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+The Project Gutenberg EBook of The Outline of Science, Vol. 1 (of 4), by
+J. Arthur Thomson
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever. You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: The Outline of Science, Vol. 1 (of 4)
+ A Plain Story Simply Told
+
+Author: J. Arthur Thomson
+
+Release Date: January 22, 2007 [EBook #20417]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK OUTLINE OF SCIENCE ***
+
+
+
+
+Produced by Brian Janes, Leonard Johnson and the Online
+Distributed Proofreading Team at http://www.pgdp.net
+
+
+
+
+
+
+
+[Illustration: THE GREAT SCARLET SOLAR PROMINENCES, WHICH ARE SUCH A
+NOTABLE FEATURE OF THE SOLAR PHENOMENA, ARE IMMENSE OUTBURSTS OF FLAMING
+HYDROGEN RISING SOMETIMES TO A HEIGHT OF 500,000 MILES]
+
+
+
+
+THE
+OUTLINE OF SCIENCE
+
+A PLAIN STORY SIMPLY TOLD
+
+
+
+EDITED BY
+J. ARTHUR THOMSON
+REGIUS PROFESSOR OF NATURAL HISTORY IN THE
+UNIVERSITY OF ABERDEEN
+
+
+
+WITH OVER 800 ILLUSTRATIONS
+OF WHICH ABOUT 40 ARE IN COLOUR
+
+
+IN FOUR VOLUMES
+
+
+
+G. P. PUTNAM'S SONS
+NEW YORK AND LONDON
+The Knickerbocker press
+
+
+
+
+Copyright, 1922
+by
+G. P. Putnam's Sons
+
+
+
+_First Printing April, 1922
+Second Printing April, 1922
+Third Printing April, 1922
+Fourth Printing April, 1922
+Fifth Printing June, 1922
+Sixth Printing June, 1922
+Seventh Printing June, 1922
+Eighth Printing June, 1922
+Ninth Printing August, 1922
+Tenth Printing September, 1922
+Eleventh Printing Sept., 1922
+Twelfth Printing, May, 1924_
+
+
+Made in the United States of America
+
+
+
+
+INTRODUCTORY NOTE
+
+By Professor J. Arthur Thomson
+
+
+Was it not the great philosopher and mathematician Leibnitz who said
+that the more knowledge advances the more it becomes possible to
+condense it into little books? Now this "Outline of Science" is
+certainly not a little book, and yet it illustrates part of the meaning
+of Leibnitz's wise saying. For here within reasonable compass there is a
+library of little books--an outline of many sciences.
+
+It will be profitable to the student in proportion to the discrimination
+with which it is used. For it is not in the least meant to be of the
+nature of an Encyclopædia, giving condensed and comprehensive articles
+with a big full stop at the end of each. Nor is it a collection of
+"primers," beginning at the very beginning of each subject and working
+methodically onwards. That is not the idea.
+
+What then is the aim of this book? It is to give the intelligent
+student-citizen, otherwise called "the man in the street," a bunch of
+intellectual keys by which to open doors which have been hitherto shut
+to him, partly because he got no glimpse of the treasures behind the
+doors, and partly because the portals were made forbidding by an
+unnecessary display of technicalities. Laying aside conventional modes
+of treatment and seeking rather to open up the subject as one might on a
+walk with a friend, the work offers the student what might be called
+informal introductions to the various departments of knowledge. To put
+it in another way, the articles are meant to be clues which the reader
+may follow till he has left his starting point very far behind. Perhaps
+when he has gone far on his own he will not be ungrateful to the simple
+book of "instructions to travellers" which this "Outline of Science" is
+intended to be. The simple "bibliographies" appended to the various
+articles will be enough to indicate "first books." Each article is meant
+to be an invitation to an intellectual adventure, and the short lists of
+books are merely finger-posts for the beginning of the journey.
+
+We confess to being greatly encouraged by the reception that has been
+given to the English serial issue of "The Outline of Science." It has
+been very hearty--we might almost say enthusiastic. For we agree with
+Professor John Dewey, that "the future of our civilisation depends upon
+the widening spread and deepening hold of the scientific habit of mind."
+And we hope that this is what "The Outline of Science" makes for.
+Information is all to the good; interesting information is better still;
+but best of all is the education of the scientific habit of mind.
+Another modern philosopher, Professor L. T. Hobhouse, has declared that
+the evolutionist's mundane goal is "the mastery by the human mind of the
+conditions, internal as well as external, of its life and growth." Under
+the influence of this conviction "The Outline of Science" has been
+written. For life is not for science, but science for life. And even
+more than science, to our way of thinking, is the individual development
+of the scientific way of looking at things. Science is our legacy; we
+must use it if it is to be our very own.
+
+
+
+
+CONTENTS
+
+
+INTRODUCTION 3
+
+I. THE ROMANCE OF THE HEAVENS 7
+
+ The scale of the universe--The solar system--Regions of
+ the sun--The surface of the sun--Measuring the speed of
+ light--Is the sun dying?--The planets--Venus--Is there
+ life on Mars?--Jupiter and Saturn--The moon--The
+ mountains of the moon--Meteors and comets--Millions of
+ meteorites--A great comet--The stellar universe--The
+ evolution of stars--The age of stars--The nebular
+ theory--Spiral nebulæ--The birth and death of
+ stars--The shape of our universe--Astronomical
+ instruments.
+
+II. THE STORY OF EVOLUTION 53
+
+ The beginning of the earth--Making a home for life--The
+ first living creatures--The first plants--The first
+ animals--Beginnings of bodies--Evolution of
+ sex--Beginning of natural death--Procession of life
+ through the ages--Evolution of land animals--The flying
+ dragons--The first known bird--Evidences of
+ evolution--Factors in evolution.
+
+III. ADAPTATIONS TO ENVIRONMENT 113
+
+ The shore of the sea--The open sea--The deep sea--The
+ fresh waters--The dry land--The air.
+
+IV. THE STRUGGLE FOR EXISTENCE 135
+
+ Animal and bird mimicry and disguise--Other kinds of
+ elusiveness.
+
+V. THE ASCENT OF MAN 153
+
+ Anatomical proof of man's relationship with a Simian
+ stock--Physiological proof--Embryological proof--Man's
+ pedigree--Man's arboreal apprenticeship--Tentative
+ men--Primitive men--Races of mankind--Steps in human
+ evolution--Factors in human progress.
+
+VI. EVOLUTION GOING ON 183
+
+ Evolutionary prospect for man--The fountain of change;
+ variability--Evolution of plants--Romance of
+ wheat--Changes in animal life--Story of the
+ salmon--Forming new habits--Experiments in locomotion;
+ new devices.
+
+VII. THE DAWN OF MIND 205
+
+ A caution in regard to instinct--A useful law--Senses of
+ fishes--The mind of a minnow--The mind and senses of
+ amphibians--The reptilian mind--Mind in
+ birds--Intelligence co-operating with instinct--The
+ mind of the mammal--Instinctive aptitudes--Power of
+ association--Why is there not more intelligence?--The
+ mind of monkeys--Activity for activity's
+ sake--Imitation--The mind of man--Body and mind.
+
+VIII. FOUNDATIONS OF THE UNIVERSE 243
+
+ The world of atoms--The energy of atoms--The discovery of
+ X-rays--The discovery of radium--The discovery of the
+ electron--The electron theory--The structure of the
+ atom--The new view of matter--Other new views--The
+ nature of electricity--Electric current--The
+ dynamo--Magnetism--Ether and waves--Light--What the
+ blue "sky" means--Light without heat--Forms of
+ energy--What heat is--Substitutes for coal--Dissipation
+ of energy--What a uniform temperature would
+ mean--Matter, ether, and Einstein--The tides--Origin of
+ the moon--The earth slowing down--The day becoming
+ longer.
+
+
+
+
+ILLUSTRATIONS
+
+
+ FACING
+ PAGE
+
+THE GREAT SCARLET SOLAR PROMINENCES, WHICH ARE SUCH A
+ NOTABLE FEATURE OF THE SOLAR PHENOMENA, ARE IMMENSE
+ OUTBURSTS OF FLAMING HYDROGEN RISING SOMETIMES TO A
+ HEIGHT OF 500,000 MILES
+ _Coloured Frontispiece_
+
+LAPLACE 10
+
+PROFESSOR J. C. ADAMS 10
+ Photo: Royal Astronomical Society.
+
+PROFESSOR EDDINGTON OF CAMBRIDGE UNIVERSITY 10
+ Photo: Elliot & Fry, Ltd.
+
+THE PLANETS, SHOWING THEIR RELATIVE DISTANCES AND
+ DIMENSIONS 11
+
+THE MILKY WAY 14
+ Photo: Harvard College Observatory.
+
+THE MOON ENTERING THE SHADOW CAST BY THE EARTH 14
+
+THE GREAT NEBULA IN ANDROMEDA, MESSIER 31 15
+ From a photograph taken at the Yerkes Observatory.
+
+DIAGRAM SHOWING THE MAIN LAYERS OF THE SUN 18
+
+SOLAR PROMINENCES SEEN AT TOTAL SOLAR ECLIPSE, MAY 29,
+ 1919. TAKEN AT SOBRAL, BRAZIL 18
+ Photo: Royal Observatory, Greenwich.
+
+THE VISIBLE SURFACE OF THE SUN 19
+ Photo: Mount Wilson Observatory.
+
+THE SUN PHOTOGRAPHED IN THE LIGHT OF GLOWING HYDROGEN 19
+ Photo: Mount Wilson Observatory.
+
+THE AURORA BOREALIS (_Coloured Illustration_) 20
+ Reproduced from _The Forces of Nature_ (Messrs. Macmillan)
+
+THE GREAT SUN-SPOT OF JULY 17, 1905 22
+ Yerkes Observatory.
+
+SOLAR PROMINENCES 22
+ From photographs taken at the Yerkes Observatory.
+
+MARS, OCTOBER 5, 1909 23
+ Photo: Mount Wilson Observatory.
+
+JUPITER 23
+
+SATURN, NOVEMBER 19, 1911 23
+ Photo: Professor E. E. Barnard, Yerkes Observatory.
+
+THE SPECTROSCOPE, AN INSTRUMENT FOR ANALYSING LIGHT; IT
+ PROVIDES MEANS FOR IDENTIFYING SUBSTANCES (_Coloured
+ Illustration_) 24
+
+THE MOON 28
+
+MARS 29
+ Drawings by Professor Percival Lowell.
+
+THE MOON, AT NINE AND THREE QUARTER DAYS 29
+
+A MAP OF THE CHIEF PLAINS AND CRATERS OF THE MOON 32
+
+A DIAGRAM OF A STREAM OF METEORS SHOWING THE EARTH PASSING
+ THROUGH THEM 32
+
+COMET, SEPTEMBER 29, 1908 33
+ Photo: Royal Observatory, Greenwich.
+
+COMET, OCTOBER 3, 1908 33
+ Photo: Royal Observatory, Greenwich.
+
+TYPICAL SPECTRA 36
+ Photo: Harvard College Observatory.
+
+A NEBULAR REGION SOUTH OF ZETA ORIONIS 37
+ Photo: Mount Wilson Observatory.
+
+STAR CLUSTER IN HERCULES 37
+ Photo: Astrophysical Observatory, Victoria, British Columbia.
+
+THE GREAT NEBULA IN ORION 40
+ Photo: Yerkes Observatory.
+
+GIANT SPIRAL NEBULA, MARCH 23, 1914 41
+ Photo: Lick Observatory.
+
+A SPIRAL NEBULA SEEN EDGE-ON 44
+ Photo: Mount Wilson Observatory.
+
+100-INCH TELESCOPE, MOUNT WILSON 45
+ Photo: H. J. Shepstone.
+
+THE YERKES 40-INCH REFRACTOR 48
+
+THE DOUBLE-SLIDE PLATE-HOLDER ON YERKES 40-INCH REFRACTING
+ TELESCOPE 49
+ Photo: H. J. Shepstone.
+
+MODERN DIRECT-READING SPECTROSCOPE 49
+ By A. Hilger, Ltd.
+
+CHARLES DARWIN 56
+ Photo: Rischgitz Collection.
+
+LORD KELVIN 56
+ Photo: Rischgitz Collection.
+
+A GIANT SPIRAL NEBULA 57
+ Photo: Lick Observatory.
+
+METEORITE WHICH FELL NEAR SCARBOROUGH AND IS NOW TO BE SEEN
+ IN THE NATURAL HISTORY MUSEUM 57
+ Photo: Natural History Museum.
+
+A LIMESTONE CANYON 60
+ Reproduced from the Smithsonian Report, 1915.
+
+GEOLOGICAL TREE OF ANIMALS 61
+
+DIAGRAM OF AMOEBA 61
+
+A PIECE OF A REEF-BUILDING CORAL, BUILT UP BY A LARGE
+ COLONY OF SMALL SEA-ANEMONE-LIKE POLYPS, EACH OF WHICH
+ FORMS FROM THE SALTS OF THE SEA A SKELETON OR SHELL OF
+ LIME 64
+ From the Smithsonian Report, 1917.
+
+A GROUP OF CHALK-FORMING ANIMALS, OR FORAMINIFERA, EACH
+ ABOUT THE SIZE OF A VERY SMALL PIN'S HEAD 65
+ Photo: J. J. Ward, F.E.S.
+
+A COMMON FORAMINIFER (POLYSTOMELLA) SHOWING THE SHELL IN
+ THE CENTRE AND THE OUTFLOWING NETWORK OF LIVING MATTER,
+ ALONG WHICH GRANULES ARE CONTINUALLY TRAVELLING, AND BY
+ WHICH FOOD PARTICLES ARE ENTANGLED AND DRAWN IN 65
+ Reproduced by permission of the Natural History Museum
+ (after Max Schultze).
+
+A PLANT-LIKE ANIMAL, OR ZOOPHYTE, CALLED OBELIA 68
+ Photo: J. J. Ward, F.E.S.
+
+TRYPANOSOMA GAMBIENSE 69
+ Reproduced by permission of _The Quart. Journ. Mic. Sci._
+
+VOLVOX 69
+
+PROTEROSPONGIA 69
+
+GREEN HYDRA 72
+ Photo: J. J. Ward, F.E.S.
+
+DIAGRAM ILLUSTRATING THE BEGINNING OF INDIVIDUAL LIFE 72
+
+EARTHWORM 72
+ Photo: J. J. Ward, F.E.S.
+
+GLASS MODEL OF A SEA-ANEMONE 72
+ Reproduced from the Smithsonian Report, 1917.
+
+THIS DRAWING SHOWS THE EVOLUTION OF THE BRAIN FROM FISH TO
+ MAN 73
+
+OKAPI AND GIRAFFE (_Coloured Illustration_) 74
+
+DIAGRAM OF A SIMPLE REFLEX ARC IN A BACKBONELESS ANIMAL
+ LIKE AN EARTHWORM 76
+
+THE YUCCA MOTH 76
+ Photo: British Museum (Natural History).
+
+INCLINED PLANE OF ANIMAL BEHAVIOUR 76
+
+VENUS' FLY-TRAP 77
+ Photo: J. J. Ward, F.E.S.
+
+A SPIDER SUNNING HER EGGS 77
+ Reproduced by permission from _The Wonders of Instinct_ by
+ J. H. Fabre.
+
+THE HOATZIN INHABITS BRITISH GUIANA 82
+
+PERIPATUS 83
+ Photograph, from the British Museum (Natural History), of a
+ drawing by Mr. E. Wilson.
+
+ROCK KANGAROO CARRYING ITS YOUNG IN A POUCH 83
+ Photo: W. S. Berridge, F.Z.S.
+
+PROFESSOR THOMAS HENRY HUXLEY (1825-95) 86
+ Photo: Rischgitz.
+
+BARON CUVIER, 1769-1832 86
+
+AN ILLUSTRATION SHOWING VARIOUS METHODS OF FLYING AND
+ SWOOPING 87
+
+ANIMALS OF THE CAMBRIAN PERIOD 90
+ From Knipe's _Nebula to Man_.
+
+A TRILOBITE 90
+ Photo: J. J. Ward, F.E.S.
+
+THE GAMBIAN MUD-FISH, PROTOPTERUS 91
+ Photo: British Museum (Natural History).
+
+THE ARCHÆOPTERYX 91
+ After William Leche of Stockholm.
+
+WING OF A BIRD, SHOWING THE ARRANGEMENT OF THE FEATHERS 91
+
+PICTORIAL REPRESENTATION OF STRATA OF THE EARTH'S CRUST,
+ WITH SUGGESTIONS OF CHARACTERISTIC FOSSILS (_Coloured
+ Illustration_) 92
+
+FOSSIL OF A PTERODACTYL OR EXTINCT FLYING DRAGON 94
+ Photo: British Museum (Natural History).
+
+PARIASAURUS: AN EXTINCT VEGETARIAN TRIASSIC REPTILE 94
+ From Knipe's _Nebula to Man_.
+
+TRICERATOPS: A HUGE EXTINCT REPTILE 95
+ From Knipe's _Nebula to Man_.
+
+THE DUCKMOLE OR DUCK-BILLED PLATYPUS OF AUSTRALIA 95
+ Photo: _Daily Mail_.
+
+SKELETON OF AN EXTINCT FLIGHTLESS TOOTHED BIRD, HESPERORNIS 100
+ After Marsh.
+
+SIX STAGES IN THE EVOLUTION OF THE HORSE, SHOWING GRADUAL
+INCREASE IN SIZE 101
+ After Lull and Matthew.
+
+DIAGRAM SHOWING SEVEN STAGES IN THE EVOLUTION OF THE
+ FORE-LIMBS AND HIND-LIMBS OF THE ANCESTORS OF THE MODERN
+ HORSE, BEGINNING WITH THE EARLIEST KNOWN PREDECESSORS OF
+ THE HORSE AND CULMINATING WITH THE HORSE OF TO-DAY 104
+ After Marsh and Lull.
+
+WHAT IS MEANT BY HOMOLOGY? ESSENTIAL SIMILARITY OF
+ ARCHITECTURE, THOUGH THE APPEARANCES MAY BE VERY
+ DIFFERENT 105
+
+AN EIGHT-ARMED CUTTLEFISH OR OCTOPUS ATTACKING A SMALL CRAB 116
+
+A COMMON STARFISH, WHICH HAS LOST THREE ARMS AND IS
+ REGROWING THEM 116
+ After Professor W. C. McIntosh.
+
+THE PAPER NAUTILUS (ARGONAUTA), AN ANIMAL OF THE OPEN SEA 117
+ Photo: J. J. Ward, F.E.S.
+
+A PHOTOGRAPH SHOWING A STARFISH (_Asterias Forreri_) WHICH
+ HAS CAPTURED A LARGE FISH 117
+
+TEN-ARMED CUTTLEFISH OR SQUID IN THE ACT OF CAPTURING A FISH 118
+
+GREENLAND WHALE 118
+
+MINUTE TRANSPARENT EARLY STAGE OF A SEA-CUCUMBER 119
+
+AN INTRICATE COLONY OF OPEN-SEA ANIMALS (_Physophora
+ Hydrostatica_) RELATED TO THE PORTUGUESE MAN-OF-WAR 119
+ Photo: British Museum (Natural History).
+
+A SCENE IN THE GREAT DEPTHS 119
+
+SEA-HORSE IN SARGASSO WEED 120
+
+LARGE MARINE LAMPREYS (_Petromyzon Marinus_) 120
+
+THE DEEP-SEA FISH _Chiasmodon Niger_ 120
+
+DEEP-SEA FISHES 120
+
+FLINTY SKELETON OF VENUS' FLOWER BASKET (_Euplectella_), A
+ JAPANESE DEEP-SEA SPONGE 121
+
+EGG DEPOSITORY OF _Semotilus Atromaculatus_ 121
+
+THE BITTERLING (_Rhodeus Amarus_) 124
+
+WOOLLY OPOSSUM CARRYING HER FAMILY 124
+ Photo: W. S. Berridge.
