desiccation is an indispensable condition for the development of 
the egg. 

Note 78, page 175. The geographical distribution of the species of 
Apus and Branchipus, for instance, offers many singularities. The eggs 
are minute and can certainly be easily transported by a high wind to a 
great distance, and even more easily by migratory birds, such as dicks, 
snipe, &c. We should therefore suppose that both these genera would 
have a wide geographical distribution like Cypris and Daphnia, of 
which I found several species in tropical countries extremely like those 
of the European continent, though perhaps specifically distinct. But 
so far as I have sought for Apus and Branchipus in fresh water I have 
found none either in the Philippines or in the Pelew Islands in the 
Pacific. Godefroy’s catalogue mentions no species of these genera as 
coming from the tropical islands of Polynesia, and I find none mentioned 
as belonging to South America, Central America, or India. This may 
perbaps be attributed to the fact that circumstances have not been 
favourable to travellers; it is well known that we may often seek in 


NOTES. 445 


vain, for years, for Apus in spots where it had previously been found in 
swarms. The best method of filling up this gap in our knowledge will 
be the transmission of mud, with exact information as to the place 
where it was collected, to scientific experts, as to Professor Von Siebold 
at Munich or Professor Brauer at Vienna ; and by this means the amount 
of material in the form of animals for investigation will also be in- 
creased in a considerable and very desirable degree. 


CHAPTER VI. 


Note 79, page 178. It was Leydig, the founder of all truly scientific 
—that is to say comparative—histology, who first pointed out’ that 
excessively fine ramifications from the trachee traverse every portion 
of the body of insects and lie between all their constituent parts. He 
showed that even in the eye, in the ganglia of the brain, and in many 
glands, &c., trachez are to be found between the cells of the organs, 
that they constantly lie quite close to them and not unfrequently end 
in a peculiar manner. Thus Leydig first discovered the vesicular ends 
of the trachez among the constituent parts of the dioptric apparatus 
of the fly, in the crystalline spheres. Even the cells of the fatty tissue 
on the presence of which the survival of many insect-larve through 
the winter seems to depend—are in direct connection with the tips of 
the trachez. 

Note 80, page 179. From what is stated in the text it might seem 
to follow that the distinction between arterial and venous blood can- 
not exist in Insects, which breathe by trachez, since by that mode of 
respiration the air is distributed to every part, and consequently the 
afferent and efferent vessels may contain blood which in each is equally 
rich in oxygen, It must not, however, be forgotten that even in Mam- 
malia the difference between the two kinds of blood—the highly oxy- 
genated arterial blood and the poorly oxygenated venous blood—is 
essentially occasioned solely by the absolute, or relative, localisation 
of the function of respiration in organs especially fitted for it—the 
gills and lungs. Thus, if in insects also there should be organs whose 
sole task it was to extract more oxygen from the trachex than other 
parts could, or—when the respiration is effected by water—could 
deposit it more abundantly in the trachez at one place than another, 
such arrangements would certainly contribute to make the blood richer 
in oxygenated particles in such spots than elsewhere; and hence, if we 


446 NOTES. 


could prove such a difference in the degree of oxygenation of the blood 
in different portions of the insect, we must speak physiologically of 
arteries and veins. Now this, it would seem, is sometimes the case, as 
in the external breathing organs of many insect-larve living in the 
water, or in the very curious conical structures at the termination of 
the intestine of flies, which, on account of their extraordinary develop- 
ment of trachez, there is now a very general disposition to regard as in- 
testinal branchie. In this last instance certainly it is not very clear 
how they are to act as organs of respiration, since it is certain that 
flies do not carry oxygenated water in the end of the intestine, and no 
observations as to the inhalation of air through it have been made to 
my knowledge. However, a small difference in the oxygenation of the 
fluids must, no doubt, exist in different portions of the body even in 
insects. 

Note 81, page 181. 1 will here briefly describe an observation acci- 
dentally made, but frequently repeated, which suggests the idea that 
some animals, and especially Infusoria, may possibly be capable of 
absorbing (and even assimilating ?) carbonic acid. In infusions pre- 
pared with the water procurable at Wiirzburg, which contains a great 
deal of lime, an excessively thin film is rapidly formed of carbonate of 
lime, beneath which various Infusoria crowd in masses. If the water 
is slightly shaken, the fragments of this film roll up into little cylinders, 
thus enclosing a minute quantity of air, as may be seen by examination 
with a microscope ; since these can be obtained only from the surface of 
the water, they must certainly be rich in carbonic acid. If now we trans- 
fer these little tubes containing air with some infusoria to a moist 
chamber, we see th*t they are not unfrequently consumed by the infu- 
soria, and if we then watch for some length of time one of the specimens 
which has just fed, we shall soon detect that the air in the lime-tubes 
disappears, and finally the tubes and the air contained in them are com- 
pletely absorbed. I have frequently repeated this observation, and 
have particularly noted whether or no the air might not escape in the 
form of minute bubbles from the oral opening or be removed in the 
pellets of food; but this was never the case, and I can most positively 
assert that all the air was perfectly absorbed. . Of course this is not 
hereby proved ; still, though I was not able to carry the observation 
any further, I regard it as sufficiently interesting to be recorded here. 
Bert says very decidedly in one of his papers read at the Sorbonne, ‘On 
the Influence of Light on Living Beings :’ ‘ On the other hand, infusoria 
containing green matter decompose carbonic acid in the same way as 
vegetable cells.” On what ground of exact experiment this bold asser- 
tion is made I do not know. 

Note 82, page 181. Only an incidental reference is made in the text 
to those other gaseous constituents of the atmosphere which, like car- 
bonic acid, are endurable in small proportions, but extremely injurious 


NOTES. 447 


in large ones. To these belong all the effluvia of decaying matter, sul- 
phuretted hydrogen, kc. But besides the fact that many insects actually 
depend on such effluvia for their existence, and many larve of insects, 
though air-breathers, live in putrefying matters, we must conclude that 
such gases are not universally injurious to all animals alike. Even 
among the vertebrata the difference in this respect is considerable. I 
once kept, in Manila, a large sea-snake at least two feet long, in a glass 
vessel hermetically closed, and three parts full of water; the water ina 
few days became putrid, but the snake lived for twenty-one dajs in the 
*pestilential atmosphere of the vessel containing it. It iseven a question 
whether then death ensued from the direct evil effects of the mephitic 
vapour, or not merely from the lack of oxygen after the absorption of 
the small portion contained in the creature’s lungs, and in the air enclosed 
in the glass. 

The reader can also compare the observations of P. Bert, Physiologie 
comparée de la Respiration, with those of Milne-Edwards, Legons d’Ana- 
tomie et de Physiologie comparée. 

Note 83, page 184. The view is sometimes put forward that certain 
stripes running parallel to the mouth of univalve shells—of the Helicide, 
for instance—afford an indication as to the age of the animal, each stripe 
being supposed to correspond to a year’s growth (like the annual rings 
inatree). This may perhaps—but only perhaps—be true with regard 
to our northern forms; but even among the land-snails of the Mediter- 
ranean, it ceases to have any application. I myself saw that in Spain 
and the Balearic Islands, after a summer’s rest of about two months, or 
even more, almost all the species began to couple, to lay eggs, and to 
grow again as soon as the autumn rains fell in September. Now, as the 
eggs of land-snails develope very rapidly, and never remain, like those 
of many insects, undeveloped through the winter, the young must be 
hatched out in the autumn; their growth is probably interrupted during 
the winter, as is, in fact, not unfrequently indicated by the presence of 
a Stripe. They begin growing again in the spring, and apparently 
deposit their first eggs before the summer drought comes on ; after their 
summer rest they lay eggs a second time, but nevertheless continue to 
grow, and thus form a second line of growth. This, at least, would 
seem to be the inference from the fact that in the autumn, along with 
the fully grown specimens, small ones are to be found with only one 
stripe, and which seem to have been hatched out in the spring. So far 
as I know, no attention has hitherto been paid to this circumstance. 

Note 84, page 186. Planarian worms are worms of low type and 
simple structure, for the most part flat, living chiefly in the water; in 
the sea they often attain a considerable size, and exhibit the most bril- 
liant hues, The first discovery of a land Planarian was made by the 
well-known Danish zoologist, O. F. Miiller, but his remarks on Planaria 
terzestris excited little attention till Darwin published his observations 


448 NOTES. 


on the land Planarians of South America. Since then these animals 
have been examined anatomically, particularly by Schultze, Metschnikoff, 
and Moseley, and we are acquainted with a great number of such forms 
through the efforts of travelling naturalists (Sclimarda, Moseley, F. 
Miller, and others). It may be stated that they are generally tropical 
animals, though three species have already been discovered in Europe ; 
here they live only in damp soil, under stones, while in tropical regions 
they take long walks in the early morning, on trees, rocks, and houses. 
I found most of, those that I collected in the Philippine and Pelew 
Islands—about 12 or 14 species—in such situations, and among them 
a few of really colossal size. Of the genus Bipaliwm, represented in 
fig. 53, Ihave one species which attains the enormous length of four 
inches. A complete list of all the land Planarians hitherto described 
is to be found in H. Moseley’s ‘ Notes on the Structure of several Forms 
of Land Planarians, with a Description of Two New Genera, &c.’ 
(Quart. Journ. Mic, Sci., new ser., vol. xvii.). 