+
+SURINAM TOAD (_Pipa Americana_) WITH YOUNG ONES HATCHING
+ OUT OF LITTLE POCKETS ON HER BACK 125
+
+STORM PETREL OR MOTHER CAREY'S CHICKEN (_Procellaria
+ Pelagica_) 125
+
+ALBATROSS: A CHARACTERISTIC PELAGIC BIRD OF THE SOUTHERN
+ SEA 128
+
+THE PRAYING MANTIS (_Mantis Religiosa_) 138
+
+PROTECTIVE COLORATION: A WINTER SCENE IN NORTH SCANDINAVIA 138
+
+THE VARIABLE MONITOR (_Varanus_) 139
+ Photo: A. A. White.
+
+BANDED KRAIT: A VERY POISONOUS SNAKE WITH ALTERNATING
+ YELLOW AND DARK BANDS 140
+ Photo: W. S. Berridge, F.Z.S.
+
+THE WARTY CHAMELEON 140
+ Photos: W. S. Berridge, F.Z.S.
+
+SEASONAL COLOUR-CHANGE: SUMMER SCENE IN NORTH SCANDINAVIA 141
+
+PROTECTIVE RESEMBLANCE 142
+ Photo: J. J. Ward, F.E.S.
+
+WHEN ONLY A FEW DAYS OLD, YOUNG BITTERN BEGIN TO STRIKE THE
+ SAME ATTITUDE AS THEIR PARENTS, THRUSTING THEIR BILLS
+ UPWARDS AND DRAWING THEIR BODIES UP SO THAT THEY RESEMBLE
+ A BUNCH OF REEDS 143
+
+PROTECTIVE COLORATION OR CAMOUFLAGING, GIVING ANIMALS A
+ GARMENT OF INVISIBILITY (_Coloured Illustration_) 144
+
+ANOTHER EXAMPLE OF PROTECTIVE COLORATION (_Coloured
+ Illustration_) 144
+
+DEAD-LEAF BUTTERFLY (_Kallima Inachis_) FROM INDIA 146
+
+PROTECTIVE RESEMBLANCE BETWEEN A SMALL SPIDER (_to the
+ left_) AND AN ANT (_to the right_) 146
+
+THE WASP BEETLE, WHICH, WHEN MOVING AMONGST THE BRANCHES,
+ GIVES A WASP-LIKE IMPRESSION 147
+ Photo: J. J. Ward, F.E.S.
+
+HERMIT-CRAB WITH PARTNER SEA-ANEMONES 147
+
+CUCKOO-SPIT 147
+ Photo: G. P. Duffus.
+
+CHIMPANZEE, SITTING 156
+ Photo: New York Zoological Park.
+
+CHIMPANZEE, ILLUSTRATING WALKING POWERS 156
+ Photo: New York Zoological Park.
+
+SURFACE VIEW OF THE BRAINS OF MAN AND CHIMPANZEE 157
+
+SIDE-VIEW OF CHIMPANZEE'S HEAD 157
+ Photo: New York Zoological Park.
+
+PROFILE VIEW OF HEAD OF PITHECANTHROPUS, THE JAVA APE-MAN,
+ RECONSTRUCTED FROM THE SKULL-CAP 157
+ After a model by J. H. McGregor.
+
+THE FLIPPER OF A WHALE AND THE HAND OF A MAN 157
+
+THE GORILLA, INHABITING THE FOREST TRACT OF THE GABOON IN
+ AFRICA (_Coloured Illustration_) 158
+
+"DARWIN'S POINT" ON HUMAN EAR 160
+
+PROFESSOR SIR ARTHUR KEITH, M.D., LL.D., F.R.S. 161
+ Photo: J. Russell & Sons.
+
+SKELETONS OF THE GIBBON, ORANG, CHIMPANZEE, GORILLA, MAN 161
+ After T. H. Huxley (by permission of Messrs. Macmillan).
+
+SIDE-VIEW OF SKULL OF MAN AND GORILLA 164
+
+THE SKULL AND BRAIN-CASE OF PITHECANTHROPUS, THE JAVA
+ APE-MAN, AS RESTORED BY J. H. MCGREGOR FROM THE SCANTY
+ REMAINS 164
+
+SUGGESTED GENEALOGICAL TREE OF MAN AND ANTHROPOID APES 165
+
+THE GIBBON IS LOWER THAN THE OTHER APES AS REGARDS ITS
+ SKULL AND DENTITION, BUT IT IS HIGHLY SPECIALIZED IN THE
+ ADAPTATION OF ITS LIMBS TO ARBOREAL LIFE 166
+ Photo: New York Zoological Park.
+
+THE ORANG HAS A HIGH ROUNDED SKULL AND A LONG FACE 166
+ Photo: New York Zoological Park.
+
+COMPARISONS OF THE SKELETONS OF HORSE AND MAN 167
+ Photo: British Museum (Natural History).
+
+A RECONSTRUCTION OF THE JAVA MAN (_Coloured Illustration_) 168
+
+PROFILE VIEW OF THE HEAD OF PITHECANTHROPUS, THE JAVA
+ APE-MAN--AN EARLY OFFSHOOT FROM THE MAIN LINE OF MAN'S
+ ASCENT 170
+ After a model by J. H. McGregor.
+
+PILTDOWN SKULL 170
+ From the reconstruction by J. H. McGregor.
+
+SAND-PIT AT MAUER, NEAR HEIDELBERG: DISCOVERY SITE OF THE
+ JAW OF HEIDELBERG MAN 171
+ Reproduced by permission from Osborn's
+ _Men of the Old Stone Age_.
+
+PAINTINGS ON THE ROOF OF THE ALTAMIRA CAVE IN NORTHERN
+ SPAIN, SHOWING A BISON AND A GALLOPING BOAR (_Coloured
+ Illustration_) 172
+
+PILTDOWN MAN, PRECEDING NEANDERTHAL MAN, PERHAPS 100,000 TO
+ 150,000 YEARS AGO 174
+ After the restoration modelled by J. H. McGregor.
+
+THE NEANDERTHAL MAN OF LA CHAPELLE-AUX-SAINTS 175
+ After the restoration modelled by J. H. McGregor.
+
+RESTORATION BY A. FORESTIER OF THE RHODESIAN MAN WHOSE
+ SKULL WAS DISCOVERED IN 1921 176-177
+
+SIDE VIEW OF A PREHISTORIC HUMAN SKULL DISCOVERED IN 1921
+ IN BROKEN HILL CAVE, NORTHERN RHODESIA 178
+ Photo: British Museum (Natural History).
+
+A CROMAGNON MAN OR CROMAGNARD, REPRESENTATIVE OF A STRONG
+ ARTISTIC RACE LIVING IN THE SOUTH OF FRANCE IN THE UPPER
+ PLEISTOCENE, PERHAPS 25,000 YEARS AGO 178
+ After the restoration modelled by J. H. McGregor.
+
+PHOTOGRAPH SHOWING A NARROW PASSAGE IN THE CAVERN OF
+ FONT-DE-GAUME ON THE BEUNE 179
+ Reproduced by permission from Osborn's
+ _Men of the Old Stone Age_.
+
+A MAMMOTH DRAWN ON THE WALL OF THE FONT-DE-GAUME CAVERN 179
+
+A GRAZING BISON, DELICATELY AND CAREFULLY DRAWN, ENGRAVED
+ ON A WALL OF THE ALTAMIRA CAVE, NORTHERN SPAIN 179
+
+PHOTOGRAPH OF A MEDIAN SECTION THROUGH THE SHELL OF THE
+ PEARLY NAUTILUS 186
+
+PHOTOGRAPH OF THE ENTIRE SHELL OF THE PEARLY NAUTILUS 186
+
+NAUTILUS 186
+
+SHOEBILL 187
+ Photo: W. S. Berridge.
+
+THE WALKING-FISH OR MUD-SKIPPER (_Periophthalmus_), COMMON
+ AT THE MOUTHS OF RIVERS IN TROPICAL AFRICA, ASIA, AND
+ NORTH-WEST AUSTRALIA 190
+
+THE AUSTRALIAN MORE-PORK OR PODARGUS 190
+ Photo: _The Times_.
+
+PELICAN'S BILL, ADAPTED FOR CATCHING AND STORING FISHES 191
+
+SPOONBILL'S BILL, ADAPTED FOR SIFTING THE MUD AND CATCHING
+ THE SMALL ANIMALS, E.G. FISHES, CRUSTACEANS, INSECT
+ LARVÆ, WHICH LIVE THERE 191
+
+AVOCET'S BILL, ADAPTED FOR A CURIOUS SIDEWAYS SCOOPING IN
+ THE SHORE-POOLS AND CATCHING SMALL ANIMALS 191
+
+HORNBILL'S BILL, ADAPTED FOR EXCAVATING A NEST IN A TREE,
+ AND ALSO FOR SEIZING AND BREAKING DIVERSE FORMS OF FOOD,
+ FROM MAMMALS TO TORTOISES, FROM ROOTS TO FRUITS 191
+
+FALCON'S BILL, ADAPTED FOR SEIZING, KILLING, AND TEARING
+ SMALL MAMMALS AND BIRDS 191
+
+PUFFIN'S BILL, ADAPTED FOR CATCHING SMALL FISHES NEAR THE
+ SURFACE OF THE SEA, AND FOR HOLDING THEM WHEN CAUGHT AND
+ CARRYING THEM TO THE NEST 191
+
+LIFE-HISTORY OF A FROG 192
+
+HIND-LEG OF WHIRLIGIG BEETLE WHICH HAS BECOME BEAUTIFULLY
+ MODIFIED FOR AQUATIC LOCOMOTION 192
+ Photo: J. J. Ward, F.E.S.
+
+THE BIG ROBBER-CRAB (_Birgus Latro_), THAT CLIMBS THE
+ COCONUT PALM AND BREAKS OFF THE NUTS 193
+
+EARLY LIFE-HISTORY OF THE SALMON 196
+
+THE SALMON LEAPING AT THE FALL IS A MOST FASCINATING SPECTACLE 197
+
+DIAGRAM OF THE LIFE-HISTORY OF THE COMMON EEL (_Anguilla
+ Vulgaris_) 200
+
+CASSOWARY 201
+ Photo: Gambier Bolton.
+
+THE KIWI, ANOTHER FLIGHTLESS BIRD, OF REMARKABLE
+ APPEARANCE, HABITS, AND STRUCTURE 201
+ Photo: Gambier Bolton.
+
+THE AUSTRALIAN FRILLED LIZARD, WHICH IS AT PRESENT TRYING
+ TO BECOME A BIPED 202
+
+A CARPET OF GOSSAMER 202
+
+THE WATER SPIDER 203
+
+JACKDAW BALANCING ON A GATEPOST 208
+ Photo: O. J. Wilkinson.
+
+TWO OPOSSUMS FEIGNING DEATH 208
+ From Ingersoll's _The Wit of the Wild_.
+
+MALE OF THREE-SPINED STICKLEBACK, MAKING A NEST OF
+ WATER-WEED, GLUED TOGETHER BY VISCID THREADS SECRETED
+ FROM THE KIDNEYS AT THE BREEDING SEASON 209
+
+A FEMALE STICKLEBACK ENTERS THE NEST WHICH THE MALE HAS
+ MADE, LAYS THE EGGS INSIDE, AND THEN DEPARTS 209
+
+HOMING PIGEON 212
+ Photo: Imperial War Museum.
+
+CARRIER PIGEON 212
+ Photo: Imperial War Museum.
+
+YELLOW-CROWNED PENGUIN 213
+ Photo: James's Press Agency.
+
+PENGUINS ARE "A PECULIAR PEOPLE" 213
+ Photo: Cagcombe & Co.
+
+HARPY-EAGLE 216
+ Photo: W. S. Berridge.
+
+THE DINGO OR WILD DOG OF AUSTRALIA, PERHAPS AN INDIGENOUS
+ WILD SPECIES, PERHAPS A DOMESTICATED DOG THAT HAS GONE
+ WILD OR FERAL 216
+ Photo: W. S. Berridge, F.Z.S.
+
+WOODPECKER HAMMERING AT A COTTON-REEL, ATTACHED TO A TREE 217
+
+THE BEAVER 220
+
+THE THRUSH AT ITS ANVIL 221
+ Photo: F. R. Hinkins & Son.
+
+ALSATIAN WOLF-DOG 226
+ Photo: Lafayette.
+
+THE POLAR BEAR OF THE FAR NORTH 227
+ Photo: W. S. Berridge.
+
+AN ALLIGATOR "YAWNING" IN EXPECTATION OF FOOD 227
+ From the Smithsonian Report, 1914.
+
+BABY ORANG 232
+ Photo: W. P. Dando.
+
+ORANG-UTAN 232
+ Photo: Gambier Bolton.
+
+CHIMPANZEE 233
+ Photo: James's Press Agency.
+
+BABY ORANG-UTAN 233
+ Photo: James's Press Agency.
+
+ORANG-UTAN 233
+ Photo: James's Press Agency.
+
+BABY CHIMPANZEES 233
+ Photo: James's Press Agency.
+
+CHIMPANZEE 238
+ Photo: W. P. Dando.
+
+YOUNG CHEETAHS, OR HUNTING LEOPARDS 238
+ Photo: W. S. Berridge.
+
+COMMON OTTER 239
+ Photo: C. Reid.
+
+SIR ERNEST RUTHERFORD 246
+ Photo: Elliott & Fry.
+
+J. CLERK-MAXWELL 246
+ Photo: Rischgitz Collection.
+
+SIR WILLIAM CROOKES 247
+ Photo: Ernest H. Mills.
+
+PROFESSOR SIR W. H. BRAGG 247
+ Photo: Photo Press.
+
+COMPARATIVE SIZES OF MOLECULES 250
+
+INCONCEIVABLE NUMBERS AND INCONCEIVABLY SMALL PARTICLES 250
+
+WHAT IS A MILLION? 250
+
+THE BROWNIAN MOVEMENT 251
+
+A SOAP BUBBLE (_Coloured Illustration_) 252
+ Reproduced from _The Forces of Nature_ (Messrs. Macmillan).
+
+DETECTING A SMALL QUANTITY OF MATTER 254
+ From _Scientific Ideas of To-day_.
+
+THIS X-RAY PHOTOGRAPH IS THAT OF A HAND OF A SOLDIER
+ WOUNDED IN THE GREAT WAR 254
+ Reproduced by permission of X-Rays Ltd.
+
+AN X-RAY PHOTOGRAPH OF A GOLF BALL, REVEALING AN IMPERFECT
+ CORE 254
+ Photo: National Physical Laboratory.
+
+A WONDERFUL X-RAY PHOTOGRAPH 255
+ Reproduced by permission of X-Rays Ltd.
+
+ELECTRIC DISCHARGE IN A VACUUM TUBE 258
+
+THE RELATIVE SIZES OF ATOMS AND ELECTRONS 258
+
+ELECTRONS STREAMING FROM THE SUN TO THE EARTH 259
+
+PROFESSOR SIR J. J. THOMSON 262
+
+ELECTRONS PRODUCED BY PASSAGE OF X-RAYS THROUGH AIR 262
+ From the Smithsonian Report, 1915.
+
+MAGNETIC DEFLECTION OF RADIUM RAYS 263
+
+PROFESSOR R. A. MILLIKAN'S APPARATUS FOR COUNTING ELECTRONS 263
+ Reproduced by permission of _Scientific American_.
+
+MAKING THE INVISIBLE VISIBLE 266
+
+THE THEORY OF ELECTRONS 267
+
+ARRANGEMENTS OF ATOMS IN A DIAMOND 267
+
+DISINTEGRATION OF ATOMS 270
+
+SILK TASSEL ELECTRIFIED 270
+ Reproduced by permission from _The Interpretation of Radium_
+ (John Murray).
+
+SILK TASSEL DISCHARGED BY THE RAYS FROM RADIUM 270
+
+A HUGE ELECTRIC SPARK 271
+
+ELECTRICAL ATTRACTION BETWEEN COMMON OBJECTS 271
+ From _Scientific Ideas of To-day_.
+
+AN ELECTRIC SPARK 274
+ Photo: Leadbeater.
+
+AN ETHER DISTURBANCE AROUND AN ELECTRON CURRENT 275
+ From _Scientific Ideas of To-day_.
+
+LIGHTNING 278
+ Photo: H. J. Shepstone.
+
+LIGHT WAVES 279
+
+THE MAGNETIC CIRCUIT OF AN ELECTRIC CURRENT 279
+
+THE MAGNET 279
+
+ROTATING DISC OF SIR ISAAC NEWTON FOR MIXING COLOURS
+ (_Coloured Illustration_) 280
+
+WAVE SHAPES 282
+
+THE POWER OF A MAGNET 282
+
+THE SPEED OF LIGHT 283
+ Photo: The Locomotive Publishing Co., Ltd.
+
+ROTATING DISC OF SIR ISAAC NEWTON FOR MIXING COLOURS 283
+
+NIAGARA FALLS 286
+
+TRANSFORMATION OF ENERGY 287
+ Photo: Stephen Cribb.
+
+"BOILING" A KETTLE ON ICE 287
+ Photo: Underwood & Underwood.
+
+THE CAUSE OF TIDES 290
+
+THE AEGIR ON THE TRENT 290
+ Photo: G. Brocklehurst.
+
+A BIG SPRING TIDE, THE AEGIR ON THE TRENT 291
+ Photo: G. Brocklehurst.
+
+
+
+
+The Outline of Science
+
+
+
+
+INTRODUCTION
+
+
+There is abundant evidence of a widened and deepened interest in modern
+science. How could it be otherwise when we think of the magnitude and
+the eventfulness of recent advances?
+
+But the interest of the general public would be even greater than it is
+if the makers of new knowledge were more willing to expound their
+discoveries in ways that could be "understanded of the people." No one
+objects very much to technicalities in a game or on board a yacht, and
+they are clearly necessary for terse and precise scientific description.
+It is certain, however, that they can be reduced to a minimum without
+sacrificing accuracy, when the object in view is to explain "the gist of
+the matter." So this OUTLINE OF SCIENCE is meant for the general reader,
+who lacks both time and opportunity for special study, and yet would
+take an intelligent interest in the progress of science which is making
+the world always new.
+
+The story of the triumphs of modern science is one of which Man may well
+be proud. Science reads the secret of the distant star and anatomises
+the atom; foretells the date of the comet's return and predicts the
+kinds of chickens that will hatch from a dozen eggs; discovers the laws
+of the wind that bloweth where it listeth and reduces to order the
+disorder of disease. Science is always setting forth on Columbus
+voyages, discovering new worlds and conquering them by understanding.
+For Knowledge means Foresight and Foresight means Power.
+
+The idea of Evolution has influenced all the sciences, forcing us to
+think of _everything_ as with a history behind it, for we have travelled
+far since Darwin's day. The solar system, the earth, the mountain
+ranges, and the great deeps, the rocks and crystals, the plants and
+animals, man himself and his social institutions--all must be seen as
+the outcome of a long process of Becoming. There are some eighty-odd
+chemical elements on the earth to-day, and it is now much more than a
+suggestion that these are the outcome of an inorganic evolution, element
+giving rise to element, going back and back to some primeval stuff, from
+which they were all originally derived, infinitely long ago. No idea has
+been so powerful a tool in the fashioning of New Knowledge as this
+simple but profound idea of Evolution, that the present is the child of
+the past and the parent of the future. And with the picture of a
+continuity of evolution from nebula to social systems comes a promise of
+an increasing control--a promise that Man will become not only a more
+accurate student, but a more complete master of his world.
+
+It is characteristic of modern science that the whole world is seen to
+be more vital than before. Everywhere there has been a passage from the
+static to the dynamic. Thus the new revelations of the constitution of
+matter, which we owe to the discoveries of men like Professor Sir J. J.
+Thomson, Professor Sir Ernest Rutherford, and Professor Frederick Soddy,
+have shown the very dust to have a complexity and an activity heretofore
+unimagined. Such phrases as "dead" matter and "inert" matter have gone
+by the board.
+
+The new theory of the atom amounts almost to a new conception of the
+universe. It bids fair to reveal to us many of nature's hidden secrets.
+The atom is no longer the indivisible particle of matter it was once
+understood to be. We know now that there is an atom within the
+atom--that what we thought was elementary can be dissociated and broken
+up. The present-day theories of the atom and the constitution of matter
+are the outcome of the comparatively recent discovery of such things as
+radium, the X-rays, and the wonderful revelations of such instruments as
+the spectroscope and other highly perfected scientific instruments.
+
+The advent of the electron theory has thrown a flood of light on what
+before was hidden or only dimly guessed at. It has given us a new
+conception of the framework of the universe. We are beginning to know
+and realise of what matter is made and what electric phenomena mean. We
+can glimpse the vast stores of energy locked up in matter. The new
+knowledge has much to tell us about the origin and phenomena, not only
+of our own planet, but other planets, of the stars, and the sun. New
+light is thrown on the source of the sun's heat; we can make more than
+guesses as to its probable age. The great question to-day is: is there
+_one_ primordial substance from which all the varying forms of matter
+have been evolved?
+
+But the discovery of electrons is only one of the revolutionary changes
+which give modern science an entrancing interest.
+
+As in chemistry and physics, so in the science of living creatures there
+have been recent advances that have changed the whole prospect. A good
+instance is afforded by the discovery of the "hormones," or chemical
+messengers, which are produced by ductless glands, such as the thyroid,
+the supra-renal, and the pituitary, and are distributed throughout the
+body by the blood. The work of physiologists like Professor Starling and
+Professor Bayliss has shown that these chemical messengers regulate what
+may be called the "pace" of the body, and bring about that regulated
+harmony and smoothness of working which we know as health. It is not too
+much to say that the discovery of hormones has changed the whole of
+physiology. Our knowledge of the human body far surpasses that of the
+past generation.
+
+The persistent patience of microscopists and technical improvements like
+the "ultramicroscope" have greatly increased our knowledge of the
+invisible world of life. To the bacteria of a past generation have been
+added a multitude of microscopic _animal_ microbes, such as that which
+causes Sleeping Sickness. The life-histories and the weird ways of many
+important parasites have been unravelled; and here again knowledge means
+mastery. To a degree which has almost surpassed expectations there has
+been a revelation of the intricacy of the stones and mortar of the house
+of life, and the microscopic study of germ-cells has wonderfully
+supplemented the epoch-making experimental study of heredity which began
+with Mendel. It goes without saying that no one can call himself
+educated who does not understand the central and simple ideas of
+Mendelism and other new departures in biology.
+
+The procession of life through the ages and the factors in the sublime
+movement; the peopling of the earth by plants and animals and the
+linking of life to life in subtle inter-relations, such as those between
+flowers and their insect-visitors; the life-histories of individual
+types and the extraordinary results of the new inquiry called
+"experimental embryology"--these also are among the subjects with which
+this OUTLINE will deal.
+
+The behaviour of animals is another fascinating study, leading to a
+provisional picture of the dawn of mind. Indeed, no branch of science
+surpasses in interest that which deals with the ways and habits--the
+truly wonderful devices, adaptations, and instincts--of insects, birds,
+and mammals. We no longer deny a degree of intelligence to some members
+of the animal world--even the line between intelligence and reason is
+sometimes difficult to find.
+
+Fresh contacts between physiology and the study of man's mental life;
+precise studies of the ways of children and wild peoples; and new
+methods like those of the psycho-analyst must also receive the attention
+they deserve, for they are giving us a "New Psychology" and the claims
+of psychical research must also be recognised by the open-minded.
+
+The general aim of the OUTLINE is to give the reader a clear and concise
+view of the essentials of present-day science, so that he may follow
+with intelligence the modern advance and share appreciatively in man's
+continued conquest of his kingdom.
+
+J. ARTHUR THOMSON.
+
+
+
+
+I
+
+THE ROMANCE OF THE HEAVENS
+
+
+
+
+THE SCALE OF THE UNIVERSE--THE SOLAR SYSTEM
+
+
+§ 1
+
+The story of the triumphs of modern science naturally opens with
+Astronomy. The picture of the Universe which the astronomer offers to us
+is imperfect; the lines he traces are often faint and uncertain. There
+are many problems which have been solved, there are just as many about
+which there is doubt, and notwithstanding our great increase in
+knowledge, there remain just as many which are entirely unsolved.
+
+ The problem of the structure and duration of the universe [said the
+ great astronomer Simon Newcomb] is the most far-reaching with which
+ the mind has to deal. Its solution may be regarded as the ultimate
+ object of stellar astronomy, the possibility of reaching which has
+ occupied the minds of thinkers since the beginning of civilisation.
+ Before our time the problem could be considered only from the
+ imaginative or the speculative point of view. Although we can to-day
+ attack it to a limited extent by scientific methods, it must be
+ admitted that we have scarcely taken more than the first step toward
+ the actual solution.... What is the duration of the universe in
+ time? Is it fitted to last for ever in its present form, or does it
+ contain within itself the seeds of dissolution? Must it, in the
+ course of time, in we know not how many millions of ages, be
+ transformed into something very different from what it now is? This
+ question is intimately associated with the question whether the
+ stars form a system. If they do, we may suppose that system to be
+ permanent in its general features; if not, we must look further for
+ our conclusions.
+
+
+The Heavenly Bodies
+
+The heavenly bodies fall into two very distinct classes so far as their
+relation to our Earth is concerned; the one class, a very small one,
+comprises a sort of colony of which the Earth is a member. These bodies
+are called _planets_, or wanderers. There are eight of them, including
+the Earth, and they all circle round the sun. Their names, in the order
+of their distance from the sun, are Mercury, Venus, Earth, Mars,
+Jupiter, Saturn, Uranus, Neptune, and of these Mercury, the nearest to
+the sun, is rarely seen by the naked eye. Uranus is practically
+invisible, and Neptune quite so. These eight planets, together with the
+sun, constitute, as we have said, a sort of little colony; this colony
+is called the Solar System.
+
+The second class of heavenly bodies are those which lie _outside_ the
+solar system. Every one of those glittering points we see on a starlit
+night is at an immensely greater distance from us than is any member of
+the Solar System. Yet the members of this little colony of ours, judged
+by terrestrial standards, are at enormous distances from one another. If
+a shell were shot in a straight line from one side of Neptune's orbit to
+the other it would take five hundred years to complete its journey. Yet
+this distance, the greatest in the Solar System as now known (excepting
+the far swing of some of the comets), is insignificant compared to the
+distances of the stars. One of the nearest stars to the earth that we
+know of is Alpha Centauri, estimated to be some twenty-five million
+millions of miles away. Sirius, the brightest star in the firmament, is
+double this distance from the earth.
+
+We must imagine the colony of planets to which we belong as a compact
+little family swimming in an immense void. At distances which would take
+our shell, not hundreds, but millions of years to traverse, we reach
+the stars--or rather, a star, for the distances between stars are as
+great as the distance between the nearest of them and our Sun. The
+Earth, the planet on which we live, is a mighty globe bounded by a crust
+of rock many miles in thickness; the great volumes of water which we
+call our oceans lie in the deeper hollows of the crust. Above the
+surface an ocean of invisible gas, the atmosphere, rises to a height of
+about three hundred miles, getting thinner and thinner as it ascends.
+
+[Illustration: LAPLACE
+
+One of the greatest mathematical astronomers of all time and the
+originator of the nebular theory.]
+
+[Illustration: _Photo: Royal Astronomical Society._
+
+PROFESSOR J. C. ADAMS
+
+who, anticipating the great French mathematician, Le Verrier, discovered
+the planet Neptune by calculations based on the irregularities of the
+orbit of Uranus. One of the most dramatic discoveries in the history of
+Science.]
+
+[Illustration: _Photo: Elliott & Fry, Ltd._
+
+PROFESSOR EDDINGTON
+
+Professor of Astronomy at Cambridge. The most famous of the English
+disciples of Einstein.]
+
+[Illustration: FIG. 1.--DIAGRAMS OF THE SOLAR SYSTEM
+
+THE COMPARATIVE DISTANCES OF THE PLANETS
+
+(Drawn approximately to scale)
+
+The isolation of the Solar System is very great. On the above scale the
+_nearest_ star (at a distance of 25 trillions of miles) would be over
+_one half mile_ away. The hours, days, and years are the measures of
+time as we use them; that is: Jupiter's "Day" (one rotation of the
+planet) is made in ten of _our hours_; Mercury's "Year" (one revolution
+of the planet around the Sun) is eighty-eight of _our days_. Mercury's
+"Day" and "Year" are the same. This planet turns always the same side to
+the Sun.]
+
+[Illustration: THE COMPARATIVE SIZES OF THE SUN AND THE PLANETS (Drawn
+approximately to scale)
+
+On this scale the Sun would be 17-1/2 inches in diameter; it is far
+greater than all the planets put together. Jupiter, in turn, is greater
+than all the other planets put together.]
+
+Except when the winds rise to a high speed, we seem to live in a very
+tranquil world. At night, when the glare of the sun passes out of our
+atmosphere, the stars and planets seem to move across the heavens with a
+stately and solemn slowness. It was one of the first discoveries of
+modern astronomy that this movement is only apparent. The apparent
+creeping of the stars across the heavens at night is accounted for by
+the fact that the earth turns upon its axis once in every twenty-four
+hours. When we remember the size of the earth we see that this implies a
+prodigious speed.
+
+In addition to this the earth revolves round the sun at a speed of more
+than a thousand miles a minute. Its path round the sun, year in year
+out, measures about 580,000,000 miles. The earth is held closely to this
+path by the gravitational pull of the sun, which has a mass 333,432
+times that of the earth. If at any moment the sun ceased to exert this
+pull the earth would instantly fly off into space straight in the
+direction in which it was moving at the time, that is to say, at a
+tangent. This tendency to fly off at a tangent is continuous. It is the
+balance between it and the sun's pull which keeps the earth to her
+almost circular orbit. In the same way the seven other planets are held
+to their orbits.
+
+Circling round the earth, in the same way as the earth circles round the
+sun, is our moon. Sometimes the moon passes directly between us and the
+sun, and cuts off the light from us. We then have a total or partial
+eclipse of the sun. At other times the earth passes directly between the
+sun and the moon, and causes an eclipse of the moon. The great ball of
+the earth naturally trails a mighty shadow across space, and the moon is
+"eclipsed" when it passes into this.
+
+The other seven planets, five of which have moons of their own, circle
+round the sun as the earth does. The sun's mass is immensely larger than
+that of all the planets put together, and all of them would be drawn
+into it and perish if they did not travel rapidly round it in gigantic
+orbits. So the eight planets, spinning round on their axes, follow their
+fixed paths round the sun. The planets are secondary bodies, but they
+are most important, because they are the only globes in which there can
+be life, as we know life.
+
+If we could be transported in some magical way to an immense distance in
+space above the sun, we should see our Solar System as it is drawn in
+the accompanying diagram (Fig. 1), except that the planets would be mere
+specks, faintly visible in the light which they receive from the sun.
+(This diagram is drawn approximately to scale.) If we moved still
+farther away, trillions of miles away, the planets would fade entirely
+out of view, and the sun would shrink into a point of fire, a star. And
+here you begin to realize the nature of the universe. _The sun is a
+star. The stars are suns._ Our sun looks big simply because of its
+comparative nearness to us. The universe is a stupendous collection of
+millions of stars or suns, many of which may have planetary families
+like ours.
+
+
+§ 2
+
+The Scale of the Universe
+
+How many stars are there? A glance at a photograph of star-clouds will
+tell at once that it is quite impossible to count them. The fine
+photograph reproduced in Figure 2 represents a very small patch of that
+pale-white belt, the Milky Way, which spans the sky at night. It is true
+that this is a particularly rich area of the Milky Way, but the entire
+belt of light has been resolved in this way into masses or clouds of
+stars. Astronomers have counted the stars in typical districts here and
+there, and from these partial counts we get some idea of the total
+number of stars. There are estimated to be between two and three
+thousand million stars.
+
+Yet these stars are separated by inconceivable distances from each
+other, and it is one of the greatest triumphs of modern astronomy to
+have mastered, so far, the scale of the universe. For several centuries
+astronomers have known the relative distances from each other of the sun
+and the planets. If they could discover the actual distance of any one
+planet from any other, they could at once tell all the distances within
+the Solar System.
+
+The sun is, on the latest measurements, at an average distance of
+92,830,000 miles from the earth, for as the orbit of the earth is not a
+true circle, this distance varies. This means that in six months from
+now the earth will be right at the opposite side of its path round the
+sun, or 185,000,000 miles away from where it is now. Viewed or
+photographed from two positions so wide apart, the nearest stars show a
+tiny "shift" against the background of the most distant stars, and that
+is enough for the mathematician. He can calculate the distance of any
+star near enough to show this "shift." We have found that the nearest
+star to the earth, a recently discovered star, is twenty-five trillion
+miles away. Only thirty stars are known to be within a hundred trillion
+miles of us.
+
+This way of measuring does not, however, take us very far away in the
+heavens. There are only a few hundred stars within five hundred trillion
+miles of the earth, and at that distance the "shift" of a star against
+the background (parallax, the astronomer calls it) is so minute that
+figures are very uncertain. At this point the astronomer takes up a new
+method. He learns the different types of stars, and then he is able to
+deduce more or less accurately the distance of a star of a known type
+from its faintness. He, of course, has instruments for gauging their
+light. As a result of twenty years work in this field, it is now known
+that the more distant stars of the Milky Way are at least a hundred
+thousand trillion (100,000,000,000,000,000) miles away from the sun.
+
+Our sun is in a more or less central region of the universe, or a few
+hundred trillion miles from the actual centre. The remainder of the
+stars, which are all outside our Solar System, are spread out,
+apparently, in an enormous disc-like collection, so vast that even a ray
+of light, which travels at the rate of 186,000 miles a second, would
+take 50,000 years to travel from one end of it to the other. This, then
+is what we call our universe.
+
+
+Are there other Universes?
+
+Why do we say "our universe"? Why not _the_ universe? It is now believed
+by many of our most distinguished astronomers that our colossal family
+of stars is only one of many universes. By a universe an astronomer
+means any collection of stars which are close enough to control each
+other's movements by gravitation; and it is clear that there might be
+many universes, in this sense, separated from each other by profound
+abysses of space. Probably there are.
+
+For a long time we have been familiar with certain strange objects in
+the heavens which are called "spiral nebulæ" (Fig 4). We shall see at a
+later stage what a nebula is, and we shall see that some astronomers
+regard these spiral nebulæ as worlds "in the making." But some of the
+most eminent astronomers believe that they are separate
+universes--"island-universes" they call them--or great collections of
+millions of stars like our universe. There are certain peculiarities in
+the structure of the Milky Way which lead these astronomers to think
+that our universe may be a spiral nebula, and that the other spiral
+nebulæ are "other universes."
+
+[Illustration: _Photo: Harvard College Observatory._
+
+FIG. 2.--THE MILKY WAY
+
+Note the cloud-like effect.]
+
+[Illustration: FIG. 3--THE MOON ENTERING THE SHADOW CAST BY THE EARTH
+
+The diagram shows the Moon partially eclipsed.]
+
+[Illustration: _From a photograph taken at the Yerkes Observatory_
+
+FIG. 4.--THE GREAT NEBULA IN ANDROMEDA, MESSIER 31]
+
+Vast as is the Solar System, then, it is excessively minute in
+comparison with the Stellar System, the universe of the Stars, which is
+on a scale far transcending anything the human mind can apprehend.
+
+
+THE SOLAR SYSTEM
+
+THE SUN
+
+
+§ 1
+
+But now let us turn to the Solar System, and consider the members of our
+own little colony.
+
+Within the Solar System there are a large number of problems that
+interest us. What is the size, mass, and distance of each of the
+planets? What satellites, like our Moon, do they possess? What are their
+temperatures? And those other, sporadic members of our system, comets
+and meteors, what are they? What are their movements? How do they
+originate? And the Sun itself, what is its composition, what is the
+source of its heat, how did it originate? Is it running down?
+
+These last questions introduce us to a branch of astronomy which is
+concerned with the physical constitution of the stars, a study which,
+not so very many years ago, may well have appeared inconceivable. But
+the spectroscope enables us to answer even these questions, and the
+answer opens up questions of yet greater interest. We find that the
+stars can be arranged in an order of development--that there are stars
+at all stages of their life-history. The main lines of the evolution of
+the stellar universe can be worked out. In the sun and stars we have
+furnaces with temperatures enormously high; it is in such conditions
+that substances are resolved into their simplest forms, and it is thus
+we are enabled to obtain a knowledge of the most primitive forms of
+matter. It is in this direction that the spectroscope (which we shall
+refer to immediately) has helped us so much. It is to this wonderful
+instrument that we owe our knowledge of the composition of the sun and
+stars, as we shall see.
+
+ "That the spectroscope will detect the millionth of a milligram of
+ matter, and on that account has discovered new elements, commands
+ our admiration; but when we find in addition that it will detect the
+ nature of forms of matter trillions of miles away, and moreover,
+ that it will measure the velocities with which these forms of matter
+ are moving with an absurdly small per cent. of possible error, we
+ can easily acquiesce in the statement that it is the greatest
+ instrument ever devised by the brain and hand of man."
+
+Such are some of the questions with which modern astronomy deals. To
+answer them requires the employment of instruments of almost incredible
+refinement and exactitude and also the full resources of mathematical
+genius. Whether astronomy be judged from the point of view of the
+phenomena studied, the vast masses, the immense distances, the æons of
+time, or whether it be judged as a monument of human ingenuity,
+patience, and the rarest type of genius, it is certainly one of the
+grandest, as it is also one of the oldest, of the sciences.
+
+
+The Solar System
+
+In the Solar System we include all those bodies dependent on the sun
+which circulate round it at various distances, deriving their light and
+heat from the sun--the planets and their moons, certain comets and a
+multitude of meteors: in other words, all bodies whose movements in
+space are determined by the gravitational pull of the sun.
+
+
+The Sun
+
+Thanks to our wonderful modern instruments and the ingenious methods
+used by astronomers, we have to-day a remarkable knowledge of the sun.
+
+Look at the figure of the sun in the frontispiece. The picture
+represents an eclipse of the sun; the dark body of the moon has screened
+the sun's shining disc and taken the glare out of our eyes; we see a
+silvery halo surrounding the great orb on every side. It is the sun's
+atmosphere, or "crown" (corona), stretching for millions of miles into
+space in the form of a soft silvery-looking light; probably much of its
+light is sunlight reflected from particles of dust, although the
+spectroscope shows an element in the corona that has not so far been
+detected anywhere else in the universe and which in consequence has been
+named Coronium.
+
+We next notice in the illustration that at the base of the halo there
+are red flames peeping out from the edges of the hidden disc. When one
+remembers that the sun is 866,000 miles in diameter, one hardly needs to
+be told that these flames are really gigantic. We shall see what they
+are presently.
+
+
+Regions of the Sun
+
+The astronomer has divided the sun into definite concentric regions or
+layers. These layers envelop the nucleus or central body of the sun
+somewhat as the atmosphere envelops our earth. It is through these
+vapour layers that the bright white body of the sun is seen. Of the
+innermost region, the heart or nucleus of the sun, we know almost
+nothing. The central body or nucleus is surrounded by a brilliantly
+luminous envelope or layer of vaporous matter which is what we see when
+we look at the sun and which the astronomer calls the photosphere.
+
+Above--that is, overlying--the photosphere there is a second layer of
+glowing gases, which is known as the reversing layer. This layer is
+cooler than the underlying photosphere; it forms a veil of smoke-like
+haze and is of from 500 to 1,000 miles in thickness.