Note 85, page 187. Nemertidz are also for the most part water 
worms, moving in the water by means of the microscopic cilia on their 
skin. They are systematically allicd to the Planarians, but distinguished 
from them externally by their perfectly circular, elongated form, and 
particularly by a proboscis opening at the fore end, which is wanting in 
the Planarians. 

Note 86, page 188. This Balearic species seems to be Jalitrus platy- 
cheles, Guirin. I have seen species of true Orchestia, both in the Pelew 
Islands and in the Philippines, where they live far from water, under 
stones and brushwood, in damp woods. 

Note 87, page 188. The arboreal Neritine usually live in mangrove 
swamps, high up on trees. I never saw them in the water, but they 
deposit their eggs on the surface of water, so that they are, at any rate 
occasionally, touched or covered by brackish water. I found the follow- 
ing species in the Philippines: Neritina dubia, communis, cornea, subsul- 
cata, zic-zac, Cumingiana, plumbea, and a few new and undescribed 
species. 

Note 88, page 189. Giinther divides these fishes into the following 
families: Luciocephalide, Labyrinthici, and Ophiocephalide. Of these 
the first two have both the secondary cavities of the branchial cavities 
furnished with convoluted labyrinthine folds; the species of the third 
family have only simple secondary cavities with feebly developed folds, 
ornone. The species of Saccobranchus, allied to the Shad, and Amphi- 
pnous cuchia, an eel-like form, allied to the Symbranchide, have also 
a subsidiary sac to the branchial cavity, but without any folds. 

Note 89, page 190. The observations of Sir Francis Day are to be 
found in the Pree. Zovl. Soe., London, 1868, Part IT. p. 274. 

Note 90, page 192. I have,in the Philippines, frequently had the 
opportunity of observing these creatures alive, and I can assert decidedly 


NOTES. 449 


—what, to my knowledge, other travellers had already described—that 
the Ampullarie breathe not merely with both gills and lungs, but that 
they do so in regular alternation; for a certain time they inhale the air 
at the surface of the water, forming a hollow elongated tube by incurv- 
ing the margin of the mantle, so that the hollow surface is enclosed 
against the water, and open only at the top. When they have thus 
sucked in a sufficient quantity of air, they reverse the margin of the 
mantle, opening the tube into which the water streams. The changes are 
tolerably frequent, once or twice in a few minutes, depending, pro- 
bably, on the temperature. No physiological explanation of these 
rhythmic alternations can, however, be at present assigned. 

Note 91, page 192. Fritz Miller, the well-known naturalist in 
Brazil, in his admirable essay, Facts and Arguments for Darwin, has 
given us a quantity of observations on the mode and way in which 
crabs breathe air. The modes by which this is accomplished are very 
various; and even the structural relations implicated in the process, 
which are sometimes extremely peculiar, irresistibly prove that the 
different air-breathing Crustaceans no more constitute a natural family 
than do the Labyrinthici among fishes. 

Note 92, page 193. In a few works, distinguished for their dogmatic 

’ sty'e, and intended for the use of students in medicine, it is stated that 
these land-crabs, and above all Birgus latvo, breathe no air, but only 
water, and that the branchie are exclusively the organs of respiration. 
I cannot understand how so incorrect a statement can have become so 
common, for the authors of these works cannot adduce a single experi- 
ment which proves that in fact the introduction of oxygen into the 
blood takes place exclusively by means of water, and through the 
branchiw. Since absolutely no physiological experiments exist on this 
subject, this erroneous view can only rest on an interpretation—which 
is acknowledged to be insufficient—of the morphological features. So 
far as here regards Birgus latro, I have shown in the text, and in the 
cut on page 5, that the views hitherto entertainéd as to the structure of 
the branchial lungs of this animal are altogether false, and that every 
morphological attribute is to be found in them which we should expect 
to find in a true lung. 

Note 93, page 195. I have before alluded to the injury that may 
sometimes result from a superabundance of air in water, by which so 
large a supply may be taken in by a fish (a stickleback or an Axolotl) 
that it may become ‘lighter than the water and so unable to find its 
food at the bottom. 

Note 94, page 198. Dr. August Pauly. With regard to the last 
point alluded to in the text I must be allowed to make a few notes. 
Pauly says that Lymne, when they are kept under water and 
have no opportunity of inhaling air-bubbles into their lungs, keep the 
lungs closed. A mollusc, having its lungs filled with air, will absorb 


450 NOTES. 


the oxygen contained in that air ifit is kept under water, and instead 
of it carbonic acid will be deposited in the lungs, This gas, being 
positively injurious to the creature, must presently be expelled, and 
consequently the lungs must soon become empty and so collapse, or 
they must be replenished with air or water. In Pauly’s experiments 
the former was the case; he expressly says the lung-cavity was empty, 
«as could be seen by its shrunken aspect from outside,’ But he con- 
ducted the experiment in a somewhat energetic fashion. He forced 
the animal to expel all the air from its lung-cavity under water. Now, 
he himself says that the expulsion of the air in consequence of irrita- 
tion may sometimes even occasion the death of the animal; hence the 
question is allowable whether the persistent closing of the lung may 
not have been a diseased result of the irritation in itself so unendurable. 
The experiment must be repeated in some way differing from Pauly’s 
before it can be regarded as perfectly conclusive evidence of the infer- 
ences given above in the text. 


CHAPTER VII. 


Note 95, page 203, Some mollusca, as Patella and Navicella, are im- 
moveably attached to the rock for the whole period of their existence ; 
they never quit one spot, and not unfrequently make a more or less con- 
spicuous impression in the stone. How? This is not known. 

When they are not disturbed, they usually lift the fore-part of the 
shell just so much as is requisite to admit a fresh supply of water to the 
branchial cavity, and of food to the mouth, If they are touched they 
shrink back, and the shell adheres so closely to the stone that it is im- 
possible to loosen it from its hold without injuring it. I have often 
endeavoured to loosen a Navicella hardly an inch in length from its 
situation on a stone in a swift mountain torrent, by lateral pressure on 
the shell, not by insinuating a knife under it; but I most rarely suc. 
ceeded—never, indeed, unless I took the creature by surprise ; if it were 
in any way on the alert, I could not do it but by application of the 
knife, and « consequent injury to the shell. The case is the same 
with Patella (limpet), and many other mollusca; even the creeping 
kinds, as Chiton, can adhere uncommonly tightly by suction, and in 
every case the foot is the organ employed. 

Note 96, page 205. A few deep-water Siphonophora have lately been 
described by Studer. Two species were dredged up from a depth of 
from 800 to 1,000 fathoms, belonging to the genus Rhizuphysa, which, 


NOTES. 451 


like many others, has an ait-bladder at the upper end of the sac. The 
air-bladder has an opening at the top by means of which, when the 
creatures are kept in small vessels, the air easily escapes. Iam asto- 
nished to find that Studer does not seem to have particularly remarked 
this feature—and yet it seemed to offer so obvious a parallel with the 
fishes living at great depths, and provided with air-bladders! In these, 
as has long been known, the air contained in the bladders is exposed to 
very considerable pressure ; if this is suddenly removed by the fish being 
rapidly brought to the surface, the air, previously compressed, expands 
and distends the belly; a prick allows the air to escape, the air-bladder 
collapses, and the fish, restored to its natural size, can swim again. It is 
evident that the air in the deep-sea Rhizophyse must also be much 
compressed, but in them the perforation already exists by which it can 
escape when the animal is raised to the surface, and the expanding air 
threatens to burst the bladder. Studer says nothing about this very 
conspicuous expansion of the bladder, and we may therefore suppose that 
the Rhizophysz had parted with the chief portion of their air before 
reaching the surface. This filling and emptying of the air-bladder, which 
must undoubtedly exist in the Rhizophyse to enable them to rise or sink, 
recalls the hydrostatic vacuoles of the Arcellide. 