+
+A third layer or envelope immediately lying over the last one is the
+region known as the chromosphere. The chromosphere extends from 5,000
+to 10,000 miles in thickness--a "sea" of red tumultuous surging fire.
+Chief among the glowing gases is the vapour of hydrogen. The intense
+white heat of the photosphere beneath shines through this layer,
+overpowering its brilliant redness. From the uppermost portion of the
+chromosphere great fiery tongues of glowing hydrogen and calcium vapour
+shoot out for many thousands of miles, driven outward by some prodigious
+expulsive force. It is these red "prominences" which are such a notable
+feature in the picture of the eclipse of the sun already referred to.
+
+During the solar eclipse of 1919 one of these red flames rose in less
+than seven hours from a height of 130,000 miles to more than 500,000
+miles above the sun's surface. This immense column of red-hot gas, four
+or five times the thickness of the earth, was soaring upward at the rate
+of 60,000 miles an hour.
+
+These flaming jets or prominences shooting out from the chromosphere are
+not to be seen every day by the naked eye; the dazzling light of the sun
+obscures them, gigantic as they are. They can be observed, however, by
+the spectroscope any day, and they are visible to us for a very short
+time during an eclipse of the sun. Some extraordinary outbursts have
+been witnessed. Thus the late Professor Young described one on September
+7, 1871, when he had been examining a prominence by the spectroscope:
+
+ It had remained unchanged since noon of the previous day--a long,
+ low, quiet-looking cloud, not very dense, or brilliant, or in any
+ way remarkable except for its size. At 12:30 p.m. the Professor left
+ the spectroscope for a short time, and on returning half an hour
+ later to his observations, he was astonished to find the gigantic
+ Sun flame shattered to pieces. The solar atmosphere was filled with
+ flying debris, and some of these portions reached a height of
+ 100,000 miles above the solar surface. Moving with a velocity which,
+ even at the distance of 93,000,000 miles, was almost perceptible to
+ the eye, these fragments doubled their height in ten minutes. On
+ January 30, 1885, another distinguished solar observer, the late
+ Professor Tacchini of Rome, observed one of the greatest prominences
+ ever seen by man. Its height was no less than 142,000
+ miles--eighteen times the diameter of the earth. Another mighty
+ flame was so vast that supposing the eight large planets of the
+ solar system ranged one on top of the other, the prominence would
+ still tower above them.[1]
+
+ [1] _The Romance of Astronomy_, by H. Macpherson.
+
+[Illustration: FIG. 5.--DIAGRAM SHOWING THE MAIN LAYERS OF THE SUN
+
+Compare with frontispiece.]
+
+[Illustration: _Photo: Royal Observatory, Greenwich._
+
+FIG. 6.--SOLAR PROMINENCES SEEN AT TOTAL SOLAR ECLIPSE, May 29, 1919.
+TAKEN AT SOBRAL, BRAZIL.
+
+The small Corona is also visible.]
+
+[Illustration: FIG. 7.--THE VISIBLE SURFACE OF THE SUN
+
+A photograph taken at the Mount Wilson Observatory of the Carnegie
+Institution at Washington.]
+
+[Illustration: FIG. 8.--THE SUN
+
+Photographed in the light of glowing hydrogen, at the Mount Wilson
+Observatory of the Carnegie Institution of Washington: vortex phenomena
+near the spots are especially prominent.]
+
+The fourth and uppermost layer or region is that of the corona, of
+immense extent and fading away into the surrounding sky--this we have
+already referred to. The diagram (Fig. 5) shows the dispositions of
+these various layers of the sun. It is through these several transparent
+layers that we see the white light body of the sun.
+
+
+§ 2
+
+The Surface of the Sun
+
+Here let us return to and see what more we know about the
+photosphere--the sun's surface. It is from the photosphere that we have
+gained most of our knowledge of the composition of the sun, which is
+believed not to be a solid body. Examination of the photosphere shows
+that the outer surface is never at rest. Small bright cloudlets come and
+go in rapid succession, giving the surface, through contrasts in
+luminosity, a granular appearance. Of course, to be visible at all at
+92,830,000 miles the cloudlets cannot be small. They imply enormous
+activity in the photosphere. If we might speak picturesquely the sun's
+surface resembles a boiling ocean of white-hot metal vapours. We have
+to-day a wonderful instrument, which will be described later, which
+dilutes, as it were, the general glare of the sun, and enables us to
+observe these fiery eruptions at any hour. The "oceans" of red-hot gas
+and white-hot metal vapour at the sun's surface are constantly driven by
+great storms. Some unimaginable energy streams out from the body or
+muscles of the sun and blows its outer layers into gigantic shreds, as
+it were.
+
+The actual temperature at the sun's surface, or what appears to us to be
+the surface--the photosphere--is, of course, unknown, but careful
+calculation suggests that it is from 5,000° C. to 7,000° C. The interior
+is vastly hotter. We can form no conception of such temperatures as must
+exist there. Not even the most obdurate solid could resist such
+temperatures, but would be converted almost instantaneously into gas.
+But it would not be gas as we know gases on the earth. The enormous
+pressures that exist on the sun must convert even gases into thick
+treacly fluids. We can only infer this state of matter. It is beyond our
+power to reproduce it.
+
+
+Sun-spots
+
+It is in the brilliant photosphere that the dark areas known as
+sun-spots appear. Some of these dark spots--they are dark only by
+contrast with the photosphere surrounding them--are of enormous size,
+covering many thousands of square miles of surface. What they are we
+cannot positively say. They look like great cavities in the sun's
+surface. Some think they are giant whirlpools. Certainly they seem to be
+great whirling streams of glowing gases with vapours above them and
+immense upward and downward currents within them. Round the edges of the
+sun-spots rise great tongues of flame.
+
+Perhaps the most popularly known fact about sun-spots is that they are
+somehow connected with what we call magnetic storms on earth. These
+magnetic storms manifest themselves in interruptions of our telegraphic
+and telephonic communications, in violent disturbances of the mariner's
+compass, and in exceptional auroral displays. The connection between the
+two sets of phenomena cannot be doubted, even although at times there
+may be a great spot on the sun without any corresponding "magnetic
+storm" effects on the earth.
+
+A surprising fact about sun-spots is that they show definite periodic
+variations in number. The best-defined period is one of about eleven
+years. During this period the spots increase to a maximum in number and
+then diminish to a minimum, the variation being more or less regular.
+Now this can only mean one thing. To be periodic the spots must have
+some deep-seated connection with the fundamental facts of the sun's
+structure and activities. Looked at from this point of view their
+importance becomes great.
+
+[Illustration: _Reproduction from "The Forces of Nature"_ (_Messrs.
+Macmillan_)
+
+THE AURORA BOREALIS
+
+The aurora borealis is one of the most beautiful spectacles in the sky.
+The colours and shape change every instant; sometimes a fan-like cluster
+of rays, at other times long golden draperies gliding one over the
+other. Blue, green, yellow, red, and white combine to give a glorious
+display of colour. The theory of its origin is still, in part, obscure,
+but there can be no doubt that the aurora is related to the magnetic
+phenomena of the earth and therefore is connected with the electrical
+influence of the sun.]
+
+It is from the study of sun-spots that we have learned that the sun's
+surface does not appear to rotate all at the same speed. The
+"equatorial" regions are rotating quicker than regions farther north or
+south. A point forty-five degrees from the equator seems to take about
+two and a half days longer to complete one rotation than a point on the
+equator. This, of course, confirms our belief that the sun cannot be a
+solid body.
+
+What is its composition? We know that there are present, in a gaseous
+state, such well-known elements as sodium, iron, copper, zinc, and
+magnesium; indeed, we know that there is practically every element in
+the sun that we know to be in the earth. How do we know?
+
+It is from the photosphere, as has been said, that we have won most of
+our knowledge of the sun. The instrument used for this purpose is the
+spectroscope; and before proceeding to deal further with the sun and the
+source of its energy it will be better to describe this instrument.
+
+
+A WONDERFUL INSTRUMENT AND WHAT IT REVEALS
+
+The spectroscope is an instrument for analysing light. So important is
+it in the revelations it has given us that it will be best to describe
+it fully. Every substance to be examined must first be made to glow,
+made luminous; and as nearly everything in the heavens _is_ luminous the
+instrument has a great range in Astronomy. And when we speak of
+analysing light, we mean that the light may be broken up into waves of
+different lengths. What we call light is a series of minute waves in
+ether, and these waves are--measuring them from crest to crest, so to
+say--of various lengths. Each wave-length corresponds to a colour of the
+rainbow. The shortest waves give us a sensation of violet colour, and
+the largest waves cause a sensation of red. The rainbow, in fact, is a
+sort of natural spectrum. (The meaning of the rainbow is that the
+moisture-laden air has sorted out these waves, in the sun's light,
+according to their length.) Now the simplest form of spectroscope is a
+glass prism--a triangular-shaped piece of glass. If white light
+(sunlight, for example) passes through a glass prism, we see a series of
+rainbow-tinted colours. Anyone can notice this effect when sunlight is
+shining through any kind of cut glass--the stopper of a wine decanter,
+for instance. If, instead of catching with the eye the coloured lights
+as they emerge from the glass prism, we allow them to fall on a screen,
+we shall find that they pass, by continuous gradations, from red at the
+one end of the screen, through orange, yellow, green, blue, and indigo,
+to violet at the other end. _In other words, what we call white light is
+composed of rays of these several colours. They go to make up the effect
+which we call white._ And now just as water can be split up into its two
+elements, oxygen and hydrogen, so sunlight can be broken up into its
+primary colours, which are those we have just mentioned.
+
+This range of colours, produced by the spectroscope, we call the solar
+spectrum, and these are, from the spectroscopic point of view, primary
+colours. Each shade of colour has its definite position in the spectrum.
+That is to say, the light of each shade of colour (corresponding to its
+wave-length) is reflected through a certain fixed angle on passing
+through the glass prism. Every possible kind of light has its definite
+position, and is denoted by a number which gives the wave-length of the
+vibrations constituting that particular kind of light.
+
+Now, other kinds of light besides sunlight can be analysed. Light
+from any substance which has been made incandescent may be observed with
+the spectroscope in the same way, and each element can be thus
+separated. It is found that each substance (in the same conditions of
+pressure, etc.) gives a constant spectrum of its own. _Each metal
+displays its own distinctive colour. It is obvious, therefore, that the
+spectrum provides the means for identifying a particular substance._ It
+was by this method that we discovered in the sun the presence of such
+well-known elements as sodium, iron, copper, zinc, and magnesium.
+
+[Illustration: _Yerkes Observatory._
+
+FIG. 9.--THE GREAT SUN-SPOT OF JULY 17, 1905]
+
+[Illustration: _From photographs taken at the Yerkes Observatory._
+
+FIG. 10.--SOLAR PROMINENCES
+
+These are about 60,000 miles in height. The two photographs show the
+vast changes occurring in ten minutes. October 10, 1910.]
+
+[Illustration: _Photo: Mount Wilson Observatory._
+
+FIG. 11.--MARS, October 5, 1909
+
+Showing the dark markings and the Polar Cap.]
+
+[Illustration: FIG. 12.--JUPITER
+
+Showing the belts which are probably cloud formations.]
+
+[Illustration: _Photo: Professor E. E. Barnard, Yerkes Observatory._
+
+FIG. 13.--SATURN, November 19, 1911
+
+Showing the rings, mighty swarms of meteorites.]
+
+Every chemical element known, then, has a distinctive spectrum of its
+own when it is raised to incandescence, and this distinctive spectrum is
+as reliable a means of identification for the element as a human face is
+for its owner. Whether it is a substance glowing in the laboratory or in
+a remote star makes no difference to the spectroscope; if the light of
+any substance reaches it, that substance will be recognised and
+identified by the characteristic set of waves.
+
+The spectrum of a glowing mass of gas will consist in a number of bright
+lines of various colours, and at various intervals; corresponding to
+each kind of gas, there will be a peculiar and distinctive arrangement
+of bright lines. But if the light from such a mass of glowing gas be
+made to pass through a cool mass of the _same_ gas it will be found that
+dark lines replace the bright lines in the spectrum, the reason for this
+being that the cool gas absorbs the rays of light emitted by the hot
+gas. Experiments of this kind enable us to reach the important general
+statement that every gas, when cold, absorbs the same rays of light
+which it emits when hot.
+
+Crossing the solar spectrum are hundreds and hundreds of dark lines.
+These could not at first be explained, because this fact of
+discriminative absorption was not known. We understand now. The sun's
+white light comes from the photosphere, but between us and the
+photosphere there is, as we have seen, another solar envelope of
+relatively cooler vapours--the reversing layer. Each constituent
+element in this outer envelope stops its own kind of light, that is, the
+kind of light made by incandescent atoms of the same element in the
+photosphere. The "stoppages" register themselves in the solar spectrum
+as dark lines placed exactly where the corresponding bright lines would
+have been. The explanation once attained, dark lines became as
+significant as bright lines. The secret of the sun's composition was
+out. We have found practically every element in the sun that we know to
+be in the earth. We have identified an element in the sun before we were
+able to isolate it on the earth. We have been able even to point to the
+coolest places on the sun, the centres of sun-spots, where alone the
+temperature seems to have fallen sufficiently low to allow chemical
+compounds to form.
+
+It is thus we have been able to determine what the stars, comets, or
+nebulæ are made of.
+
+
+A Unique Discovery
+
+In 1868 Sir Norman Lockyer detected a light coming from the prominences
+of the sun which was not given by any substance known on earth, and
+attributed this to an unknown gas which he called helium, from the Greek
+_helios_, the sun. _In 1895 Sir William Ramsay discovered in certain
+minerals the same gas identified by the spectroscope._ We can say,
+therefore, that this gas was discovered in the sun nearly thirty years
+before it was found on earth; this discovery of the long-lost heir is as
+thrilling a chapter in the detective story of science as any in the
+sensational stories of the day, and makes us feel quite certain that our
+methods really tell us of what elements sun and stars are built up. The
+light from the corona of the sun, as we have mentioned indicates a gas
+still unknown on earth, which has been christened Coronium.
+
+
+Measuring the Speed of Light
+
+But this is not all; soon a new use was found for the spectroscope. We
+found that we could measure with it the most difficult of all speeds
+to measure, speed in the line of sight. Movement at right angles to the
+direction in which one is looking is, if there is sufficient of it, easy
+to detect, and, if the distance of the moving body is known, easy to
+measure. But movement in the line of vision is both difficult to detect
+and difficult to measure. Yet, even at the enormous distances with which
+astronomers have to deal, the spectroscope can detect such movement and
+furnish data for its measurement. If a luminous body containing, say,
+sodium is moving rapidly towards the spectroscope, it will be found that
+the sodium lines in the spectrum have moved slightly from their usual
+definite positions towards the violet end of the spectrum, the amount of
+the change of position increasing with the speed of the luminous body.
+If the body is moving away from the spectroscope the shifting of the
+spectral lines will be in the opposite direction, towards the red end of
+the spectrum. In this way we have discovered and measured movements that
+otherwise would probably not have revealed themselves unmistakably to us
+for thousands of years. In the same way we have watched, and measured
+the speed of, tremendous movements on the sun, and so gained proof that
+the vast disturbances we should expect there actually do occur.
+
+[Illustration: THE SPECTROSCOPE IS AN INSTRUMENT FOR ANALYSING LIGHT; IT
+PROVIDES THE MEANS FOR IDENTIFYING DIFFERENT SUBSTANCES
+
+This pictorial diagram illustrates the principal of Spectrum Analysis,
+showing how sunlight is decomposed into its primary colours. What we
+call white light is composed of seven different colours. The diagram is
+relieved of all detail which would unduly obscure the simple process by
+which a ray of light is broken up by a prism into different
+wave-lengths. The spectrum rays have been greatly magnified.]
+
+
+IS THE SUN DYING?
+
+§ 3
+
+Now let us return to our consideration of the sun.
+
+To us on the earth the most patent and most astonishing fact about the
+sun is its tremendous energy. Heat and light in amazing quantities pour
+from it without ceasing.
+
+Where does this energy come from? Enormous jets of red glowing gases can
+be seen shooting outwards from the sun, like flames from a fire, for
+thousands of miles. Does this argue fire, as we know fire on the earth?
+On this point the scientist is sure. The sun is not burning, and
+combustion is not the source of its heat. Combustion is a chemical
+reaction between atoms. The conditions that make it possible are known
+and the results are predictable and measurable. But no chemical reaction
+of the nature of combustion as we know it will explain the sun's energy,
+nor indeed will any ordinary chemical reaction of any kind. If the sun
+were composed of combustible material throughout and the conditions of
+combustion as we understand them were always present, the sun would burn
+itself out in some thousands of years, with marked changes in its heat
+and light production as the process advanced. There is no evidence of
+such changes. There is, instead, strong evidence that the sun has been
+emitting light and heat in prodigious quantities, not for thousands, but
+for millions of years. Every addition to our knowledge that throws light
+on the sun's age seems to make for increase rather than decrease of its
+years. This makes the wonder of its energy greater.
+
+And we cannot avoid the issue of the source of the energy by saying
+merely that the sun is gradually radiating away an energy that
+originated in some unknown manner, away back at the beginning of things.
+Reliable calculations show that the years required for the mere cooling
+of a globe like the sun could not possibly run to millions. In other
+words, the sun's energy must be subject to continuous and more or less
+steady renewal. However it may have acquired its enormous energy in the
+past, it must have some source of energy in the present.
+
+The best explanation that we have to-day of this continuous accretion of
+energy is that it is due to shrinkage of the sun's bulk under the force
+of gravity. Gravity is one of the most mysterious forces of nature, but
+it is an obvious fact that bodies behave as if they attracted one
+another, and Newton worked out the law of this attraction. We may say,
+without trying to go too deeply into things, that every particle of
+matter attracts every other throughout the universe. If the diameter of
+the sun were to shrink by one mile all round, this would mean that all
+the millions of tons in the outer one-mile thickness would have a
+straight drop of one mile towards the centre. And that is not all,
+because obviously the layers below this outer mile would also drop
+inwards, each to a less degree than the one above it. What a tremendous
+movement of matter, however slowly it might take place! And what a
+tremendous energy would be involved! Astronomers calculate that the
+above shrinkage of one mile all round would require fifty years for its
+completion, assuming, reasonably, that there is close and continuous
+relationship between loss of heat by radiation and shrinkage. Even if
+this were true we need not feel over-anxious on this theory; before the
+sun became too cold to support life many millions of years would be
+required.
+
+It was suggested at one time that falls of meteoric matter into the sun
+would account for the sun's heat. This position is hardly tenable now.
+The mere bulk of the meteoric matter required by the hypothesis, apart
+from other reasons, is against it. There is undoubtedly an enormous
+amount of meteoric matter moving about within the bounds of the solar
+system, but most of it seems to be following definite routes round the
+sun like the planets. The stray erratic quantities destined to meet
+their doom by collision with the sun can hardly be sufficient to account
+for the sun's heat.
+
+Recent study of radio-active bodies has suggested another factor that
+may be working powerfully along with the force of gravitation to
+maintain the sun's store of heat. In radio-active bodies certain atoms
+seem to be undergoing disintegration. These atoms appear to be splitting
+up into very minute and primitive constituents. But since matter may be
+split up into such constituents, may it not be built up from them?
+
+The question is whether these "radio-active" elements are undergoing
+disintegration, or formation, in the sun. If they are undergoing
+disintegration--and the sun itself is undoubtedly radio-active--then we
+have another source of heat for the sun that will last indefinitely.
+
+
+
+
+THE PLANETS
+
+LIFE IN OTHER WORLDS?