Note 97, page 205. Lymnza and Planorbis are frequently to be seen 
with the sole—so to speak—of the foot spread out on the upper surface 
of the water, and thus swimming in an inverted position; but this 
swimming is more accurately described as creeping on the under surface 
of the air, the plane of contact of the air and water. At first we are 
inclined to imagine that the adhesion of the foot to this surface is strong 
enough to bear the whole weight of the animal ard its shell. This, how- 
ever, is not the case; for if the snail is induced to retract its foot so 
slowly that no air-bubbles are expelled from the branchial orifice during 
the process, the animal turns over in the water, but it remains floating 
at the surface, so that at that moment its specific gravity must be less 
than that of water. 

Note 98, page 207. It may, perhaps, surprise many readers to hear of 
the fins of birds. But most water-birds do in fact use their wings for 
swimming in the water quite as well as for flying in the air. The wings 
of ducks, divers, cormorants, &c., are not the less true wings. In the 
penguins, however, the same limbs, morphologically speaking, have 
become true fins, which the creature can use in the water, but can no 
longer use in the air as wings. In them, although the portions of the 
skeleton still correspond in all essentials with those of a true wing, 
there is absolutely no external resemblance to the true wings of other 
birds. Anyone may convince himself of this in a zoological collection. 

Note 99, page 209. This assertion is founded on a careful anatomical 
investigation of a good many different species—at least six—of the 
genus Navicella, Naturally there are visible differences between it and 


452 NOTES. 


Neritina, otherwise the two groups would long since have been united 
in one genus. But the differences in their structure, irrespective of the 
form of the shell and the structure and situation of the operculum, are 
so trifling that we are justified in regarding the Navicella as a modified 
form of Neritina. 

Note 100, page 212. It is in accordance with this view that we find 
great variability in the forms of the operculum of Navicella. We know 
that truly rudimentary organs which have lost their principal function 
and have not become serviceable for any other well-defined function are, 
as arule, remarkably variable. That is the case here. While the oper- 
cula of the most dissimilar Neritine are of very uniform structure, in 
those of Navicella we find the widest dissimilarity. My own researches— 
which certainly are not yet completed—make it seem probable that the 
deviations from the normal form in a species, even in individuals, may 
sometimes be very considerable. It is, moreover, a very interesting fact 
that even the operculum of the male may differ from that of the female. 

Note 101, page 213. The structure and origin of the shells of mol- 
lusca are not, even at the present, thoroughly understood ; for although, 
as to the first point, a vast mass of interesting researches lies before us, no 
one has yet succeeded by purely histological investigation, and a know- 
ledge, however exact, of the minutest structure of the shell itself, in 
establishing any fundamental character as common to all shells. At 
the same time too much attention has been paid to the general relations 
of the shell when fully developed, and not enough to the development 
and bearings of those relations. We know, for instance, that in the fresh- 
water mollusca three distinct and very dissimilar layers are to be found. 
The external layer is a purely organic cuticle; next comes the prismatic 
layer; and inside this the nacreous layer. It is admitted that the pris- 
matic layer is often wanting. Now, if we are considering merely the 
details of structure, the correctness of this view cannot be doubted, but 
it would be quite an error to suppose that every shell in which the pris- 
matic layer was wanting therefore consisted of two layers only, the outer 
cuticle and the internal nacreous layer. In point of fact, these shells 
also have three separate layers, and that lying next beneath the organic 
cuticle differs from a true prismatic layer only by its deviation from it 
in the physical process of formation; the material composing it does 
not form distinct prisms as it is secreted. But the spot whence it 
originates is, on the other hand, an essential character. The organic 
cuticle and the prismatic layer—the external calcareous layer—are sc- 
creted only and invariably at the margin of the mantle ; the former fre- 
quently between two lobes, or folds of the margin of the mantle, the 
second from the narrow edge between the margin of the mantle, and a 
line, not always present, which indicates the insertion of the small 
muscle of the mantle. From this line as far as to the top of the shell, 
nothing can besecreted but nacreous material; the growth in thickness 


NOTES. 453 


of the shell, therefore, must depend greatly, or exclusively, on the de- 
velopment of the nacreous layer. It is only when the secreting power 
of the surface of the mantle is extremely small, while that of the narrow 
edge which secretes the prismatic layer is unusually strong, that the 
thickness of the shell can be determined by the growth of the prismatic 
layer. In such exceptional cases the margin of the shellis always thicker 
than the main part. Any more minute description of these facts seems 
to me to be here out of place, and unfortunately I cannot refer the 
reader to other works, as I have not published anything on the subject, 
nor am I ut present likely to do so. 

Note 102, page 217. Graeffe suys that he has sometimes found a whole 
family in these galls or cysts. It is not quite clear to me how a family 
of the Crustaceans could find room in a cavity which is hardly twice the 
size of the fully-grown Hapalocarcinus. It is possible that young larvee 
might be found there before their escape, but thisis not clearly expressed 
in the text. 

Note 103, page 221. This crab, living in Trachyphyliia, a West Indian 
coral, is extremely like Cryptochirus, and perhaps belongs to the same 
genus ; this can only be determined by future and more exact exami- 
nation. But the ‘cave dwelling’ of this West Indian crab is perfectly 
unlike that of the Eastern species, which is found from the Red Sea as 
far as the Pacific Ocean; it is not cylindrical, but has one side quite 
flat, so that its transverse section is almost exactly a half-circle; the 
under side of the crab rests against the flat side of the cavity. With 
regard to the pouch, I have not yet been able to make any investiga- 
tions. 

Note 104, page 223. The conditions here described will under some 
circumstances be available for enabling us to form a much more exact 
estimate of the rate at which a block of coral grows than has hitherto 
been possible. The data here given, and presently to be worked out 
more fully, are certainly hardly to be regarded as the result of perfectly 
exact investigations ; the only really exact observation—carried out, too, 
in minute detail—is that of Le Conte (in Silliman’s Journal, series 3, 
vol, x., 1875), and he found that a coral plateau in Key West (West 
Indies) exhibited a perfectly regular dependence on the height of the 
water at different seasons, so that it was always possible to ascertain 
with great accuracy the rate of growth of the one species of coral ob- 
served there—a kind of madrepore—which was about 3} inches in the 
year. Now, if the period of growth of a specimen of Cryptochirus could 
be exactly determined, the rate of growth of the coral to which it 
belonged could also be determined with mathematical accuracy, much 
more exactly than it could be ascertained by direct measurement of the 
coral itself. If we assame—what at present, it is true, cannot be proved, 
though it is not improbable—that the Cryptuchirus acquires the first six 
millimétres of its length in the first year, that would give an average 


454 NOTES. 


rate of growth for the massive Astreide of 18 feet in a thousand years, 
whereas Dana allows at the utmost 5 feet. It is not, however, to be 
supposed that either of these estimates is universally applicable, since 
the different species of corals, like all other animals, have different rates 
of growth, and the rapidity might also vary under different circum- 
stances. It would, indeed, be extremely interesting if only the maximum 
rapidity of growth in individual corals—as those of different reefs— 
could be established by observation ; but to do this would be a highly 
complicated and difficult task, since the vigour of growth of the animals 
must depend on a great number of different influences which combine 
to affect it. 

Note 105, page 229. Ihave been at great pains in seeking in books 
of travels or descriptions of the different species of corals for dataas to 
the various forms which coral-blocks are capable of assuming in diffe- 
rent situations, but the results of my search have been terribly meagre. 
I found, in fact, only the observations made by Ehrenberg to the effect 
that Stephanocora Hemprichii, Ebren., in the Red Sea, forms branched 
or flattened stocks according to whether it lives in still or in rough 
water (see Ehren. Corals of the Red Sea). This, in my opinion, is 
the inevitable result of the faulty methods of investigation hitherto 
applied to these creatures; naturalists are desirous of distinguishing 
the species, and accordingly they have above everything paid attention 
to the distinctive character—as with insects, shells, &c.—and at the 
time—like Dana in his magnificent work on corals, connected with 
Wilkes’s expedition—they have bestowed the utmcst pains in ascertain- 
ing the limits of variability for individual species, as he has done with 
the greatest care in regard to certain madrepores. Klunzinger’s new 
work on the corals of the Red Sea supplies an abundance of material 
of this kind. But up to the present time no systematic observa- 
tions have been carried out bearing on the question which we are 
especially studying—as to how far currents in the sea, variations 
in temperature, or the saline constituents and other physico-chemical 
influences, may affect each species individually. The excuse to be 
offered is evidently this: that the fundamental essence of Darwin's 
theory is only now beginning to exert its influence, and that we are only 
now beginning to recognise the necessity for not merely putting off 
these reacting conditions with an attempt at a hypothetical explanation, 
bat for throwing on them the light of carefully conducted research, and, 
wherever it is possible, of actual experiment. Another and a very 
serious hindrance lies in the difficulty of obtaining the living material 
that is indispensable for such investigations; stationary zoologists, 
qualified to conduct them, are not many in the tropics, and travellers 
can never have time enough to make any really valuable observations 
of this kind. We must hope that we may ere long see a few zoological 
stations established in the tropical seas, such as that inaugurated with 


NOTES. 455 


so much energy and talent at Naples by Dohrn; for it is only in such 
institutions—which supply, as it seems to me, a real want —that it can 
be pessible to carry on aseries of observations through successive years, 
which is indispensable for clearing up biological questions. Mean- 
while let us be thankful that we have that of Dohrn, and a few others 
recently established, here in Europe. I cannot omit to record my 
satisfaction that Dohrn has decided henceforth to publish a special 
journal of his own Transactions, for I am convinced that the Institute 
itself, as well as zoologists at a distance, who desire information about 
it, will find it advantageous. The complaint that it constitutes a new 
scientific journal seems to me ill founded, for such an objection is never 
raised against a new book, and the work begun and continued in such 
an institute appears to me to constitute a whole, quite as coherent as 
the different chapters of a book, or indeed of many monographs, and 
often of much greater value. 


CHAPTER VIII. 


Note 106, page 259. According to Wiechmann the rocks of the Kokeal 
formation contained the following fossils :— 

Tridacna, Strombus, Mactra, Cyprina, Madrepora, Serpula, Lucina, 
Tellina, Venus, Spondylus, Fistulana, Balanus. 

He regards the eruptive rocks as of tertiary or post-tertiary date. 

Note 107, page 275. It would be highly advantageous now to criticise 
the Theory of Subsidence not merely in its application to a particular 
instance, as I have done, but in its universal bearings, so as to come to 
some conclusion as to whether my theory of* currents, sit renia verbo, 
deserves, or does not deserve, general preference. This, however, is not 
the place for such a discussion. I will only observe that I believe that, 
in fact, my theory presents fewer difficulties than the Theory of Subsi- 
dence, and may therefore be regarded as more in accordance with 
nature. On the other hand, I readily concede that sometimes—as, for 
instance, in the Andaman Islands—an atoll may be formed during a 
period of subsidence, and yet it may not be exclusively the result of the 
subsidence. Still, under the assumption that absolutely no influence of 
the nature above indicated could have formed atolls among the Anda- 
mans, this could only have been possible if the subsidence had through- 
out been slower than the growth of the coral. This appears to be 
sometimes the case ; for the Andamans are said to be sinking at the 
rate of a foot in a century, while Le Conte gives the maximum growth 


456 NOTES. 


of a coral as one foot in three and a half years, and another observation, 
in Port Darwin, gives one foot in twelve years. On the other hand, how- 
ever, there are other islands which prove that the upward growth of 
corals is certainly never so rapid, and is often remarkably slow. In the 
Sandwich Islands—which, according to Dana, are sinking—all the corals 
live at several fathoms below the level of the water, and the case is the 
same in the Galapagos and the Gulf of Panama. Here, by assuming 2 
subsidence, the growth upwards is less rapid than the rate of subsidence, 
and it must be even slower, much slower, if we assume an upheaval as 
going on in these islands. Hence I regard it as quite possible that 
under certain circumstances a subsidence may be combined with the 
formation of atolls, and even that it may once have been the sole 
cause of their formation ; but I cannol admit that subsidence is alone 
sufficient to explain all the conditions and relations of coral-reefs, or 
even of predominant importance. 

The following letter from Mr. Charles Darwin to the author refers 
to the subject under consideration :— 

‘October 2, 1879. 

“My dear Professor Semper,—I thank you for your extremely kind 
letter of the 19th and for the proof-sheets. I believe that I understand 
all, excepting one or two sentences where my imperfect knowledge of 
German has interfered. This is my sole excuse for the mistake which 
I made in the second edition of my Coral-book. Your account of the 
Pelew Islands is a fine addition to our knowledge on coral reefs. I 
have very little to say on the subject: even if I had formerly read your 
account and seen your maps, but had known nothing of the proofs of 
recent elevation, and of your belief that the islands have not since 
subsided, I have no doubt that I should have considered them as formed 
during subsidence. But I should have been much troubled in my mind 
by the sea not being so deep as it usually is round atolls, and by the 
reef on one side sloping so gradually beneath the sea; for this latter 
fact, as far as my memory serves me, is a very unusual and almost un- 
paralleled case. I always foresaw that a bank at the proper depth 
beneath the surface would give rise to a reef which could not be dis- 
tinguished from an atoll formed during subsidence. I must still adhere 
to my opinion that the atolls and barrier-reefs in the middle of the 
Pacific and Indian Oceans indicate subsidence; but J fully agree with 
you that such cases as that of the Pelew Islands, if of at all frequent 
occurrence, would make my gencral conclusions of very little value. 
Future observers must decide between us. It will be a strange fact if 
there has not been subsidence of the bed of the great oceans, and if 
this has not affected the forms of the coral reefs. 

* Yours very sincerely, 
‘CHARLES DARWIN.’ 


NOTES. 457 


CHAPTER IX. 


Note 108, page 284. Zoologists and geologists alike are wont to re- 
gard all the land mollusca, or rather their shells, as peculiarly fitted to 
indicate the affinities and relationship of living and extinct faunas. 
Now, I do not dispute that they may sometimes be of the greatest 
utility in this respect, but I must here express my conviction—a convic- 
tion derived from years of study of the animals as well as of their 
shells—that in many cases we have absolutely no right whatever to 
avail ourselves of the shells of land mollusca for such comparisons ; and, 
moreover, that their classification by the shells, which is universally 
adopted by conchologists and geologists, and which they have accepted 
as anatural one, is absolutely and totally worthless and unnatural. Thus 
every argument based on the assumption that the genera and sub- 
genera as at present distributed are natural divisions, indicating the 
true affinity of the syecies they include, must be purely imaginary, a 
mere castle in the air (such, for example, as Geotrochus, Bulimus, 
Rachis, Homorus, Hapalus, Nanina, Leweochroa, &c., &c.; comp. Wallace, 
Geog. Dist. Animals, ii. 512 et seq.). 

Note 109, page 287. The careful investigations which I pursued for 
years, extending over many hundred species, have brought me more and 
more to the idea that it may be possible to determine the route of migra- 
tion followed by many genera of land mollusca by a diligent examina- 
tion of their natural affinities. This evidently cannot be done by 
an examination of the shells exclusively. These, of course, must not be 
neglected, but their systematic value has hitherto been greatly over- 
estimated, especially by geologists, and without a close familiarity with 
the animals themselves we can but very rarely determine the affinities 
of the species with any certainty. Hence our first task must be to sepa- 
rate those groups of the land mollusca whose shells do, in fact, afford a 
sure indication of their systematic position from those in which the 
shell is quite or almost useless for such a purpose. To what a great 
degree this is often the case is shown by the Philippine genus Cochlo- 
styla, of which the shells are so excessively variable—in spite of the 
similarity of structure in the animals themselves—that no conchologist 
could possibly describe the genus from the shells. Hitherto we have 
always hada genus under the name of Vitrina, but species were in- 
cluded in it which belong not merely to different genera, but even to 
different families; these are so much alike as to the shells that, 
according to that character alone, it was inevitable that they should 
get classed together. In my work on the land mollusca I have shown 
that almost all the shells of the Philippines known as Vitrina belong to 
the genus Helicarion and the family Zonitide, while Vitrina is one of 


458 NOTES. 


the Limacide. From mere external resemblance a host of shells from 
India, Persia, &c., have constantly been described as belonging to 
Helicarion which, so far as it has hitherto been possible to inves- 
tigate the creatures anatomically, all belong to the typical genera of 
the neighbouring Indian mainland, to which indeed they often ex- 
hibit but little similarity even in their shells. The Philippine molluse 
Pfeifferia micans is often placed under Jitvina, and the shell certainly 
has some likeness to that of Vitvina, but the animal is in every par- 
ticular a true Cochlustyla (and ihus a true Helix), and is one of the 
innumerable variations of this variable genus, the structure of the shell 
completely disguising its true character. It these and the other 200 or 
so of species of Cochlostyla could be discovered somewhere in a fossil 
state, geologists would undoubtedly make at least eight distinct 
genera of them. This instance must here suffice to justify the assertion 
I have made. 