+
+§ 1
+
+It is quite clear that there cannot be life on the stars. Nothing solid
+or even liquid can exist in such furnaces as they are. Life exists only
+on planets, and even on these its possibilities are limited. Whether all
+the stars, or how many of them, have planetary families like our sun, we
+cannot positively say. If they have, such planets would be too faint and
+small to be visible tens of trillions of miles away. Some astronomers
+think that our sun may be exceptional in having planets, but their
+reasons are speculative and unconvincing. Probably a large proportion at
+least of the stars have planets, and we may therefore survey the globes
+of our own solar system and in a general way extend the results to the
+rest of the universe.
+
+In considering the possibility of life as we know it we may at once rule
+out the most distant planets from the sun, Uranus and Neptune. They are
+probably intrinsically too hot. We may also pass over the nearest planet
+to the sun, Mercury. We have reason to believe that it turns on its axis
+in the same period as it revolves round the sun, and it must therefore
+always present the same side to the sun. This means that the heat on the
+sunlit side of Mercury is above boiling-point, while the cold on the
+other side must be between two and three hundred degrees below
+freezing-point.
+
+
+The Planet Venus
+
+The planet Venus, the bright globe which is known to all as the morning
+and evening "star," seems at first sight more promising as regards the
+possibility of life. It is of nearly the same size as the earth, and it
+has a good atmosphere, but there are many astronomers who believe that,
+like Mercury, it always presents the same face to the sun, and it would
+therefore have the same disadvantage--a broiling heat on the sunny side
+and the cold of space on the opposite side. We are not sure. The
+surface of Venus is so bright--the light of the sun is reflected to us
+by such dense masses of cloud and dust--that it is difficult to trace
+any permanent markings on it, and thus ascertain how long it takes to
+rotate on its axis. Many astronomers believe that they have succeeded,
+and that the planet always turns the same face to the sun. If it does,
+we can hardly conceive of life on its surface, in spite of the
+cloud-screen.
+
+[Illustration: FIG. 14.--THE MOON
+
+Showing a great plain and some typical craters. There are thousands of
+these craters, and some theories of their origin are explained on page
+34.]
+
+[Illustration: FIG. 15.--MARS
+
+ 1} Drawings by Prof. Lowell to accompany actual photographs of Mars
+ showing many of the
+ 2} canals. Taken in 1907 by Mr. E. C. Slipher of the Lowell Observatory.
+ 3 Drawing by Prof. Lowell made January 6, 1914.
+ 4 Drawing by Prof. Lowell made January 21, 1914.
+
+Nos. 1 and 2 show the effect of the planet's rotation. Nos. 3 and 4
+depict quite different sections. Note the change in the polar snow-caps
+in the last two.]
+
+[Illustration: FIG. 16.--THE MOON, AT NINE AND THREE-QUARTER DAYS
+
+Note the mysterious "rays" diverging from the almost perfectly circular
+craters indicated by the arrows (Tycho, upper; Copernicus, lower), and
+also the mountains to the right with the lunar dawn breaking on them.]
+
+We turn to Mars; and we must first make it clear why there is so much
+speculation about life on Mars, and why it is supposed that, if there
+_is_ life on Mars, it must be more advanced than life on the earth.
+
+
+Is there Life on Mars?
+
+The basis of this belief is that if, as we saw, all the globes in our
+solar system are masses of metal that are cooling down, the smaller will
+have cooled down before the larger, and will be further ahead in their
+development. Now Mars is very much smaller than the earth, and must have
+cooled at its surface millions of years before the earth did. Hence, if
+a story of life began on Mars at all, it began long before the story of
+life on the earth. We cannot guess what sort of life-forms would be
+evolved in a different world, but we can confidently say that they would
+tend toward increasing intelligence; and thus we are disposed to look
+for highly intelligent beings on Mars.
+
+But this argument supposes that the conditions of life, namely air and
+water, are found on Mars, and it is disputed whether they are found
+there in sufficient quantity. The late Professor Percival Lowell, who
+made a lifelong study of Mars, maintained that there are hundreds of
+straight lines drawn across the surface of the planet, and he claimed
+that they are beds of vegetation marking the sites of great channels or
+pipes by means of which the "Martians" draw water from their polar
+ocean. Professor W. H. Pickering, another high authority, thinks that
+the lines are long, narrow marshes fed by moist winds from the poles.
+There are certainly white polar caps on Mars. They seem to melt in the
+spring, and the dark fringe round them grows broader.
+
+Other astronomers, however, say that they find no trace of water-vapour
+in the atmosphere of Mars, and they think that the polar caps may be
+simply thin sheets of hoar-frost or frozen gas. They point out that, as
+the atmosphere of Mars is certainly scanty, and the distance from the
+sun is so great, it may be too cold for the fluid water to exist on the
+planet.
+
+If one asks why our wonderful instruments cannot settle these points,
+one must be reminded that Mars is never nearer than 34,000,000 miles
+from the earth, and only approaches to this distance once in fifteen or
+seventeen years. The image of Mars on the photographic negative taken in
+a big telescope is very small. Astronomers rely to a great extent on the
+eye, which is more sensitive than the photographic plate. But it is easy
+to have differences of opinion as to what the eye sees, and so there is
+a good deal of controversy.
+
+In August, 1924, the planet will again be well placed for observation,
+and we may learn more about it. Already a few of the much-disputed
+lines, which people wrongly call "canals," have been traced on
+photographs. Astronomers who are sceptical about life on Mars are often
+not fully aware of the extraordinary adaptability of life. There was a
+time when the climate of the whole earth, from pole to pole, was
+semi-tropical for millions of years. No animal could then endure the
+least cold, yet now we have plenty of Arctic plants and animals. If the
+cold came slowly on Mars, as we have reason to suppose, the population
+could be gradually adapted to it. On the whole, it is possible that
+there is advanced life on Mars, and it is not impossible, in spite of
+the very great difficulties of a code of communication, that our "elder
+brothers" may yet flash across space the solution of many of our
+problems.
+
+
+§ 2
+
+Jupiter and Saturn
+
+Next to Mars, going outward from the sun, is Jupiter. Between Mars and
+Jupiter, however, there are more than three hundred million miles of
+space, and the older astronomers wondered why this was not occupied by a
+planet. We now know that it contains about nine hundred "planetoids," or
+small globes of from five to five hundred miles in diameter. It was at
+one time thought that a planet might have burst into these fragments (a
+theory which is not mathematically satisfactory), or it may be that the
+material which is scattered in them was prevented by the nearness of the
+great bulk of Jupiter from uniting into one globe.
+
+For Jupiter is a giant planet, and its gravitational influence must
+extend far over space. It is 1,300 times as large as the earth, and has
+nine moons, four of which are large, in attendance on it. It is
+interesting to note that the outermost moons of Jupiter and Saturn
+revolve round these planets in a direction contrary to the usual
+direction taken by moons round planets, and by planets round the sun.
+But there is no life on Jupiter.
+
+The surface which we see in photographs (Fig. 12) is a mass of cloud or
+steam which always envelops the body of the planet. It is apparently
+red-hot. A red tinge is seen sometimes at the edges of its cloud-belts,
+and a large red region (the "red spot"), 23,000 miles in length, has
+been visible on it for half a century. There may be a liquid or solid
+core to the planet, but as a whole it is a mass of seething vapours
+whirling round on its axis once in every ten hours. As in the case of
+the sun, however, different latitudes appear to rotate at different
+rates. The interior of Jupiter is very hot, but the planet is not
+self-luminous. The planets Venus and Jupiter shine very brightly, but
+they have no light of their own; they reflect the sunlight.
+
+Saturn is in the same interesting condition. The surface in the
+photograph (Fig. 13) is steam, and Saturn is so far away from the sun
+that the vaporisation of its oceans must necessarily be due to its own
+internal heat. It is too hot for water to settle on its surface. Like
+Jupiter, the great globe turns on its axis once in ten hours--a
+prodigious speed--and must be a swirling, seething mass of metallic
+vapours and gases. It is instructive to compare Jupiter and Saturn in
+this respect with the sun. They are smaller globes and have cooled down
+more than the central fire.
+
+Saturn is a beautiful object in the telescope because it has ten moons
+(to include one which is disputed) and a wonderful system of "rings"
+round it. The so-called rings are a mighty swarm of meteorites--pieces
+of iron and stone of all sorts and sizes, which reflect the light of the
+sun to us. This ocean of matter is some miles deep, and stretches from a
+few thousand miles from the surface of the planet to 172,000 miles out
+in space. Some astronomers think that this is volcanic material which
+has been shot out of the planet. Others regard it as stuff which would
+have combined to form an eleventh moon but was prevented by the nearness
+of Saturn itself. There is no evidence of life on Saturn.
+
+
+THE MOON
+
+Mars and Venus are therefore the only planets, besides the earth, on
+which we may look for life; and in the case of Venus, the possibility is
+very faint. But what about the moons which attend the planets? They
+range in size from the little ten-miles-wide moons of Mars, to Titan, a
+moon of Saturn, and Ganymede, a satellite of Jupiter, which are about
+3,000 miles in diameter. May there not be life on some of the larger of
+these moons? We will take our own moon as a type of the class.
+
+
+A Dead World
+
+The moon is so very much nearer to us than any other heavenly body that
+we have a remarkable knowledge of it. In Fig. 14 you have a photograph,
+taken in one of our largest telescopes, of part of its surface. In a
+sense such a telescope brings the moon to within about fifty miles of
+us. We should see a city like London as a dark, sprawling blotch on the
+globe. We could just detect a Zeppelin or a Diplodocus as a moving speck
+against the surface. But we find none of these things. It is true that a
+few astronomers believe that they see signs of some sort of feeble life
+or movement on the moon. Professor Pickering thinks that he can trace
+some volcanic activity. He believes that there are areas of vegetation,
+probably of a low order, and that the soil of the moon may retain a
+certain amount of water in it. He speaks of a very thin atmosphere, and
+of occasional light falls of snow. He has succeeded in persuading some
+careful observers that there probably are slight changes of some kind
+taking place on the moon.
+
+[Illustration: FIG. 17.--A MAP OF THE CHIEF PLAINS AND CRATERS OF THE
+MOON
+
+The plains were originally supposed to be seas: hence the name "Mare."]
+
+[Illustration: FIG. 18.--A DIAGRAM OF A STREAM OF METEORS SHOWING THE
+EARTH PASSING THROUGH THEM] [Illustration: _Photo: Royal Observatory,
+Greenwich._
+
+FIG. 19.--COMET, September 29, 1908
+
+Notice the tendency to form a number of tails. (See photograph below.)]
+
+[Illustration: _Photo: Royal Observatory, Greenwich._
+
+FIG. 20.--COMET, October 3, 1908
+
+The process has gone further and a number of distinct tails can now be
+counted.]
+
+But there are many things that point to absence of air on the moon. Even
+the photographs we reproduce tell the same story. The edges of the
+shadows are all hard and black. If there had been an appreciable
+atmosphere it would have scattered the sun's light on to the edges and
+produced a gradual shading off such as we see on the earth. This
+relative absence of air must give rise to some surprising effects. There
+will be no sounds on the moon, because sounds are merely air waves. Even
+a meteor shattering itself to a violent end against the surface of the
+moon would make no noise. Nor would it herald its coming by glowing into
+a "shooting star," as it would on entering the earth's atmosphere. There
+will be no floating dust, no scent, no twilight, no blue sky, no
+twinkling of the stars. The sky will be always black and the stars will
+be clearly visible by day as by night. The sun's wonderful corona, which
+no man on earth, even by seizing every opportunity during eclipses, can
+hope to see for more than two hours in all in a long lifetime, will be
+visible all day. So will the great red flames of the sun. Of course,
+there will be no life, and no landscape effects and scenery effects due
+to vegetation.
+
+The moon takes approximately twenty-seven of our days to turn once on
+its axis. So for fourteen days there is continuous night, when the
+temperature must sink away down towards the absolute cold of space. This
+will be followed without an instant of twilight by full daylight. For
+another fourteen days the sun's rays will bear straight down, with no
+diffusion or absorption of their heat, or light, on the way. It does not
+follow, however, that the temperature of the moon's surface must rise
+enormously. It may not even rise to the temperature of melting ice.
+Seeing there is no air there can be no check on radiation. The heat that
+the moon gets will radiate away immediately. We know that amongst the
+coldest places on the earth are the tops of very high mountains, the
+points that have reared themselves nearest to the sun but farthest out
+of the sheltering blanket of the earth's atmosphere. The actual
+temperature of the moon's surface by day is a moot point. It may be
+below the freezing-point or above the boiling-point of water.
+
+
+The Mountains of the Moon
+
+The lack of air is considered by many astronomers to furnish the
+explanation of the enormous number of "craters" which pit the moon's
+surface. There are about a hundred thousand of these strange rings, and
+it is now believed by many that they are spots where very large
+meteorites, or even planetoids, splashed into the moon when its surface
+was still soft. Other astronomers think that they are the remains of
+gigantic bubbles which were raised in the moon's "skin," when the globe
+was still molten, by volcanic gases from below. A few astronomers think
+that they are, as is popularly supposed, the craters of extinct
+volcanoes. Our craters, on the earth, are generally deep cups, whereas
+these ring-formations on the moon are more like very shallow and broad
+saucers. Clavius, the largest of them, is 123 miles across the interior,
+yet its encircling rampart is not a mile high.
+
+The mountains on the moon (Fig. 16) rise to a great height, and are
+extraordinarily gaunt and rugged. They are like fountains of lava,
+rising in places to 26,000 and 27,000 feet. The lunar Apennines have
+three thousand steep and weird peaks. Our terrestrial mountains are
+continually worn down by frost acting on moisture and by ice and water,
+but there are none of these agencies operating on the moon. Its
+mountains are comparatively "everlasting hills."
+
+The moon is interesting to us precisely because it is a dead world. It
+seems to show how the earth, or any cooling metal globe, will evolve in
+the remote future. We do not know if there was ever life on the moon,
+but in any case it cannot have proceeded far in development. At the most
+we can imagine some strange lowly forms of vegetation lingering here and
+there in pools of heavy gas, expanding during the blaze of the sun's
+long day, and frozen rigid during the long night.
+
+
+METEORS AND COMETS
+
+We may conclude our survey of the solar system with a word about
+"shooting stars," or meteors, and comets. There are few now who do not
+know that the streak of fire which suddenly lights the sky overhead at
+night means that a piece of stone or iron has entered our atmosphere
+from outer space, and has been burned up by friction. It was travelling
+at, perhaps, twenty or thirty miles a second. At seventy or eighty miles
+above our heads it began to glow, as at that height the air is thick
+enough to offer serious friction and raise it to a white heat. By the
+time the meteor reached about twenty miles or so from the earth's
+surface it was entirely dissipated, as a rule in fiery vapour.
+
+
+Millions of Meteorites
+
+It is estimated that between ten and a hundred million meteorites enter
+our atmosphere and are cremated, every day. Most of them weigh only an
+ounce or two, and are invisible. Some of them weigh a ton or more, but
+even against these large masses the air acts as a kind of "torpedo-net."
+They generally burst into fragments and fall without doing damage.
+
+It is clear that "empty space" is, at least within the limits of our
+solar system, full of these things. They swarm like fishes in the seas.
+Like the fishes, moreover, they may be either solitary or gregarious.
+The solitary bit of cosmic rubbish is the meteorite, which we have just
+examined. A "social" group of meteorites is the essential part of a
+comet. The nucleus, or bright central part, of the head of a comet (Fig.
+19) consists of a swarm, sometimes thousands of miles wide, of these
+pieces of iron or stone. This swarm has come under the sun's
+gravitational influence, and is forced to travel round it. From some
+dark region of space it has moved slowly into our system. It is not then
+a comet, for it has no tail. But as the crowded meteors approach the
+sun, the speed increases. They give off fine vapour-like matter and the
+fierce flood of light from the sun sweeps this vapour out in an
+ever-lengthening tail. Whatever way the comet is travelling, the tail
+always points away from the sun.
+
+
+A Great Comet
+
+The vapoury tail often grows to an enormous length as the comet
+approaches the sun. The great comet of 1843 had a tail two hundred
+million miles long. It is, however, composed of the thinnest vapours
+imaginable. Twice during the nineteenth century the earth passed through
+the tail of a comet, and nothing was felt. The vapours of the tail are,
+in fact, so attenuated that we can hardly imagine them to be white-hot.
+They may be lit by some electrical force. However that may be, the comet
+dashes round the sun, often at three or four hundred miles a second,
+then may pass gradually out of our system once more. It may be a
+thousand years, or it may be fifty years, before the monarch of the
+system will summon it again to make its fiery journey round his throne.
+
+[Illustration: _Photo: Harvard College Observatory._
+
+FIG. 21.--TYPICAL SPECTRA
+
+Six main types of stellar spectra. Notice the lines they have in common,
+showing what elements are met with in different types of stars. Each of
+these spectra corresponds to a different set of physical and chemical
+conditions.] [Illustration: _Photo: Mount Wilson Observatory._
+
+FIG. 22.--A NEBULAR REGION SOUTH OF ZETA ORIONIS
+
+Showing a great projection of "dark matter" cutting off the light from
+behind.]
+
+[Illustration: _Photo: Astrophysical Observatory, Victoria, British
+Columbia._
+
+FIG. 23.--STAR CLUSTER IN HERCULES
+
+A wonderful cluster of stars. It has been estimated that the distance of
+this cluster is such that it would take light more than 100,000 years to
+reach us.]
+
+
+THE STELLAR UNIVERSE
+
+§ 1
+
+The immensity of the Stellar Universe, as we have seen, is beyond our
+apprehension. The sun is nothing more than a very ordinary star, perhaps
+an insignificant one. There are stars enormously greater than the sun.
+One such, Betelgeux, has recently been measured, and its diameter is
+more than 300 times that of the sun.
+
+
+The Evolution of Stars
+
+The proof of the similarity between our sun and the stars has come to us
+through the spectroscope. The elements that we find by its means in the
+sun are also found in the same way in the stars. Matter, says the
+spectroscope, is essentially the same everywhere, in the earth and the
+sun, in the comet that visits us once in a thousand years, in the star
+whose distance is incalculable, and in the great clouds of "fire-mist"
+that we call nebulæ.
+
+In considering the evolution of the stars let us keep two points clearly
+in mind. The starting-point, the nebula, is no figment of the scientific
+imagination. Hundreds of thousands of nebulæ, besides even vaster
+irregular stretches of nebulous matter, exist in the heavens. But the
+stages of the evolution of this stuff into stars are very largely a
+matter of speculation. Possibly there is more than one line of
+evolution, and the various theories may be reconciled. And this applies
+also to the theories of the various stages through which the stars
+themselves pass on their way to extinction.
+
+The light of about a quarter of a million stars has been analysed in the
+spectroscope, and it is found that they fall into about a dozen classes
+which generally correspond to stages in their evolution (Fig. 21).
+
+
+The Age of Stars
+
+In its main lines the spectrum of a star corresponds to its colour, and
+we may roughly group the stars into red, yellow, and white. This is also
+the order of increasing temperature, the red stars being the coolest and
+the white stars the hottest. We might therefore imagine that the white
+stars are the youngest, and that as they grow older and cooler they
+become yellowish, then red, and finally become invisible--just as a
+cooling white-hot iron would do. But a very interesting recent research
+shows that there are two kinds of red stars; some of them are amongst
+the oldest stars and some are amongst the youngest. The facts appear to
+be that when a star is first formed it is not very hot. It is an immense
+mass of diffuse gas glowing with a dull-red heat. It contracts under the
+mutual gravitation of its particles, and as it does so it grows hotter.
+It acquires a yellowish tinge. As it continues to contract it grows
+hotter and hotter until its temperature reaches a maximum as a white
+star. At this point the contraction process does not stop, but the
+heating process does. Further contraction is now accompanied by cooling,
+and the star goes through its colour changes again, but this time in the
+inverse order. It contracts and cools to yellow and finally to red. But
+when it again becomes a red star it is enormously denser and smaller
+than when it began as a red star. Consequently the red stars are divided
+into two classes called, appropriately, Giants and Dwarfs. This theory,
+which we owe to an American astronomer, H. N. Russell, has been
+successful in explaining a variety of phenomena, and there is
+consequently good reason to suppose it to be true. But the question as
+to how the red giant stars were formed has received less satisfactory
+and precise answers.