Note 110, page 289. Wagner’s phrase, which I have somewhat altered 
in the text, runs as follows: ‘ Hach closed cycle of forms (a species or 
constaut variety) originates in a mechanical process of isolation and 
colony-formation by individual emigrants from a parent-stock capable 
of variation ; the indispensable conditions of the formation of such a 
cycle are variability and inheritance. The sum of morphological cha- 
racters which distinguish it are the result from the sum of differences in 
the external conditions of life on the one part—such as food, climate, 
character of the soil—as supplied by the habitat of the isolated colony, 
when compared with the native province of the old stock, and from the 
sum of phyletic and individual capacity for variation on the other part 
as imported by the colonist itself, and transmitted as morphological 
characters to its-progeny and posterity by direct descent. The constancy 
of the new form always depends on a long-continued period of iso- 
lation.’ 

Note 111, page 291. I cannot understand in any other sense the vari- 
ous passages in which Wagner distinctly opposes his theory of ‘ isolation’ 
to the ‘struggle for existence.’ I will here quote only one passage: 
‘The Achatinelle are harmless vegetable-feeders, content with any 
situation, and their overwhelming multiplication is kept within bounds 
not by the pursuit of enemies, but by epidemics. They have no vital 
struggle to carry on for food, since this is supplied in any quantity by 
the abundant herbage of the soil, nor can we discover that any struggle 
for propagation can take place among them, since each animal is herm- 
aphrodite, and pairs with any other. If here and there one of these snails, 
which generally find sufticient shelter by a rapid retreat into their shell, 
is by chance devoured by a bird or a predatory beetle, or accidentally 
crushed by a grazing beast, these are purely accidental occurrences, which 
would be far less likely to reduce their numbers than the constant per- 
secution to which the ladybird, for instance, is exposed. Nature has at 


NOTES. 459 


her disposal an all-sufficient means of reducing the too exuberant mul- 
tiplication of all very fertile species, in epidemic diseases, and no com- 
petition is needed or available here.’ Now, it seems to me that this 
sentence can have no other meaning than that I have attributed to it; 
according to Wagner the ‘struggle for existence’ means nothing else 
than a competition between two animals for a certain possession. But 
“is there no struggle for existence when a snail endeavours to escape the 
causes which produce an epidemic? Epidemics among land-snails are 
commonly caused by too great moisture or drought; those that cannot 
escape rapidly enough perish; those that cannot endure drought are 
destroyed. In an epidemic of rot, or rather of saturation, the old and 
feeble individuals will perish first ; parching heat is least endurable to 
the young animals, as their shell and diaphragm are not thick enough to 
protect them against desiccation. Nay, even a direct struggle is not 
always entirely avoided. 1n order to escape from drought, many land-- 
mollusca creep into cracks and fissures in the recks; the first-comers are 
the best off, for they can creep furthest in, and those that come after 
close up the opening and prevent the escape of the moisture. Thus, 
during the dry season in the countries of the Mediterranean, for 
instance, we find the outer rows of snails almost invariably dead, while 
any considerable number of living ones are only found at some depth. 
The conditions are reversed when the rainy season comes.on. All the 
rifts and crevices are filled with water ; those lowest down are the first 
to be immersed, and strive to escape the too abundant supply that soaks 
their skin ; but the dead shells remain attached above them, or those still 
living, but not yet aroused by the wet, hinder them from creeping out, the 
water penetrates their pores, and in a few hours they are so ‘ water-logged’ 
and dropsical as to be incapable of any rapid movement. (It is a great 
error to suppose that a snail cannot have too much moisture; if one 
js plunged into water and prevented from escaping within twenty-four 
hours, it is so completely sodden as to be quite incapable of crawling.) 
Is not this a struggle for existence? It seems to me that it is a very 
obstinate struggle for existence when one snail, even after its death, can 
bar the road to life’and freedom to one of its companions. But many 
of the premisses in the passage above quoted from Wagner are false or 
quite unfounded. He says that Achatinella is one of the very fecund 
species of which the overwhelming multiplication is more effectually 
hindered by epidemics than by competition or rivalry. This is either 
false or devoid of foundation; Achatinella is oviparous, and produces 
only a few young at a time ; how often in the year is perfectly unknown. 
The assertion that every mature hermaphrodite individual is always ready 
to pair is certainly not proved. It has never been actually disproved 
by observation that many snails die without pairing from antipathy, 
though fully grown and mature, and the extraordinary convolutions and 
gymnastics performed by snails before pairing lead to the conclvsion 


460 NOTES. 


that even in these apathetic organisms liking and dislike play a certain 
part. Whether Achatinella is, in fact, spared all struggle in the matter 
of food, cannot possibly be determined, to judge from the investigations 
of Gulich. The merely hypothetical superabundance of food is no proof 
of its real sufficiency; if, for instance, the space where this abundance 
is supplied is very limited, the animals desirous of feeding will get in 
each other’s way, and it is possible that this might give rise to some’ 
quite unknown psychical influence. Many animals, as is well known, eat 
freely only in solitude. This is very certainly not the case with snails, 
but they may nevertheless desire a certain amount of elbow-room, a 
point that has never been observed, or even thought of, But even sup- 
posing that these positions of Wagner’s were all proved to the utmost 
extent of their very positive assertions—which is by no means the case 
—still, the struggle of the Achatinelle against the causes of the epi- 
demics that decimate them is necessarily a competition, though of course 
not in the same sense as is a duel fought for life or death between two 
individuals. 

Note 112, page 252. Notwithstanding that Darwin’s works are univer- 
sally accessible, I will here quote the passages to which I particularly 
refer. In the first place, with reference to the external conditions of 
existence, I offer a few extracts: ‘Neither migration nor isolation in 
themselves can do anything. These principles come into play only 
by bringing organisms into new relations with each other, and ina lesser 
dezree with the surrounding physical conditions ’ (Origin of Species). 

‘Hence, though it must be admitted that new conditions of existence 
do sometimes definitely affect organic beings, it may be doubted 
whether well-marked races have often been produced by the direct 
action of changed conditions without the aid of selection either by 
man or nature’ (Animals under Domestication, ch. xxiii.). 

‘Such changes are manifestly due not to any one pair, but to all the 
individuals having been subjected to the same conditions, aided, per- 
haps, by the principle of reversion’ (Descent of Man, i. 236). ‘ Although 
with our present knowledge we cannot account for the strongly marked 
differences in colour between the races of man, either through correla- 
tion with constitutional peculiarities or through the direct action of 
climates, yet we must not quite ignore the latter agency, for there is 
good reason to believe that some inherited effect is thus produced’ 
(Descent of Man, i. 245, and on p. 246 he adduces reasons in support of 
this statement). 

One more quotation : ‘There can, however, be no doubt that changed 
conditions induce an almost indefinite amount of fluctuating variability 
by which the whole organisation is rendered in some degree plastic’ 
(ibid. i. p. 113). Compare with this what Darwin says as to the direct 
external influences which affect the skull (ibid. p. 147). But it seems 
to me to be proved by numerous passages in Darwin’s works that he 


NOTES. 461 


regards the principle of isolation of new forms and the hindering of in- 
breeding with the parent form as an integral portion of his thecry. I 
will here quote the most striking examples. He says (in Animals under 
Domestication, ch. xv.): ‘The prevention of free crossing and the 
intentional matching of individual animals are the corner-stone of the 
breeder’s art. No man in his senses would expect to improve or 
modify a breed in a particular manner or keep an old breed true and 
distinct unless he separated his animals. The killing of inferior 
animals in each generation comes to the same thing as their separation.’ 
The struggle for existence combined with other causes produces the 
isolation which is indispensable to the phenomena of a race or species 
by causing the necessary separation from the parent form and by hin- 
dering its crossing with it again. Elsewhere Darwin says: ‘A country 
having species, genera, and whole families peculiar to it, will be the 
necessary result of its having been isolated for a long period, sufficient 
for many series of species to have been created on the type of pre- 
existing ones.’ 

Note 113, page 293. Gitinther on the Tortoises of Mauritius and 
Galapagos, Ann. Mag. Nat. Hist. 1874, vol. xiv.; Silliman’s Journal, 1874, 
ser. 3, vol. viii. p. 403. These tortoises are also interesting from their 
enormous size. Giinther comes to the conclusion that those of the 
Mauritius and those of Galapagos must have originated independently. 

Note 114, page 294. The question whether similar forms can have a 
polyphyletic origin (be derived, that is, from independent parent stocks) 
or no, has gradually become the corner-stone of that extreme and 
dogmatic form of Darwinism which in Germany has been designated 
as Haechkclism.* Haeckel himself, the founder of this creed, allows of no 
doubt that all those characters collectively of a species or of a genus 
which present themselves to us as identical can only and uncondition- 
ally, as being identical, have been derived from a single parent-stock. 
This is known as Monophyletic descent, and according to this view all 
the species of a genus must have descended from one parent species, 
all the genera of a family from one parent genus, and so forth. In 
opposition to this is the view that a polyphyletic origin may be possible, 
i.e. that the forms comprehended by us in a genus or a family may 
have descended from more than one parent species or genus. 

The theoretical correctness of the monophyletic hypothesis may 
be conceded unconditionally without any necessity for admitting its 
practical correctness. Fundamentally the only correct view is that 
any definite phenomenon must have had a definite cause, and that 


* Many of Haeckel’s works are known to English readers through 
excellent translations, as The Nat. Hist. of Creation, and the Kvolution of 
Man (C. Kegan Paul & Co.). 


462 NOTES. 


a repetition of the same causes in the realm of or,’anic nature is 
simply impossible; thus the twofold origination of one and the 
same specics from different parent forms brought into existence 
by dissimilar causes is physically inconceivable and hence impos- 
sible. Granted. The error and logically false conclusion involved 
in Haeckelism does not lie in this but in the presumption which 
asserts that the forms or individuals which it declares to belong to the 
same species must be identical. This they certainly are not, and 
though zoologists may include them under the concept of a ‘species’ 
this is done on extremely various grounds that are without exception of 
a subjective character. No one is competent to deliver an objective 
decision as to whether these or those individuals actually constitute 
only one, or two, or more species; the criteria for such a determi- 
nation are wholly wanting. Moreover, the monophyletic hypothesis 
entirely ignores the fact that in by far the greater number of cases two 
individuals are needed for the propagation of new individuals, and these, 
irrespective of their sexual differences, certainly veed ‘not invariably 
belong to the same species; the possibility of hybridisation, i.e. the 
fertile union of two individuals of different species, is fully established. 
We know, moreover, that hybridisation is a favourite method employed 
by Nature for the origination of new forms—perhaps, indeed, the most 
powerful means at her command, Now, if the hybrid union of a 
species, A, with three others, B, C, D, results in each case in an arialogous 
but different deviation from both parents, if this new character, com- 
mon to the three families of hybrid progeny, A B, AC, and A D, justifies 
us, according to our subjective views, in establishing a new genus, we 
here have three different species of a second genus derived from the 
three originally different species, B, c, D; they have originated by a 
polyphyletic process. The Amphioxus is one of the cosmopolitan 
species, but the specimens from different localities exhibit some not 
inconsiderable differences. Now,if new forms were to arise from these 
dissimilar individuals, these might still possibly belong to one and 
the same genus; still, the Brazilian, the Philippine, the American, and 
the Australian species of this new genus would not have originated 
from a transformation of the descendants of a single pair, as the mono- 
phyletic hypothesis requires. I can make this discussion quite in- 
telligible simply by quoting the following lines from Darwin: ‘TI will 
only remark,’ he says, ‘that if two species of two closely allied genera 
produced a number of new and divergent species, I can believe that 
these new forms might sometimes approach each other so closely that 
they would for convenience’ sake be classed in the same genus, and thus 
two genera would converge into one.’ Thus Darwin regards it as 
possible that the species of one and the same genus may have 
been derived from species not merely of one but of two diferent 
genera. All the most careful and recent investigations make it seem 


NOTES. 463 


probable that the polypbyletic hypothesis is nearer to the truth than 
that which opposes it. 

Note 115, page 301. These words had long been written when, quite 
lately, a paper came into my hands by Huxley on the affinities of fresh- 
water Crustaceans. According to him, the river Crustaceans of the 
northern hemisphere belong to one family, called by him Potamobiide, 
while those of the south he calls Parastacide. He points out that the 
two groups are easily distinguished by certain peculiarities in the struc- 
ture of the gills, but he nevertheless suggests that the two forms, in 
themselves so distinct, might have descended from a common primitive 
form which peopled the tropical seas—where they are now for the most 
part wanting—and in their migrations into the rivers of the islands and 
continents of the north diverged into the structure of the Potamobiida, 
and in the south into that of the Parastacide. 

Note 116, page 302. Anotice of Tyndall’s recent investigations may 
be found in Wature for 1877. The unprejudiced reader will here find, 
as it seems to me; an irrefutable disproof of ‘ Abiogenesis,’ as it is called, 
and will be greatly interested in following the course of brilliant expe- 
riments, and the crowd of new facts elicited by them. Any further 
details are not here to the purpose, as Tyndall’s experiments were made 
on the development of the germs of Fungi. 

Note 117, page 312. In the centre of Mindanao, on the upper course 
of the Agusan, among the Manobos living there, I found a fcssil 
elephant’s tooth, which was worn by the Baganis, or chiefs, of that can- 
nibal race on solemn occasions, such as going out to battle, strang on to 
a necklace with other objects, as small images of gods, crocodiles’ teeth, 
&c. When a foe is killed, his breast is opened with the sacred sword, 
and all these objects, sacred to the god of war, are dipped in his blood ; 
and it is not till the god has thus slaked his thirst in the blood of the 
enemy that the Bagani may eat a portion of the heart or lungs. Both 
the specimens of fossil elephant-teeth that I brought thence are now in 
the Ethnclogical Museum at Dresden. 


CHAPTER XI. 


Note 118, page 332. Compare Note 18 to Chap. IIT. 

Note 119, page 335. It was formerly supposed that the slightly spiral 
tubes in the corals, in which the Sipunculide live, were the shells of a 
mollusc, and that the worms bad first established themselves in them, 
and then the coral had formed upon them. This view was the result of 
a superficial examination ; there can be no doubt that the worm settles 
on the coral, grows with it, and makes its own tube. 


464 


NOTES. 


Note 120, page 352. The minute Copepod here described as living 
in the stomach of Miilleria lecanora is Lecanurius intestinalis. 

Note 121, page 354. The following list is extracted from various papers 
in a German periodical, the Zoological Garden, edited by Dr. Noll, of 
Frankfurt. I have omitted such examples as have already been men- 


tioned in the text. 


Carnivora, 
Polecat and ferret. 
Wild cat and domestic cat. 
African leopard and black panther 
of Java. 


Ruminants. 
Yak and common cow (at Halle). 
Bison and black cattle. 
Ovis musimon and O. eyeloceros. 
Cervus virginianus and Cervus 
macrotis (in Cincinnati). 
Lina-sheep (in Chili), a cross be- 
tween the sheep and goat— 
somewhat doubtful. 
Cervus minor, a cross between the 
Axis and the Hog-deer. 


Pachydermata 
(with solid hoofs). 

Equus teniopus, M., and Equus 

zebra, Fem. (at Berlin). 
Horse, M., and Burchell’s zebra, F. 
Ass, M., and Burchell’s zebra, F. 
Sus scrofa persica and Sus scrava 

sondaica (at Rotterdam). 


Rodents. 


Lepus variabilis and Lepus timidus, 
in a free state. 


Birds. 


Modena pigeon, M.,and turtle-dove, 
Fem. 

Phasianus versicolor and Gold 
pheasant (at Antwerp). 

Gold pheasant and Thaumalia Am- 
herstie (at Paris). 

Anas superciliosa and Aix sponsa. 

Greenfinch and goldfinch. 


Insects, 


Phigatia pilosaria, M., and Nysia 
hispidaria, Fem. (as described 
by Midford). See Packard, 
Guide to the Study of Insects, 
p. 54. 


CHAPTER XII. 


Note 122, page 361. Similar relations exist between various other 
animals. The singular Nemertean Malacobdella lives almost every- 
where, a solitary parasite in the branchial cavity of a mollusc; but we 
here have a very plausible explanation which is almost certainly the 
correct one, being confirmed by occasional observations of the co-exist- 


NOTES. 466 


ence of two or three specimens in the same mollusc. When the young 
animal, having just found its way into its dwelling, begins to eat, it will 
catch at every organic object that is brought into the branchial cavity 
by the current, and so hinder a later comer from establishing itself in 
the same place. But an instance observed by K. Vogt is quite unin- 
telligible without some such hypothesis as ] have put forward in the 
text. Among hundreds of specimens of a species of Labrus, of which 
about 43 per cent. were attacked by a parasitic Crustacean, Leposphilus 
only two were found which had two parasites, one on each side; all the 
others had but one, sometimes on the right and sometimes on the left ; 
but the number of those that had settled on the right side was consider- 
ably greater than those on the left, as 27 to 16 per cent. They were 
always attached to the lateral line. What in this case can have hindered 
the establishment of several parasites on the same fish? As it seems to 
me, the only thing that proves unfavourable to a second parasite is some 
deterioration in the juices of the fish by the first. 