+
+The most commonly accepted theory is the nebular theory.
+
+
+THE NEBULAR THEORY
+
+§ 2
+
+Nebulæ are dim luminous cloud-like patches in the heavens, more like
+wisps of smoke in some cases than anything else. Both photography and
+the telescope show that they are very numerous, hundreds of thousands
+being already known and the number being continually added to. They are
+not small. Most of them are immensely large. Actual dimensions cannot be
+given, because to estimate these we must first know definitely the
+distance of the nebulæ from the earth. The distances of some nebulæ are
+known approximately, and we can therefore form some idea of size in
+these cases. The results are staggering. The mere visible surface of
+some nebulæ is so large that the whole stretch of the solar system would
+be too small to form a convenient unit for measuring it. A ray of light
+would require to travel for years to cross from side to side of such a
+nebula. Its immensity is inconceivable to the human mind.
+
+There appear to be two types of nebulæ, and there is evidence suggesting
+that the one type is only an earlier form of the other; but this again
+we do not know.
+
+The more primitive nebulæ would seem to be composed of gas in an
+extremely rarified form. It is difficult to convey an adequate idea of
+the rarity of nebular gases. The residual gases in a vacuum tube are
+dense by comparison. A cubic inch of air at ordinary pressure would
+contain more matter than is contained in millions of cubic inches of the
+gases of nebulæ. The light of even the faintest stars does not seem to
+be dimmed by passing through a gaseous nebula, although we cannot be
+sure on this point. The most remarkable physical fact about these gases
+is that they are luminous. Whence they derive their luminosity we do not
+know. It hardly seems possible to believe that extremely thin gases
+exposed to the terrific cold of space can be so hot as to be luminous
+and can retain their heat and their luminosity indefinitely. A cold
+luminosity due to electrification, like that of the aurora borealis,
+would seem to fit the case better.
+
+Now the nebular theory is that out of great "fire-mists," such as we
+have described, stars are born. We do not know whether gravitation is
+the only or even the main force at work in a nebula, but it is supposed
+that under the action of gravity the far-flung "fire-mists" would begin
+to condense round centres of greatest density, heat being evolved in the
+process. Of course the condensation would be enormously slow, although
+the sudden irruption of a swarm of meteors or some solid body might
+hasten matters greatly by providing large, ready-made centres of
+condensation.
+
+
+Spiral Nebulæ
+
+It is then supposed that the contracting mass of gas would begin to
+rotate and to throw off gigantic streamers, which would in their turn
+form centres of condensation. The whole structure would thus form a
+spiral, having a dense region at its centre and knots or lumps of
+condensed matter along its spiral arms. Besides the formless gaseous
+nebulæ there are hundreds of thousands of "spiral" nebulæ such as we
+have just mentioned in the heavens. They are at all stages of
+development, and they are visible to us at all angles--that is to say,
+some of them face directly towards us, others are edge on, and some are
+in intermediate positions. It appears, therefore, that we have here a
+striking confirmation of the nebular hypothesis. But we must not go so
+fast. There is much controversy as to the nature of these spiral nebulæ.
+Some eminent astronomers think they are other stellar universes,
+comparable in size with our own. In any case they are vast structures,
+and if they represent stars in process of condensation, they must be
+giving birth to huge agglomerations of stars--to star clusters at least.
+These vast and enigmatic objects do not throw much light on the origin
+of our own solar system. The nebular hypothesis, which was invented
+by Laplace to explain the origin of our solar system, has not yet met
+with universal acceptance. The explanation offers grave difficulties,
+and it is best while the subject is still being closely investigated, to
+hold all opinions with reserve. It may be taken as probable, however,
+that the universe has developed from masses of incandescent gas.
+
+[Illustration: _Photo: Yerkes Observatory._
+
+FIG. 24.--THE GREAT NEBULA IN ORION
+
+The most impressive nebula in the heavens. It is inconceivably greater
+in dimensions than the whole solar system.]
+
+[Illustration: _Photo: Lick Observatory._
+
+FIG. 25--GIANT SPIRAL NEBULA, March 23, 1914
+
+This spiral nebula is seen full on. Notice the central nucleus and the
+two spiral arms emerging from its opposite directions. Is matter flowing
+out of the nucleus into the arms or along the arms into the nucleus? In
+either case we should get two streams in opposite directions within the
+nucleus.]
+
+
+THE BIRTH AND DEATH OF STARS
+
+§ 3
+
+Variable, New, and Dark Stars: Dying Suns
+
+Many astronomers believe that in "variable stars" we have another star,
+following that of the dullest red star, in the dying of suns. The light
+of these stars varies periodically in so many days, weeks, or years. It
+is interesting to speculate that they are slowly dying suns, in which
+the molten interior periodically bursts through the shell of thick
+vapours that is gathering round them. What we saw about our sun seems to
+point to some such stage in the future. That is, however, not the
+received opinion about variable stars. It may be that they are stars
+which periodically pass through a great swarm of meteors or a region of
+space that is rich in cosmic dust of some sort, when, of course, a great
+illumination would take place.
+
+One class of these variable stars, which takes its name from the star
+Algol, is of special interest. Every third night Algol has its light
+reduced for several hours. Modern astronomy has discovered that in this
+case there are really two stars, circulating round a common centre, and
+that every third night the fainter of the two comes directly between us
+and its companion and causes an "eclipse." This was until recently
+regarded as a most interesting case in which a dead star revealed itself
+to us by passing before the light of another star. But astronomers have
+in recent years invented something, the "selenium-cell," which is even
+more sensitive than the photographic plate, and on this the supposed
+dead star registers itself as very much alive. Algol is, however,
+interesting in another way. The pair of stars which we have discovered
+in it are hundreds of trillions of miles away from the earth, yet we
+know their masses and their distances from each other.
+
+
+The Death and Birth of Stars
+
+We have no positive knowledge of dead stars; which is not surprising
+when we reflect that a dead star means an invisible star! But when we
+see so many individual stars tending toward death, when we behold a vast
+population of all conceivable ages, we presume that there are many
+already dead. On the other hand, there is no reason to suppose that the
+universe as a whole is "running down." Some writers have maintained
+this, but their argument implies that we know a great deal more about
+the universe than we actually do. The scientific man does not know
+whether the universe is finite or infinite, temporal or eternal; and he
+declines to speculate where there are no facts to guide him. He knows
+only that the great gaseous nebulæ promise myriads of worlds in the
+future, and he concedes the possibility that new nebulæ may be forming
+in the ether of space.
+
+The last, and not the least interesting, subject we have to notice is
+the birth of a "new star." This is an event which astronomers now
+announce every few years; and it is a far more portentous event than the
+reader imagines when it is reported in his daily paper. The story is
+much the same in all cases. We say that the star appeared in 1901, but
+you begin to realise the magnitude of the event when you learn that the
+distant "blaze" had really occurred about the time of the death of
+Luther! The light of the conflagration had been speeding toward us
+across space at 186,000 miles a second, yet it has taken nearly three
+centuries to reach us. To be visible at all to us at that distance the
+fiery outbreak must have been stupendous. If a mass of petroleum ten
+times the size of the earth were suddenly fired it would not be seen at
+such a distance. The new star had increased its light many hundredfold
+in a few days.
+
+There is a considerable fascination about the speculation that in such
+cases we see the resurrection of a dead world, a means of renewing the
+population of the universe. What happens is that in some region of the
+sky where no star, or only a very faint star, had been registered on our
+charts, we almost suddenly perceive a bright star. In a few days it may
+rise to the highest brilliancy. By the spectroscope we learn that this
+distant blaze means a prodigious outpour of white-hot hydrogen at
+hundreds of miles a second. But the star sinks again after a few months,
+and we then find a nebula round it on every side. It is natural to
+suppose that a dead or dying sun has somehow been reconverted in whole
+or in part into a nebula. A few astronomers think that it may have
+partially collided with another star, or approached too closely to
+another, with the result we described on an earlier page. The general
+opinion now is that a faint or dead star had rushed into one of those
+regions of space in which there are immense stretches of nebulous
+matter, and been (at least in part) vaporised by the friction.
+
+But the difficulties are considerable, and some astronomers prefer to
+think that the blazing star may merely have lit up a dark nebula which
+already existed. It is one of those problems on which speculation is
+most tempting but positive knowledge is still very incomplete. We may be
+content, even proud, that already we can take a conflagration that has
+occurred more than a thousand trillion miles away and analyse it
+positively into an outflame of glowing hydrogen gas at so many miles a
+second.
+
+
+THE SHAPE OF OUR UNIVERSE
+
+§ 4
+
+Our Universe a Spiral Nebula
+
+What is the shape of our universe, and what are its dimensions? This is
+a tremendous question to ask. It is like asking an intelligent insect,
+living on a single leaf in the midst of a great Brazilian forest, to say
+what is the shape and size of the forest. Yet man's ingenuity has proved
+equal to giving an answer even to this question, and by a method exactly
+similar to that which would be adopted by the insect. Suppose, for
+instance, that the forest was shaped as an elongated oval, and the
+insect lived on a tree near the centre of the oval. If the trees were
+approximately equally spaced from one another they would appear much
+denser along the length of the oval than across its width. This is the
+simple consideration that has guided astronomers in determining the
+shape of our stellar universe. There is one direction in the heavens
+along which the stars appear denser than in the directions at right
+angles to it. That direction is the direction in which we look towards
+the Milky Way. If we count the number of stars visible all over the
+heavens, we find they become more and more numerous as we approach the
+Milky Way. As we go farther and farther from the Milky Way the stars
+thin out until they reach a maximum sparseness in directions at right
+angles to the plane of the Milky Way. We may consider the Milky Way to
+form, as it were, the equator of our system, and the line at right
+angles to point to the north and south poles.
+
+Our system, in fact, is shaped something like a lens, and our sun is
+situated near the centre of this lens. In the remoter part of this lens,
+near its edge, or possibly outside it altogether, lies the great series
+of star clouds which make up the Milky Way. All the stars are in motion
+within this system, but the very remarkable discovery has been made that
+these motions are not entirely random. The great majority of the stars
+whose motions can be measured fall into two groups drifting past one
+another in opposite directions. The velocity of one stream relative to
+the other is about twenty-five miles per second. The stars forming these
+two groups are thoroughly well mixed; it is not a case of an inner
+stream going one way and an outer stream the other. But there are not
+quite as many stars going one way as the other. For every two stars in
+one stream there are three in the other. Now, as we have said, some
+eminent astronomers hold that the spiral nebulæ are universes like our
+own, and if we look at the two photographs (Figs. 25 and 26) we see that
+these spirals present features which, in the light of what we have just
+said about our system, are very remarkable. The nebula in Coma Berenices
+is a spiral edge-on to us, and we see that it has precisely the
+lens-shaped middle and the general flattened shape that we have found in
+our own system. The nebula in Canes Venatici is a spiral facing towards
+us, and its shape irresistibly suggests motions along the spiral arms.
+This motion, whether it is towards or away from the central, lens-shaped
+portion, would cause a double streaming motion in that central portion
+of the kind we have found in our own system. Again, and altogether apart
+from these considerations, there are good reasons for supposing our
+Milky Way to possess a double-armed spiral structure. And the great
+patches of dark absorbing matter which are known to exist in the Milky
+Way (see Fig. 22) would give very much the mottled appearance we notice
+in the arms (which we see edge-on) of the nebula in Coma Berenices. The
+hypothesis, therefore, that our universe is a spiral nebula has much to
+be said for it. If it be accepted it greatly increases our estimate of
+the size of the material universe. For our central, lens-shaped system
+is calculated to extend towards the Milky Way for more than twenty
+thousand times a million million miles, and about a third of this
+distance towards what we have called the poles. If, as we suppose, each
+spiral nebula is an independent stellar universe comparable in size with
+our own, then, since there are hundreds of thousands of spiral nebulæ,
+we see that the size of the whole material universe is indeed beyond our
+comprehension.
+
+[Illustration: _Photo: Mount Wilson Observatory._
+
+FIG. 26.--A SPIRAL NEBULA SEEN EDGE-ON
+
+Notice the lens-shaped formation of the nucleus and the arm stretching
+as a band across it. See reference in the text to the resemblance
+between this and our stellar universe.]
+
+[Illustration: _Photo: H. J. Shepstone._
+
+100-INCH TELESCOPE, MOUNT WILSON
+
+A reflecting telescope: the largest in the world. The mirror is situated
+at the base of the telescope.]
+
+[Illustration:
+
+ ________________________________________________________________
+ | |
+ | THE SOLAR SYSTEM |
+ |________________________________________________________________|
+ | | | | | |
+ | | MEAN DISTANCE | PERIOD OF | | |
+ | NAME | FROM SUN (IN | REVOLUTION | DIAMETER | NUMBER OF |
+ | | MILLIONS OF | AROUND SUN | (IN MILES) | SATELLITES |
+ | | MILES) | (IN YEARS) | | |
+ |_________|_______________|____________|____________|____________|
+ | | | | | |
+ | MERCURY | 36.0 | 0.24 | 3030 | 0 |
+ | VENUS | 67.2 | 0.62 | 7700 | 0 |
+ | EARTH | 92.9 | 1.00 | 7918 | 1 |
+ | MARS | 141.5 | 1.88 | 4230 | 2 |
+ | JUPITER | 483.3 | 11.86 | 86500 | 9 |
+ | SATURN | 886.0 | 29.46 | 73000 | 10 |
+ | URANUS | 1781.9 | 84.02 | 31900 | 4 |
+ | NEPTUNE | 2971.6 | 164.78 | 34800 | 1 |
+ | SUN | ------ | ------ | 866400 | -- |
+ | MOON | ------ | ------ | 2163 | -- |
+ |_________|_______________|____________|____________|____________|
+
+FIG. 27]
+
+[Illustration:
+
+ ______________________________________
+ | |
+ | STAR DISTANCES |
+ |______________________________________|
+ | |
+ | DISTANCE IN |
+ | STAR LIGHT-YEARS |
+ | |
+ | POLARIS 76 |
+ | CAPELLA 49.4 |
+ | RIGEL 466 |
+ | SIRIUS 8.7 |
+ | PROCYON 10.5 |
+ | REGULUS 98.8 |
+ | ARCTURUS 43.4 |
+ | [ALPHA] CENTAURI 4.29 |
+ | VEGA 34.7 |
+ |______________________________________|
+ | |
+ | SMALLER MAGELLANIC CLOUD 32,600[A] |
+ | GREAT CLUSTER IN HERCULES 108,600[A] |
+ |______________________________________|
+
+[A] ESTIMATED
+
+FIG. 28
+
+The above distances are merely approximate and are subject to further
+revision. A "light-year" is the distance that light, travelling at the
+rate of 186,000 miles per second, would cover in one year.]
+
+In this simple outline we have not touched on some of the more debatable
+questions that engage the attention of modern astronomers. Many of these
+questions have not yet passed the controversial stage; out of these will
+emerge the astronomy of the future. But we have seen enough to convince
+us that, whatever advances the future holds in store, the science of the
+heavens constitutes one of the most important stones in the wonderful
+fabric of human knowledge.
+
+
+ASTRONOMICAL INSTRUMENTS
+
+§ 1
+
+The Telescope
+
+The instruments used in modern astronomy are amongst the finest triumphs
+of mechanical skill in the world. In a great modern observatory the
+different instruments are to be counted by the score, but there are two
+which stand out pre-eminent as the fundamental instruments of modern
+astronomy. These instruments are the telescope and the spectroscope, and
+without them astronomy, as we know it, could not exist.
+
+There is still some dispute as to where and when the first telescope was
+constructed; as an astronomical instrument, however, it dates from the
+time of the great Italian scientist Galileo, who, with a very small and
+imperfect telescope of his own invention, first observed the spots on
+the sun, the mountains of the moon, and the chief four satellites of
+Jupiter. A good pair of modern binoculars is superior to this early
+instrument of Galileo's, and the history of telescope construction, from
+that primitive instrument to the modern giant recently erected on Mount
+Wilson, California, is an exciting chapter in human progress. But the
+early instruments have only an historic interest: the era of modern
+telescopes begins in the nineteenth century.
+
+During the last century telescope construction underwent an
+unprecedented development. An immense amount of interest was taken in
+the construction of large telescopes, and the different countries of the
+world entered on an exciting race to produce the most powerful possible
+instruments. Besides this rivalry of different countries there was a
+rivalry of methods. The telescope developed along two different lines,
+and each of these two types has its partisans at the present day. These
+types are known as _refractors_ and _reflectors_, and it is necessary to
+mention, briefly, the principles employed in each. The _refractor_ is
+the ordinary, familiar type of telescope. It consists, essentially, of a
+large lens at one end of a tube, and a small lens, called the eye-piece,
+at the other. The function of the large lens is to act as a sort of
+gigantic eye. It collects a large amount of light, an amount
+proportional to its size, and brings this light to a focus within the
+tube of the telescope. It thus produces a small but bright image, and
+the eye-piece magnifies this image. In the _reflector_, instead of a
+large lens at the top of the tube, a large mirror is placed at the
+bottom. This mirror is so shaped as to reflect the light that falls on
+it to a focus, whence the light is again led to an eye-piece. Thus the
+refractor and the reflector differ chiefly in their manner of gathering
+light. The powerfulness of the telescope depends on the size of the
+light-gatherer. A telescope with a lens four inches in diameter is four
+times as powerful as the one with a lens two inches in diameter, for the
+amount of light gathered obviously depends on the _area_ of the lens,
+and the area varies as the _square_ of the diameter.
+
+The largest telescopes at present in existence are _reflectors_. It is
+much easier to construct a very large mirror than to construct a very
+large lens; it is also cheaper. A mirror is more likely to get out of
+order than is a lens, however, and any irregularity in the shape of a
+mirror produces a greater distorting effect than in a lens. A refractor
+is also more convenient to handle than is a reflector. For these reasons
+great refractors are still made, but the largest of them, the great
+Yerkes' refractor, is much smaller than the greatest reflector, the one
+on Mount Wilson, California. The lens of the Yerkes' refractor measures
+three feet four inches in diameter, whereas the Mount Wilson reflector
+has a diameter of no less than eight feet four inches.
+
+[Illustration: THE YERKES 40-INCH REFRACTOR
+
+(The largest _refracting_ telescope in the world. Its big lens weighs
+1,000 pounds, and its mammoth tube, which is 62 feet long, weighs about
+12,000 pounds. The parts to be moved weigh approximately 22 tons.
+
+The great _100-inch reflector_ of the Mount Wilson reflecting
+telescope--the largest _reflecting_ instrument in the world--weighs
+nearly 9,000 pounds and the moving parts of the telescope weigh about
+100 tons.
+
+The new _72-inch reflector_ at the Dominion Astrophysical Observatory,
+near Victoria, B. C., weighs nearly 4,500 pounds, and the moving parts
+about 35 tons.)]
+
+[Illustration: _Photo: H. J. Shepstone._
+
+THE DOUBLE-SLIDE PLATE HOLDER ON YERKES 40-INCH REFRACTING TELESCOPE
+
+The smaller telescope at the top of the picture acts as a "finder"; the
+field of view of the large telescope is so restricted that it is
+difficult to recognise, as it were, the part of the heavens being
+surveyed. The smaller telescope takes in a larger area and enables the
+precise object to be examined to be easily selected.]
+
+[Illustration: MODERN DIRECT-READING SPECTROSCOPE
+
+(_By A. Hilger, Ltd._)
+
+The light is brought through one telescope, is split up by the prism,
+and the resulting spectrum is observed through the other telescope.]
+
+But there is a device whereby the power of these giant instruments,
+great as it is, can be still further heightened. That device is the
+simple one of allowing the photographic plate to take the place of the
+human eye. Nowadays an astronomer seldom spends the night with his eye
+glued to the great telescope. He puts a photographic plate there. The
+photographic plate has this advantage over the eye, that it builds up
+impressions. However long we stare at an object too faint to be seen, we
+shall never see it. With the photographic plate, however, faint
+impressions go on accumulating. As hour after hour passes, the star
+which was too faint to make a perceptible impression on the plate goes
+on affecting it until finally it makes an impression which can be made
+visible. In this way the photographic plate reveals to us phenomena in
+the heavens which cannot be seen even through the most powerful
+telescopes.
+
+Telescopes of the kind we have been discussing, telescopes for exploring
+the heavens, are mounted _equatorially_; that is to say, they are
+mounted on an inclined pillar parallel to the axis of the earth so that,
+by rotating round this pillar, the telescope is enabled to follow the
+apparent motion of a star due to the rotation of the earth. This motion
+is effected by clock-work, so that, once adjusted on a star, and the
+clock-work started, the telescope remains adjusted on that star for any
+length of time that is desired. But a great official observatory, such
+as Greenwich Observatory or the Observatory at Paris, also has _transit_
+instruments, or telescopes smaller than the equatorials and without the
+same facility of movement, but which, by a number of exquisite
+refinements, are more adapted to accurate measurements. It is these
+instruments which are chiefly used in the compilation of the _Nautical
+Almanac_. They do not follow the apparent motions of the stars. Stars
+are allowed to drift across the field of vision, and as each star
+crosses a small group of parallel wires in the eye-piece its precise
+time of passage is recorded. Owing to their relative fixity of position
+these instruments can be constructed to record the _positions_ of stars
+with much greater accuracy than is possible to the more general and
+flexible mounting of equatorials. The recording of transit is
+comparatively dry work; the spectacular element is entirely absent;
+stars are treated merely as mathematical points. But these observations
+furnish the very basis of modern mathematical astronomy, and without
+them such publications as the _Nautical Almanac_ and the _Connaissance
+du Temps_ would be robbed of the greater part of their importance.
+
+
+§ 2
+
+The Spectroscope
+
+We have already learnt something of the principles of the spectroscope,
+the instrument which, by making it possible to learn the actual
+constitution of the stars, has added a vast new domain to astronomy. In
+the simplest form of this instrument the analysing portion consists of a
+single prism. Unless the prism is very large, however, only a small
+degree of dispersion is obtained. It is obviously desirable, for
+accurate analytical work, that the dispersion--that is, the separation
+of the different parts of the spectrum--should be as great as possible.
+The dispersion can be increased by using a large number of prisms, the
+light emerging from the first prism, entering the second, and so on. In
+this way each prism produces its own dispersive effect and, when a
+number of prisms are employed, the final dispersion is considerable. A
+considerable amount of light is absorbed in this way, however, so that
+unless our primary source of light is very strong, the final spectrum
+will be very feeble and hard to decipher.
+
+Another way of obtaining considerable dispersion is by using a
+_diffraction grating_ instead of a prism. This consists essentially of a
+piece of glass on which lines are ruled by a diamond point. When the
+lines are sufficiently close together they split up light falling on
+them into its constituents and produce a spectrum. The modern
+diffraction grating is a truly wonderful piece of work. It contains
+several thousands of lines to the inch, and these lines have to be
+spaced with the greatest accuracy. But in this instrument, again, there
+is a considerable loss of light.
+
+We have said that every substance has its own distinctive spectrum, and
+it might be thought that, when a list of the spectra of different
+substances has been prepared, spectrum analysis would become perfectly
+straightforward. In practice, however, things are not quite so simple.
+The spectrum emitted by a substance is influenced by a variety of
+conditions. The pressure, the temperature, the state of motion of the
+object we are observing, all make a difference, and one of the most
+laborious tasks of the modern spectroscopist is to disentangle these
+effects from one another. Simple as it is in its broad outlines,
+spectroscopy is, in reality, one of the most intricate branches of
+modern science.
+
+
+BIBLIOGRAPHY
+
+(The following list of books may be useful to readers wishing to pursue
+further the study of Astronomy.)
+
+ BALL, _The Story of the Heavens_.
+ BALL, _The Story of the Sun_.
+ FORBES, _History of Astronomy_.
+ HINCKS, _Astronomy_.
+ KIPPAX, _Call of the Stars_.
+ LOWELL, _Mars and Its Canals_.
+ LOWELL, _Evolution of Worlds_.
+ MCKREADY, _A Beginner's Star-Book_.
+ NEWCOMB, _Popular Astronomy_.
+ NEWCOMB, _The Stars: A Study of the Universe_.
+ OLCOTT, _Field Book of the Stars_.
+ PRICE, _Essence of Astronomy_.
+ SERVISS, _Curiosities of the Skies_.
+ WEBB, _Celestial Objects for Common Telescopes_.
+ YOUNG, _Text-Book of General Astronomy_.
+
+
+
+
+II
+
+THE STORY OF EVOLUTION
+
+
+
+
+INTRODUCTORY
+
+THE BEGINNING OF THE EARTH--MAKING A HOME FOR LIFE--THE FIRST LIVING
+CREATURES
+
+
+§ 1
+
+The Evolution-idea is a master-key that opens many doors. It is a
+luminous interpretation of the world, throwing the light of the past
+upon the present. Everything is seen to be an antiquity, with a history
+behind it--a _natural history_, which enables us to understand in some
+measure how it has come to be as it is. We cannot say more than
+"understand in some measure," for while the _fact_ of evolution is
+certain, we are only beginning to discern the _factors_ that have been
+at work.
+
+The evolution-idea is very old, going back to some of the Greek
+philosophers, but it is only in modern times that it has become an
+essential part of our mental equipment. It is now an everyday
+intellectual tool. It was applied to the origin of the solar system and
+to the making of the earth before it was applied to plants and animals;
+it was extended from these to man himself; it spread to language, to
+folk-ways, to institutions. Within recent years the evolution-idea has
+been applied to the chemical elements, for it appears that uranium may
+change into radium, that radium may produce helium, and that lead is the
+final stable result when the changes of uranium are complete. Perhaps
+all the elements may be the outcome of an inorganic evolution. Not less
+important is the extension of the evolution-idea to the world within as
+well as to the world without. For alongside of the evolution of bodies
+and brains is the evolution of feelings and emotions, ideas and
+imagination.
+
+Organic evolution means that the present is the child of the past and
+the parent of the future. It is not a power or a principle; it is a
+process--a process of becoming. It means that the present-day animals
+and plants and all the subtle inter-relations between them have arisen
+in a natural knowable way from a preceding state of affairs on the whole
+somewhat simpler, and that again from forms and inter-relations simpler
+still, and so on backwards and backwards for millions of years till we
+lose all clues in the thick mist that hangs over life's beginnings.
+
+Our solar system was once represented by a nebula of some sort, and we
+may speak of the evolution of the sun and the planets. But since it has
+been _the same material throughout_ that has changed in its distribution
+and forms, it might be clearer to use some word like genesis. Similarly,
+our human institutions were once very different from what they are now,
+and we may speak of the evolution of government or of cities. But Man
+works with a purpose, with ideas and ideals in some measure controlling
+his actions and guiding his achievements, so that it is probably clearer
+to keep the good old word history for all processes of social becoming
+in which man has been a conscious agent. Now between the genesis of the
+solar system and the history of civilisation there comes the vast
+process of organic evolution. The word development should be kept for
+the becoming of the individual, the chick out of the egg, for instance.
+
+Organic evolution is a continuous natural process of racial change, by
+successive steps in a definite direction, whereby distinctively new
+individualities arise, take root, and flourish, sometimes alongside of,
+and sometimes, sooner or later, in place of, the originative stock. Our
+domesticated breeds of pigeons and poultry are the results of
+evolutionary change whose origins are still with us in the Rock Dove and
+the Jungle Fowl; but in most cases in Wild Nature the ancestral stocks
+of present-day forms are long since extinct, and in many cases they are
+unknown. Evolution is a long process of coming and going, appearing and
+disappearing, a long-drawn-out sublime process like a great piece of
+music.
+
+[Illustration: _Photo: Rischgitz Collection._
+
+CHARLES DARWIN
+
+Greatest of naturalists, who made the idea of evolution current
+intellectual coin, and in his _Origin of Species_ (1859) made the whole
+world new.]
+
+[Illustration: _Photo: Rischgitz Collection._
+
+LORD KELVIN
+
+One of the greatest physicists of the nineteenth century. He estimated
+the age of the earth at 20,000,000 years. He had not at his disposal,
+however, the knowledge of recent discoveries, which have resulted in
+this estimate being very greatly increased.]
+
+[Illustration: _Photo: Lick Observatory._
+
+A GIANT SPIRAL NEBULA
+
+Laplace's famous theory was that the planets and the earth were formed
+from great whirling nebulæ.]
+
+[Illustration: _Photo: Natural History Museum._
+
+METEORITE WHICH FELL NEAR SCARBOROUGH, AND IS NOW TO BE SEEN IN THE
+NATURAL HISTORY MUSEUM
+
+It weighs about 56 lb., and is a "stony" meteorite, i.e., an aerolite.]
+
+
+§ 2
+
+The Beginning of the Earth
+
+When we speak the language of science we cannot say "In the beginning,"
+for we do not know of and cannot think of any condition of things that
+did not arise from something that went before. But we may qualify the
+phrase, and legitimately inquire into the beginning of the earth within
+the solar system. If the result of this inquiry is to trace the sun and
+the planets back to a nebula we reach only a relative beginning. The
+nebula has to be accounted for. And even before matter there may have
+been a pre-material world. If we say, as was said long ago, "In the
+beginning was Mind," we may be expressing or trying to express a great
+truth, but we have gone BEYOND SCIENCE.
+
+
+The Nebular Hypothesis
+
+One of the grandest pictures that the scientific mind has ever thrown
+upon the screen is that of the Nebular Hypothesis. According to
+Laplace's famous form of this theory (1796), the solar system was once a
+gigantic glowing mass, spinning slowly and uniformly around its centre.
+As the incandescent world-cloud of gas cooled and its speed of rotation
+increased the shrinking mass gave off a separate whirling ring, which
+broke up and gathered together again as the first and most distant
+planet. The main mass gave off another ring and another till all the
+planets, including the earth, were formed. The central mass persisted as
+the sun.
+
+Laplace spoke of his theory, which Kant had anticipated forty-one years
+before, with scientific caution: "conjectures which I present with all
+the distrust which everything not the result of observation or of
+calculation ought to inspire." Subsequent research justified his
+distrust, for it has been shown that the original nebula need not have
+been hot and need not have been gaseous. Moreover, there are great
+difficulties in Laplace's theory of the separation of successive rings
+from the main mass, and of the condensation of a whirling gaseous ring
+into a planet.
+
+So it has come about that the picture of a hot gaseous nebula revolving
+as a unit body has given place to other pictures. Thus Sir Norman
+Lockyer pointed out (1890) that the earth is gathering to itself
+millions of meteorites every day; this has been going on for millions of
+years; in distant ages the accretion may have been vastly more rapid and
+voluminous; and so the earth has grown! Now the meteoritic contributions
+are undoubted, but they require a centre to attract them, and the
+difficulty is to account for the beginning of a collecting centre or
+planetary nucleus. Moreover, meteorites are sporadic and erratic,
+scattered hither and thither rather than collecting into unit-bodies. As
+Professor Chamberlin says, "meteorites have rather the characteristics
+of the wreckage of some earlier organisation than of the parentage of
+our planetary system." Several other theories have been propounded to
+account for the origin of the earth, but the one that has found most
+favour in the eyes of authorities is that of Chamberlin and Moulton.
+According to this theory a great nebular mass condensed to form the sun,
+from which under the attraction of passing stars planet after planet,
+the earth included, was heaved off in the form of knotted spiral nebulæ,
+like many of those now observed in the heavens.
+
+Of great importance were the "knots," for they served as collecting
+centres drawing flying matter into their clutches. Whatever part of the
+primitive bolt escaped and scattered was drawn out into independent
+orbits round the sun, forming the "planetesimals" which behave like
+minute planets. These planetesimals formed the food on which the knots
+subsequently fed.
+
+
+The Growth of the Earth
+
+It has been calculated that the newborn earth--the "earth-knot" of
+Chamberlin's theory--had a diameter of about 5,500 miles. But it grew
+by drawing planetesimals into itself until it had a diameter of over
+8,100 miles at the end of its growing period. Since then it has shrunk,
+by periodic shrinkages which have meant the buckling up of successive
+series of mountains, and it has now a diameter of 7,918 miles. But
+during the shrinking the earth became more varied.
+
+A sort of slow boiling of the internally hot earth often forced molten
+matter through the cold outer crust, and there came about a gradual
+assortment of lighter materials nearer the surface and heavier materials
+deeper down. The continents are built of the lighter materials, such as
+granites, while the beds of the great oceans are made of the heavier
+materials such as basalts. In limited areas land has often become sea,
+and sea has often given place to land, but the probability is that the
+distinction of the areas corresponding to the great continents and
+oceans goes back to a very early stage.
+
+The lithosphere is the more or less stable crust of the earth, which may
+have been, to begin with, about fifty miles in thickness. It seems that
+the young earth had no atmosphere, and that ages passed before water
+began to accumulate on its surface--before, in other words, there was
+any hydrosphere. The water came from the earth itself, to begin with,
+and it was long before there was any rain dissolving out saline matter
+from the exposed rocks and making the sea salt. The weathering of the
+high grounds of the ancient crust by air and water furnished the
+material which formed the sandstones and mudstones and other sedimentary
+rocks, which are said to amount to a thickness of over fifty miles in
+all.
+
+
+§ 3
+
+Making a Home for Life
+
+It is interesting to inquire how the callous, rough-and-tumble
+conditions of the outer world in early days were replaced by others that
+allowed of the germination and growth of that tender plant we call
+LIFE. There are very tough living creatures, but the average organism is
+ill suited for violence. Most living creatures are adapted to mild
+temperatures and gentle reactions. Hence the fundamental importance of
+the early atmosphere, heavy with planetesimal dust, in blanketing the
+earth against intensities of radiance from without, as Chamberlin says,
+and inequalities of radiance from within. This was the first preparation
+for life, but it was an atmosphere without free oxygen. Not less
+important was the appearance of pools and lakelets, of lakes and seas.
+Perhaps the early waters covered the earth. And water was the second
+preparation for life--water, that can dissolve a larger variety of
+substances in greater concentration than any other liquid; water, that
+in summer does not readily evaporate altogether from a pond, nor in
+winter freeze throughout its whole extent; water, that is such a mobile
+vehicle and such a subtle cleaver of substances; water, that forms over
+80 per cent. of living matter itself.
+
+Of great significance was the abundance of carbon, hydrogen, and oxygen
+(in the form of carbonic acid and water) in the atmosphere of the
+cooling earth, for these three wonderful elements have a unique
+_ensemble_ of properties--ready to enter into reactions and relations,
+making great diversity and complexity possible, favouring the formation
+of the plastic and permeable materials that build up living creatures.
+We must not pursue the idea, but it is clear that the stones and mortar
+of the inanimate world are such that they built a friendly home for
+life.
+
+
+Origin of Living Creatures upon the Earth
+
+During the early chapters of the earth's history, no living creature
+that we can imagine could possibly have lived there. The temperature was
+too high; there was neither atmosphere nor surface water. Therefore it
+follows that at some uncertain, but inconceivably distant date, living
+creatures appeared upon the earth. No one knows how, but it is
+interesting to consider possibilities.
+
+[Illustration: _Reproduced from the Smithsonian Report, 1915._
+
+A LIMESTONE CANYON
+
+Many fossils of extinct animals have been found in such rock
+formations.]
+
+[Illustration: GENEALOGICAL TREE OF ANIMALS
+
+Showing in order of evolution the general relations of the chief classes
+into which the world of living things is divided. This scheme represents
+the present stage of our knowledge, but is admittedly provisional.]
+
+[Illustration: DIAGRAM OF AMOEBA
+
+(Greatly magnified.)
+
+The amoeba is one of the simplest of all animals, and gives us a hint
+of the original ancestors. It looks like a tiny irregular speck of
+greyish jelly, about 1/100th of an inch in diameter. It is commonly
+found gliding on the mud or weeds in ponds, where it engulfs its
+microscopic food by means of out-flowing lobes (PS). The food vacuole
+(FV) contains ingested food. From the contractile vacuole (CV) the waste
+matter is discharged. N is the nucleus, GR, granules.]
+
+From ancient times it has been a favourite answer that the dust of the
+earth may have become living in a way which is outside scientific
+description. This answer forecloses the question, and it is far too soon
+to do that. Science must often say "Ignoramus": Science should be slow
+to say "Ignorabimus."
+
+A second position held by Helmholtz, Lord Kelvin, and others, suggests
+that minute living creatures may have come to the earth from elsewhere,
+in the cracks of a meteorite or among cosmic dust. It must be remembered
+that seeds can survive prolonged exposure to very low temperatures; that
+spores of bacteria can survive high temperature; that seeds of plants
+and germs of animals in a state of "latent life" can survive prolonged
+drought and absence of oxygen. It is possible, according to Berthelot,
+that as long as there is not molecular disintegration vital activities
+may be suspended for a time, and may afterwards recommence when
+appropriate conditions are restored. Therefore, one should be slow to
+say that a long journey through space is impossible. The obvious
+limitation of Lord Kelvin's theory is that it only shifts the problem of
+the origin of organisms (i.e. living creatures) from the earth to
+elsewhere.
+
+The third answer is that living creatures of a very simple sort may have
+emerged on the earth's surface from not-living material, e.g. from some
+semi-fluid carbon compounds activated by ferments. The tenability of
+this view is suggested by the achievements of the synthetic chemists,
+who are able artificially to build up substances such as oxalic acid,
+indigo, salicylic acid, caffeine, and grape-sugar. We do not know,
+indeed, what in Nature's laboratory would take the place of the clever
+synthetic chemist, but there seems to be a tendency to complexity.
+Corpuscles form atoms, atoms form molecules, small molecules large
+ones.
+
+Various concrete suggestions have been made in regard to the possible
+origin of living matter, which will be dealt with in a later chapter. So
+far as we know of what goes on to-day, there is no evidence of
+spontaneous generation; organisms seem always to arise from pre-existing
+organisms of the same kind; where any suggestion of the contrary has
+been fancied, there have been flaws in the experimenting. But it is one
+thing to accept the verdict "omne vivum e vivo" as a fact to which
+experiment has not yet discovered an exception and another thing to
+maintain that this must always have been true or must always remain
+true.
+
+If the synthetic chemists should go on surpassing themselves, if
+substances like white of egg should be made artificially, and if we
+should get more light on possible steps by which simple living creatures
+may have arisen from not-living materials, this would not greatly affect
+our general outlook on life, though it would increase our appreciation
+of what is often libelled as "inert" matter. If the dust of the earth
+did naturally give rise very long ago to living creatures, if they are
+in a real sense born of her and of the sunshine, then the whole world
+becomes more continuous and more vital, and all the inorganic groaning
+and travailing becomes more intelligible.
+
+
+§ 4
+
+The First Organisms upon the Earth
+
+We cannot have more than a speculative picture of the first living
+creatures upon the earth or, rather, in the waters that covered the
+earth. A basis for speculation is to be found, however, in the simplest
+creatures living to-day, such as some of the bacteria and one-celled
+animalcules, especially those called Protists, which have not taken any
+very definite step towards becoming either plants or animals. No one can
+be sure, but there is much to be said for the theory that the first
+creatures were microscopic globules of living matter, not unlike the
+simplest bacteria of to-day, but able to live on air, water, and
+dissolved salts. From such a source may have originated a race of
+one-celled marine organisms which were able to manufacture chlorophyll,
+or something like chlorophyll, that is to say, the green pigment which
+makes it possible for plants to utilise the energy of the sunlight in
+breaking up carbon dioxide and in building up (photosynthesis) carbon
+compounds like sugars and starch. These little units were probably
+encased in a cell-wall of cellulose, but their boxed-in energy expressed
+itself in the undulatory movement of a lash or flagellum, by means of
+which they propelled themselves energetically through the water. There
+are many similar organisms to-day, mostly in water, but some of
+them--simple one-celled plants--paint the tree-stems and even the
+paving-stones green in wet weather. According to Prof. A. H. Church
+there was a long chapter in the history of the earth when the sea that
+covered everything teemed with these green flagellates--the originators
+of the Vegetable Kingdom.