Note 123, page 372. Thisassertion that no mollusca but these of the 
genus Onchidium have such dorsal eyes is based on the investigations, at 
once of great extent and of extreme anatomical accuracy, conducted by 
Bergh, of naked marine mollusca, and on my own researches, carried on 
with a view to this particular, into the structure of other land and water 
mollusca. Here and there, certainly, we find eye-like specks of pigment 
on the back or sides of the body, as in Spherodoris punctata and paypil- 
lata among the naked mollusca, and Margarita in the Conchifera, but 
all Bergh’s researches and my own, with all the most modern instru- 
ments, show these to be merely concentrated spots of pigment with no 
connection in any instance with a nerve, and exhibiting no trace of 
the typical elements of a true eye. 

Note 124, page 381. A case perfectly analogous to that of the 
Onchidium described in the text occurs among fishes of the family of 
Scopelide. These are deep-sea forms, to which indeed belong some 
of those described by Giinther as having luminous organs; at the side 
of the body or on the belly they have a number—varying according 
to the species—of large silvery spots of different sizes, and which had 
already been spoken of as eyes by Leuckart in 1865. Still, until quite 
recently, the accuracy of this view had been doubted in spite of the 
statement of that very skilful naturalist. Quite lately, however, an 
exact description by Dr. Ussow (published in the Bulletin of Moscow) 
of the structure of certain eye-like spots in some bony fishes leaves no 
room for doubt, so far as I see, that Leuckart was perfectly right; all 
the attributes of true eyes are to be found in the genera Chauliodus, 
Astronesthes, and Stomias, But according to Ussow other species have 
organs in similar positions, which he designates as glands. I must con- 
fess that his representation has not convinced me of the accuracy of 
this interpretation, and I should venture to hazard an opinion on the 


21 


466 NOTES. 


contrary that these so-called glands are either illuminating organs, like 
those detected by Giinther, or sensitive organs which have not yet been 
developed into eyes. The genera in which he has discovered these sup- 
posed ‘pigmented glands’ are Scopelus, Gonostoma, and Maurolicus. 
Finally, there are two genera, Sternoptyx and Argyropelecus, in which 
the pigment-cells existing in corresponding spots in the body are said 
to have a character between pigmented glands and true eyes. 

Note 125, page 382. My brother, Georg Semper, has communicated 
to me the following case, lately observed by him, of the adaptation of 
an old species to the colour of new surroundings—or rather of its avail- 
ing itself of it for protection. During the last ten years the well-known 
white-leaved variety of Acer negundo has been largely planted in 
gardens in Hamburg, and since this the common white cabbage butterfly 
has accustomed itself to settle by preference on this shrub. It is then 
extremely difficult, as my brother informs me, to distinguish the butter- 
flies as they sit on the leaves, their yellowish colour being lost in that 
of the leaves. Here it is quite clear that the colour of the Pieris cannot 
have been produced by selection, since it had the same characteristic 
colouring long before the introduction of the white-leaved Maple. But 
if now one or another species or variety could benefit by the similarity 
of colouring which has thus accidentally arisen, in the struggle for 
existence, it would be atan advantage over any other species which was 
by any cause disqualified from availing itself of this protection, and 
thus the protective resemblance might occasion a selection among the 
different forms. In this case, beyond a question, selection had abso- 
lutely nothing to do with the origin of the protective colouring, and I 
am convinced that in many cases, if not in all, the occurrence of pro- 
tective resemblance is not to be explained by selection. Some very in- 
teresting cases of protective mimicry are mentioned in Brazil, the 
Amazons, and the Coast, by Herbert Smith (Sampson Low & Co.), 
chap. vii. 

Note 126, page 387. The pigment-forming matter—chromogene, as 
it is called—is conveyed by the blood to every organ in the body. It 
depends on local conditions whether it is in some places deposited in 
abundance and in others not at all. Consequently the primary distri- 
bution of colour depends on the structure of the organ or of that portion 
of the skin where such a deposit normally takes place. Examples to 
prove this are absolutely innumerable; they may be found in almost 
every animal. The gay colours of many shells—both bivalve and 
univalve—are in great measure produced by the deposition of pigment in 
the external organic skin, which covers the calcareous portion of the 
shell; the pigment itself is elaborated by glands which exist exclusively 
in the margin of the mantle. It is according to the regularity of the 
arrangement of these pigment-glands and the interruption in the exercise 
of their functions that certain patterns and colours occur in the shells— 


NOTES. 467 


spots, stripes, bands, or zig-zag lines, &c. In butterflies the microscopic 
scales on both surfaces of the wings have the pigment deposited in 
them ; in quadrupeds it is in the hair, in birds in the feathers ; here the 
distribution of colours must depend on the affinity of these organs for 
the chromogenes, and, consequently, indirectly on those organs which 
grow out of the skin. But the usefulness of certain colouring, which 
does not occur until later, can have no influence on the origination of 
these organs or on the different degrees of affinity of these parts to the 
chromogenes ; hence it follows that it is only by the more or less regu- 
lar arrangement of such organs that animals can acquire a mode of 
colouring which corresponds with similar regular colouring in the sur- 
rounding objects. Hence a striped butterfly can never originate 
directly from an irregularly spotted one by natural selection, since this 
presupposes a previous transformation in the organs containing the 
colours; but if, through any physiological cause acting in the organism, 
the spotted colouring had already been altered to any considerable 
extent to a striped arrangement, then selection might gradually lead to 
the extermination of the spotted variety by augmenting any protective 
resemblances the striped form might possess. But it would, of course, 
be absurd, in such a case as this, to speak of selection as the primary 
cause of the mode of colouring. 

Note 127, page 389. Sesia apiformis, respiformis, crabroniformis, &c. 

Note 128, page 397. According to Quug a Gaimaro, the species of 
Harpa, a marine univalve, possess the same peculiarity as Polydontes, 
Stenopus, and Helicarion. Although I have caught a considerable 
number of living specimens, I never discovered this by my own expe- 
rience. At any rate, the mode in which Harpa sheds its foot is quite dif- 
ferent from that of the species of Helicarion. If by some extraordinary 
accident it is unable to withdraw its foot, which is very large, into its 
shell, it pressesit against the sharp edge of the shell, and so cuts off the 
hinder portion of it in order to protect itself. 


ACR 


ACRONYCTA, effects of food on, 66 
Algz, associated with sponges, 
342, 343 

Amblystoma, note 47 

Ameeba, 15, 302 

Ampullaria, 191 

Anabas scandens, 148 

Anodonta, 36 

— distribution of, 298 

Autipathes, associated with an An- 
nelid, 341 

— with a Molluse, 337 

Aphis, effects of temperature on, 123 

Apus, 128, 175 

— distribution of, note 23 

Arcella, 205 

Artemia, 157 

Aruangel, atoll of, 234 

Ascaris nigrovenosa, 44 

Ascidians, 3 

Astacid, 301, note 115 

Atlantis, 295 

Atlantosauride, note 1 

Axolotl, 88 

—white variety of, 90 


BABELTH UAP, 234, 244, 269 
Bates on mimicry 
Bernard, C., experiments, 150 
Bert, Paul, on respiration in frogs, note 
4 


— on light and pigment, 88 

— on organs of respiration, 169, note 68 
Beudant’s experiments, 153 

— tables, note 60 

Biigus latro, 5, 193 

Bleak, anatomy of, 7 
Boleophthalmus, 189 


DAR 


Bopyrus, 147, 360 
Branchipus, effects of temperature on, 
120 


— of salt water, 155 

—— distribution of, 304 

Brauer’s experiments, 176, 806 

Braun on the casting of the shell of 
the river crayfish, 20 

Buston’s experiments on Parrots, 133 


CARTIER on the casting of the 
skin in reptiles, 20 

Cestodon, 49 

Chameleon, structure of, 4 

Chlorzea, 285, 899 

Chlorophyll in plants and in animals, 
70, 88, note 17 7 

Chromatophores, 92 

Chrysotis festiva, 67 

Cienkowsky, on yellow cells in Radio- 
laria, 74 

Cladocora, 401 

Cobitis fossilis, 171, notes 104, 105, 107 

Corals and reefs, 224 

Cordylophora, migration from the sea 
of, 149 

Coregonus hiemalis, 321 

Cossol bank, see Kossol 

Coypu, 61 

Crocodilus biporcatus, 63 

Cryptochirus, 221, 281 

— acrustacean resembling, 28, note 103 

Cymothoe, 83 


p44 on coral reefs, 231, 272 
Darwin on reef formation, 216, 
230, 272 


470 


DAR 


Darwin on external conditions of life, 87 

Dasy peltis, 55 

Day, Sir F., on the respiration of 
Labyrinthici, 190 

Desoria glacialis, 117 

Dewar on the optic current, 97 

Diplozoon, 124 

Dorcasia, 399 

Draco, parachute of, 10 

Dugong, 17 


RLOEAR SLES and Entopara- 
sites, 276, 860 

Elephants’ teeth fossil in Mindanao, 
note 117 

Emery on respiration in Amphibia, 
note 75 

Entoconcha, 348 

Erythrinus, respiration of, by the air- 
bladder, 190 

Etiolation, 88 

Euglena, 75 

FEulima, 361 

Euplectella, 137 

Euprepia, change of colour in, 67 

Eurythermal and Stenotbermal, 105 

Exocetus, 9 


Fle RIDEA associated with asponge, 
343 


Forel on the Lymnzide of the Lake 
of Geneva, 53, 197 

Frauenfeld on a triton enclosed in a 
stone, note 9 

Friedrichsen’s map of the Pelew 
Islands, 245 

Frog, skin of, 92 


(Biers on Corals, 218 
Gecarcinus, 192 

Gecko, 21 

— foot of, 22 

Gentry’s observation on the effect of 
food on the Lepidoptera, 66 

Geonemertes palaensis, 187 

Gobius Ruthensparri, 93 

Gouriet on the air-bladder, note 75 

Graeffe on Hapalocarcinus, note 102 

Giinther on Labyrinthici, note 88 

— on the tortoises of the Mauritius, 
note 113 

— on deep-sea creatures, 86, note 21 

Gulick on Achatinella, note 111 


INDEX. 


LAN 


GALCKEELS hypothesis, note 114 
Hagen’s observations, 83 

Holicore or Dugong, 17 

Halobates, 144 

Hapalocarcinus, 221, 281 

Heinche on the colours of Gobius, 98, 
note 27 

Helicarion, 285 

Helix fruticum, 400 

Heterocyathus associated with Sipun- 
culus, 335 

Heteropsammia, 835 

Higginbottom on the formation of 
pigment, 89 

— on the effects of arising tempera- 


ture, 129 
Hincks on the effects of food on 
Polyps, 66 


Holmgren on the effects of food oz tre 
stomach of the pigeon, 68 

Holothurie, 44 

— parasites on and in, 351 

Horvath on winter sleep, 112 

Hunter’s experiments on feeding gulls, 
61, 68 

Huzley on the distribution of Asta- 
cide, note 115 

Hydra viridis, 73 


NFUSORIA, distribution of, 289 
— possible decomposition of car- 
bonic acid by, note 81 
Intestinal worms, development of, 276 
Ixodes, note 8 


ACHUS vulgaris, 59 
Jobert on the respiration of fishes. 
note 75 


J LEINENBERG on Hydra viri- 
* dis, 73 

Kokeal, its structure and distribution, 
260, note 106 

Killiker’s section of the Germinal 
Layers, 30 

Kossol, 242 

Kramer on sexual characters in male 
insects, 366 

Kriangle, Atoll of, 254, 237, 267 


ABYRINTHICI, 189 
Lacerta agilis, 59 
Lantern fishes of the deep sea, 86 


INDEX. 


LAR 


Larus, gulls, stomach of, 60, 67 

Larva forms, 126 

Lecanurius, 352 

Leposphilus, note 122 

Leuckart on abdominal eyes in fishes, 
note 124 

Leydig on organs of a sixth sense, 21 

— on chlorophyll in beetles, note 17 

Lieberhiihn on the association of alge 
and sponges, 343 

Limnoria terebrans in wood and stone, 
326 

Lister's experiments on colour. 92 

Lori Rajah, 67 

Lucina philippensis, 170 

Lupea, 196 

Lymnea stagnalis, 209 

— its mode of ewimming, note 97 

— its growth, 108, 160 


ALACOBDELLA, note 122 
Mulaunavi, reef of, 227 
Mammoth cave of Kentucky, 82 
Marsh, on pneumatic bones in reptiles, 
3819 
Mauritius, tortoises of, note 113 
Maximum of temperature, note 42 
Meduse, 166 
Melania, distribution of, 298 
Meénétriés on changes of structure in 
birds, 68 
Milne Edwards's experiments, 179 
Minimum of temperature, note 4 
Mobius on the deposition of eggs, 136 
— on the sinking current, 53 
—on the effects of temperature on 
Mollusca, 136 
Mollusca, shells of, 327 
— development of, note 101 
Monotremata in New Guinea, 32 
Moreau on oxygen in the air-bladder, 
note 75 
Mozambique current, 202, 279, 302 
Miiller on the respiration of crabs, 
note 91 
Myopotamus Coypu, 61 
Myxicola, 401 


TAUPLIUS, 128 
Navicella, 208, 209 
Nematode larvex, 75 
Neritina, 209 
Nestor mirabilis, 62 
Nogaur or Angaur, 251 
Nordmann on the eggs of Tergipes, 136 


471 


RAY 


CYPODA, 196 
Onchidium, 281 
— distribution of, 375 
— dorsal eyes of, "369, 578 
Oysters, their disappearance from the 
Baltic, note 61 
— in fresh water, 148 


ACHYBDELLA, 338 
Pachydrilus, 145 

Pagurus, 331 

Paimén on the migrations of birds, 
297 

Paludina, 298 

Paulsen on carnivorous horses, 62 

Pauly on the respiration of Lymnza, 
198 

— remarks, note 94 

Pelelew, 249 

— fossils in, 262 

— elevation of, 263 

Pelews, north island, 234 

— geological structure, 234, 264 

— Andesite in the Southern reef, 260 

Periophthalmus, 189, 373 

Philippi on carnivorous horses, 62 

Philippines, Mollusca of the, 183, 284, 
288, 395 

Phosphorescent 
depths, 86 

Phyllopoda, distribution of, 304 

Pigeon, stomach of, 55, note 14 

Pigment, note 26 

Pinnotheres holothuria, 80 

Planarians, land, 187 

Planorbis, 300 

Plateau’s experiments, 151 

Platurus, 146 

Polyphyletic descent of fresh-water 
animals, note 114 

Polystomum, 134 

Porites, growth of, 225 

Pouchet on the effects of desiccation, 
174 

— on the chromatic function, 91 

— on freezing up animals, 111 

— on changes of colour, 96 

Pourtalés Plateau, 273 

Prairie dogs, 110 

Proteus, 79 

Pycnogonide, galls of, on Hydroide, 
832 


animals in great 


ADIOLARIANS, 74 
Ray Lankester on chlorophyll, 73 


472 


REA 


Réaumur on Aphides, 123 

Rhizochilus associated 
pathes, 837 

Rhysota, 285 

Rossbach, experiments on Infusoria, 
106, 119 

Rotatoria, distribution of, 305 


with <Anti- 


ACCULINA, 47, 340 
Schmankewitsch on Artemia and 

Branchipus, 156 

Schultze on the structure of the eyein 
night-birds, 77 

— on chlorophyll, 73 

Scopelidz, abdominal eyes of, note 124 

Seidlitz on the effects of food on colour, 
66 

Semper, Georg, on Pieris, note 125 

Sensitive hairs, 23 

Sightless animals, note 19 

Siphonophore of surface water, 205 

— of deep water, note 96 

Sipunculide associated with Hetero- 
psammia, 835 

— with Heterocyathus, 335 

Sorby on chlorophyll in animals, 73 

Spermophilus citillus, 112 

Sphenopus, 75 

Sponges, structure of, 73 

— take up foreign spicula, note 18 

— associated with alyw, 343 

— compared with lichens, 343 

Spongia cartilaginea, 344 

Statoblasts of Bryozoa, 112 

Stenothermal animals, 105 

Stentor viridis, 75 

Stickleback, 146 

Strix grallaria, 68 

Sudis gigas, mode of respiration, 190 


EMNOCEPHALA chilensis, 293 
Tergipes, winter eggs of, 136 
Thacker on the origin of the four limbs 

in the yertebrata, note 5 


INDEX. 


ZEA 


Trachew in insects, 180 

— their distribution, note 79 
Trematoda, 351 

Trichina, 56 

Turbellarians, 71 


NIO, 298 
Ussow on the abdominal eyes of 
the Scopelide note 124 


‘ERIEERAL column, note 2 
Vortex viridis, 73 


Ww4 GNER’S separation theory, 
290, notes 110, 111 

Wallace on the faunas of Eastern 
Asia, 283, 811 

~~ of West African Islands, 295 

— on the origin of colouring, 385 

—o0n geographical distribution of 
animals, 32 

— on mimicry, 384 

—on the action of constant strong 
currents, 315 

Weismann’s experiments, 117 

West Africa, islands of, 295 

Wiechmann on the fossils of the Pe- 
Jews, 259 

Wilder on the respiration of Ganoid 
fishes, note 75 

Wittich on the effects of light on the 
chromatophores, 95 


ESTA, 286 
— mimicking Rhysota, 395 
— Cumingi, 397 


GELLER on Diplozoon snd Poly- 
stomum, 124, 134 
Zora stage of crabs, 81 


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