+
+On another tack, however, there probably evolved a series of simple
+predatory creatures, not able to build up organic matter from air,
+water, and salts, but devouring their neighbours. These units were not
+closed in with cellulose, but remained naked, with their living matter
+or protoplasm flowing out in changeful processes, such as we see in the
+Amoebæ in the ditch or in our own white blood corpuscles and other
+amoeboid cells. These were the originators of the animal kingdom. Thus
+from very simple Protists the first animals and the first plants may
+have arisen. All were still very minute, and it is worth remembering
+that had there been any scientific spectator after our kind upon the
+earth during these long ages, he would have lamented the entire absence
+of life, although the seas were teeming. The simplest forms of life and
+the protoplasm which Huxley called the physical basis of life will be
+dealt with in the chapter on Biology in a later section of this work.
+
+
+FIRST GREAT STEPS IN EVOLUTION
+
+THE FIRST PLANTS--THE FIRST ANIMALS--BEGINNINGS OF BODIES--EVOLUTION OF
+SEX--BEGINNING OF NATURAL DEATH
+
+§ 1
+
+The Contrast between Plants and Animals
+
+However it may have come about, there is no doubt at all that one of the
+first great steps in Organic Evolution was the forking of the
+genealogical tree into Plants and Animals--the most important parting of
+the ways in the whole history of Nature.
+
+Typical plants have chlorophyll; they are able to feed at a low chemical
+level on air, water, and salts, using the energy of the sunlight in
+their photosynthesis. They have their cells boxed in by cellulose walls,
+so that their opportunities for motility are greatly restricted. They
+manufacture much more nutritive material than they need, and live far
+below their income. They have no ready way of getting rid of any
+nitrogenous waste matter that they may form, and this probably helps to
+keep them sluggish.
+
+Animals, on the other hand, feed at a high chemical level, on the
+carbohydrates (e.g. starch and sugar), fats, and proteins (e.g. gluten,
+albumin, casein) which are manufactured by other animals, or to begin
+with, by plants. Their cells have not cellulose walls, nor in most cases
+much wall of any kind, and motility in the majority is unrestricted.
+Animals live much more nearly up to their income. If we could make for
+an animal and a plant of equal weight two fractions showing the ratio of
+the upbuilding, constructive, chemical processes to the down-breaking,
+disruptive, chemical processes that go on in their respective bodies,
+the ratio for the plant would be much greater than the corresponding
+ratio for the animal. In other words, animals take the munitions which
+plants laboriously manufacture and explode them in locomotion and
+work; and the entire system of animate nature depends upon the
+photosynthesis that goes on in green plants.
+
+[Illustration: _From the Smithsonian Report, 1917_
+
+A PIECE OF A REEF-BUILDING CORAL, BUILT UP BY A LARGE COLONY OF SMALL
+SEA-ANEMONE-LIKE POLYPS, EACH OF WHICH FORMS FROM THE SALTS OF THE SEA A
+SKELETON OR SHELL OF LIME
+
+The wonderful mass of corals, which are very beautiful, are the skeleton
+remains of hundreds of these little creatures.]
+
+[Illustration: _Photo: J. J. Ward, F.E.S._
+
+THE INSET CIRCLE SHOWS A GROUP OF CHALK-FORMING ANIMALS, OR
+FORAMINIFERA, EACH ABOUT THE SIZE OF A VERY SMALL PIN'S HEAD
+
+They form a great part of the chalk cliffs of Dover and similar deposits
+which have been raised from the floor of an ancient sea.
+
+THE ENORMOUSLY ENLARGED ILLUSTRATION IS THAT OF A COMMON FORAMINIFER
+(POLYSTOMELLA) SHOWING THE SHELL IN THE CENTRE AND THE OUTFLOWING
+NETWORK OF LIVING MATTER, ALONG WHICH GRANULES ARE CONTINUALLY
+TRAVELLING, AND BY WHICH FOOD PARTICLES ARE ENTANGLED AND DRAWN IN
+
+_Reproduced by permission of the Natural History Museum_ (_after Max
+Schultze_).]
+
+As the result of much more explosive life, animals have to deal with
+much in the way of nitrogenous waste products, the ashes of the living
+fire, but these are usually got rid of very effectively, e.g. in the
+kidney filters, and do not clog the system by being deposited as
+crystals and the like, as happens in plants. Sluggish animals like
+sea-squirts which have no kidneys are exceptions that prove the rule,
+and it need hardly be said that the statements that have been made in
+regard to the contrasts between plants and animals are general
+statements. There is often a good deal of the plant about the animal, as
+in sedentary sponges, zoophytes, corals, and sea-squirts, and there is
+often a little of the animal about the plant, as we see in the movements
+of all shoots and roots and leaves, and occasionally in the parts of the
+flower. But the important fact is that on the early forking of the
+genealogical tree, i.e. the divergence of plants and animals, there
+depended and depends all the higher life of the animal kingdom, not to
+speak of mankind. The continuance of civilisation, the upkeep of the
+human and animal population of the globe, and even the supply of oxygen
+to the air we breathe, depend on the silent laboratories of the green
+leaves, which are able with the help of the sunlight to use carbonic
+acid, water, and salts to build up the bread of life.
+
+
+§ 2
+
+The Beginnings of Land Plants
+
+It is highly probable that for long ages the waters covered the earth,
+and that all the primeval vegetation consisted of simple Flagellates in
+the universal Open Sea. But contraction of the earth's crust brought
+about elevations and depressions of the sea-floor, and in places the
+solid substratum was brought near enough the surface to allow the
+floating plants to begin to settle down without getting out of the
+light. This is how Professor Church pictures the beginning of a fixed
+vegetation--a very momentous step in evolution. It was perhaps among
+this early vegetation that animals had their first successes. As the
+floor of the sea in these shallow areas was raised higher and higher
+there was a beginning of dry land. The sedentary plants already spoken
+of were the ancestors of the shore seaweeds, and there is no doubt that
+when we go down at the lowest tide and wade cautiously out among the
+jungle of vegetation only exposed on such occasions we are getting a
+glimpse of very ancient days. _This_ is the forest primeval.
+
+
+The Protozoa
+
+Animals below the level of zoophytes and sponges are called Protozoa.
+The word obviously means "First Animals," but all that we can say is
+that the very simplest of them may give us some hint of the simplicity
+of the original first animals. For it is quite certain that the vast
+majority of the Protozoa to-day are far too complicated to be thought of
+as primitive. Though most of them are microscopic, each is an animal
+complete in itself, with the same fundamental bodily attributes as are
+manifested in ourselves. They differ from animals of higher degree in
+not being built up of the unit areas or corpuscles called cells. They
+have no cells, no tissues, no organs, in the ordinary acceptation of
+these words, but many of them show a great complexity of internal
+structure, far exceeding that of the ordinary cells that build up the
+tissues of higher animals. They are complete living creatures which have
+not gone in for body-making.
+
+In the dim and distant past there was a time when the only animals were
+of the nature of Protozoa, and it is safe to say that one of the great
+steps in evolution was the establishment of three great types of
+Protozoa: (_a_) Some were very active, the Infusorians, like the slipper
+animalcule, the night-light (Noctiluca), which makes the seas
+phosphorescent at night, and the deadly Trypanosome, which causes
+Sleeping Sickness. (_b_) Others were very sluggish, the parasitic
+Sporozoa, like the malaria organism which the mosquito introduces into
+man's body. (_c_) Others were neither very active nor very passive, the
+Rhizopods, with out-flowing processes of living matter. This amoeboid
+line of evolution has been very successful; it is represented by the
+Rhizopods, such as Amoebæ and the chalk-forming Foraminifera and the
+exquisitely beautiful flint-shelled Radiolarians of the open sea. They
+have their counterparts in the amoeboid cells of most multicellular
+animals, such as the phagocytes which migrate about in the body,
+engulfing and digesting intruding bacteria, serving as sappers and
+miners when something has to be broken down and built up again, and
+performing other useful offices.
+
+
+§ 3
+
+The Making of a Body
+
+The great naturalist Louis Agassiz once said that the biggest gulf in
+Organic Nature was that between the unicellular and the multicellular
+animals (Protozoa and Metazoa). But the gulf was bridged very long ago
+when sponges, stinging animals, and simple worms were evolved, and
+showed, for the first time, a "body." What would one not give to be able
+to account for the making of a body, one of the great steps in
+evolution! No one knows, but the problem is not altogether obscure.
+
+When an ordinary Protozoon or one-celled animal divides into two or
+more, which is its way of multiplying, the daughter-units thus formed
+float apart and live independent lives. But there are a few Protozoa in
+which the daughter-units are not quite separated off from one another,
+but remain coherent. Thus Volvox, a beautiful green ball, found in some
+canals and the like, is a colony of a thousand or even ten thousand
+cells. It has almost formed a body! But in this "colony-making"
+Protozoon, and in others like it, the component cells are all of one
+kind, whereas in true multicellular animals there are different kinds
+of cells, showing division of labour. There are some other Protozoa in
+which the nucleus or kernel divides into many nuclei within the cell.
+This is seen in the Giant Amoeba (Pelomyxa), sometimes found in
+duck-ponds, or the beautiful Opalina, which always lives in the hind
+part of the frog's food-canal. If a portion of the living matter of
+these Protozoa should gather round each of the nuclei, then _that would
+be the beginning of a body_. It would be still nearer the beginning of a
+body if division of labour set in, and if there was a setting apart of
+egg-cells and sperm-cells distinct from body-cells.
+
+It was possibly in some such way that animals and plants with a body
+were first evolved. Two points should be noticed, that body-making is
+not essentially a matter of size, though it made large size possible.
+For the body of a many-celled Wheel Animalcule or Rotifer is no bigger
+than many a Protozoon. Yet the Rotifer--we are thinking of Hydatina--has
+nine hundred odd cells, whereas the Protozoon has only one, except in
+forms like Volvox. Secondly, it is a luminous fact that _every
+many-celled animal from sponge to man that multiplies in the ordinary
+way begins at the beginning again as a "single cell,"_ the fertilised
+egg-cell. It is, of course, not an ordinary single cell that develops
+into an earthworm or a butterfly, an eagle, or a man; it is a cell in
+which a rich inheritance, the fruition of ages, is somehow condensed;
+but it is interesting to bear in mind the elementary fact that every
+many-celled creature, reproduced in the ordinary way and not by budding
+or the like, starts as a fertilised egg-cell. The coherence of the
+daughter-cells into which the fertilised egg-cell divides is a
+reminiscence, as it were, of the primeval coherence of daughter-units
+that made the first body possible.
+
+
+The Beginning of Sexual Reproduction
+
+A freshwater Hydra, growing on the duckweed usually multiplies by
+budding. It forms daughter-buds, living images of itself; a check comes
+to nutrition and these daughter-buds go free. A big sea-anemone may
+divide in two or more parts, which become separate animals. This is
+asexual reproduction, which means that the multiplication takes place by
+dividing into two or many portions, and not by liberating egg-cells and
+sperm-cells. Among animals as among plants, asexual reproduction is very
+common. But it has great disadvantages, for it is apt to be
+physiologically expensive, and it is beset with difficulties when the
+body shows great division of labour, and is very intimately bound into
+unity. Thus, no one can think of a bee or a bird multiplying by division
+or by budding. Moreover, if the body of the parent has suffered from
+injury or deterioration, the result of this is bound to be handed on to
+the next generation if asexual reproduction is the only method.
+
+[Illustration: _Photos: J. J. Ward, F.E.S._
+
+A PLANT-LIKE ANIMAL, OR ZOOPHYTE, CALLED OBELIA
+
+Consisting of a colony of small polyps, whose stinging tentacles are
+well shown greatly enlarged in the lower photograph.]
+
+[Illustration: _Reproduced by permission of "The Quart. Journ. Mic.
+Sci."_
+
+TRYPANOSOMA GAMBIENSE
+
+(Very highly magnified.)
+
+The microscopic animal Trypanosome, which causes Sleeping Sickness. The
+study of these organisms has of late years acquired an immense
+importance on account of the widespread and dangerous maladies to which
+some of them give rise. It lives in the blood of man, who is infected by
+the bite of a Tse-tse fly which carries the parasite from some other
+host.]
+
+[Illustration: VOLVOX
+
+The Volvox is found in some canals and the like. It is one of the first
+animals to suggest the beginning of a body. It is a colony of a thousand
+or even ten thousand cells, but they are all cells of one kind. In
+_multicellular_ animals the cells are of _different_ kinds with
+different functions. Each of the ordinary cells (marked 5) has two
+lashes or flagella. Daughter colonies inside the Parent colony are being
+formed at 3, 4, and 2. The development of germ-cells is shown at 1.]
+
+[Illustration: PROTEROSPONGIA
+
+One of the simplest multicellular animals, illustrating the beginning of
+a body. There is a setting apart of egg-cells and sperm-cells, distinct
+from body-cells; the collared lashed cells on the margin are different
+in kind from those farther in. Thus, as in indubitable multicellular
+animals, division of labour has begun.]
+
+Splitting into two or many parts was the old-fashioned way of
+multiplying, but one of the great steps in evolution was the discovery
+of a better method, namely, sexual reproduction. The gist of this is
+simply that during the process of body-building (by the development of
+the fertilised egg-cell) certain units, _the germ-cells_, do not share
+in forming ordinary tissues or organs, but remain apart, continuing the
+full inheritance which was condensed in the fertilised egg-cell. _These
+cells kept by themselves are the originators of the future reproductive
+cells of the mature animal_; they give rise to the egg-cells and the
+sperm-cells.
+
+The advantages of this method are great. (1) The new generation is
+started less expensively, for it is easier to shed germ-cells into the
+cradle of the water than to separate off half of the body. (2) It is
+possible to start a great many new lives at once, and this may be of
+vital importance when the struggle for existence is very keen, and when
+parental care is impossible. (3) The germ-cells are little likely to be
+prejudicially affected by disadvantageous dints impressed on the body of
+the parent--little likely unless the dints have peculiarly penetrating
+consequences, as in the case of poisons. (4) A further advantage is
+implied in the formation of two kinds of germ-cells--the ovum or
+egg-cell, with a considerable amount of building material and often with
+a legacy of nutritive yolk; the spermatozoon or sperm-cell, adapted to
+move in fluids and to find the ovum from a distance, thus securing
+change-provoking cross-fertilisation.
+
+
+§ 4
+
+The Evolution of Sex
+
+Another of the great steps in organic evolution was the differentiation
+of two different physiological types, the male or sperm-producer and the
+female or egg-producer. It seems to be a deep-seated difference in
+constitution, which leads one egg to develop into a male, and another,
+lying beside it in the nest, into a female. In the case of pigeons it
+seems almost certain, from the work of Professor Oscar Riddle, that
+there are two kinds of egg, a male-producing egg and a female-producing
+egg, which differ in their yolk-forming and other physiological
+characters.
+
+In sea-urchins we often find two creatures superficially
+indistinguishable, but the one is a female with large ovaries and the
+other is a male with equally large testes. Here the physiological
+difference does not affect the body as a whole, but the reproductive
+organs or gonads only, though more intimate physiology would doubtless
+discover differences in the blood or in the chemical routine
+(metabolism). In a large number of cases, however, there are marked
+superficial differences between the sexes, and everyone is familiar with
+such contrasts as peacock and peahen, stag and hind. In such cases the
+physiological difference between the sperm-producer and the
+ovum-producer, for this is the essential difference, saturates through
+the body and expresses itself in masculine and feminine structures and
+modes of behaviour. The expression of the masculine and feminine
+characters is in some cases under the control of hormones or chemical
+messengers which are carried by the blood from the reproductive organs
+throughout the body, and pull the trigger which brings about the
+development of an antler or a wattle or a decorative plume or a capacity
+for vocal and saltatory display. In some cases it is certain that the
+female carries in a latent state the masculine features, but these are
+kept from expressing themselves by other chemical messengers from the
+ovary. Of these chemical messengers more must be said later on.
+
+Recent research has shown that while the difference between male and
+female is very deep-rooted, corresponding to a difference in gearing, it
+is not always clear-cut. Thus a hen-pigeon may be very masculine, and a
+cock-pigeon very feminine. The difference is in degree, not in kind.
+
+
+§ 5
+
+What is the meaning of the universal or almost universal inevitableness
+of death? A Sequoia or "Big Tree" of California has been known to live
+for over two thousand years, but eventually it died. A centenarian
+tortoise has been known, and a sea-anemone sixty years of age; but
+eventually they die. What is the meaning of this apparently inevitable
+stoppage of bodily life?
+
+
+The Beginning of Natural Death
+
+There are three chief kinds of death, (_a_) The great majority of
+animals come to a violent end, being devoured by others or killed by
+sudden and extreme changes in their surroundings. (_b_) When an animal
+enters a new habitat, or comes into new associations with other
+organisms, it may be invaded by a microbe or by some larger parasite to
+which it is unaccustomed and to which it can offer no resistance. With
+many parasites a "live-and-let-live" compromise is arrived at, but new
+parasites are apt to be fatal, as man knows to his cost when he is
+bitten by a tse-tse fly which infects him with the microscopic animal (a
+Trypanosome) that causes Sleeping Sickness. In many animals the
+parasites are not troublesome as long as the host is vigorous, but if
+the host is out of condition the parasites may get the upper hand, as in
+the so-called "grouse disease," and become fatal. (_c_) But besides
+violent death and microbic (or parasitic) death, there is natural death.
+This is in great part to be regarded as the price paid for a body. A
+body worth having implies complexity or division of labour, and this
+implies certain internal furnishings of a more or less stable kind in
+which the effects of wear and tear are apt to accumulate. It is not the
+living matter itself that grows old so much as the framework in which it
+works--the furnishings of the vital laboratory. There are various
+processes of rejuvenescence, e.g. rest, repair, change, reorganisation,
+which work against the inevitable processes of senescence, but sooner or
+later the victory is with ageing. Another deep reason for natural death
+is to be found in the physiological expensiveness of reproduction, for
+many animals, from worms to eels, illustrate natural death as the
+nemesis of starting new lives. Now it is a very striking fact that to a
+large degree the simplest animals or Protozoa are exempt from natural
+death. They are so relatively simple that they can continually
+recuperate by rest and repair; they do not accumulate any bad debts.
+Moreover, their modes of multiplying, by dividing into two or many
+units, are very inexpensive physiologically. It seems that in some
+measure this bodily immortality of the Protozoa is shared by some simple
+many-celled animals like the freshwater Hydra and Planarian worms. Here
+is an interesting chapter in evolution, the evolution of means of
+evading or staving off natural death. Thus there is the well-known case
+of the Paloloworm of the coral-reefs where the body breaks up in
+liberating the germ-cells, but the head-end remains fixed in a crevice
+of the coral, and buds out a new body at leisure.
+
+Along with the evolution of the ways of avoiding death should be
+considered also the gradual establishment of the length of life best
+suited to the welfare of the species, and the punctuation of the
+life-history to suit various conditions.
+
+[Illustration: _Photo: J. J. Ward, F.E.S._
+
+GREEN HYDRA
+
+A little freshwater polyp, about half an inch long, with a crown of
+tentacles round the mouth. It is seen giving off a bud, a clear
+illustration of asexual reproduction. When a tentacle touches some small
+organism the latter is paralysed and drawn into the mouth.]
+
+[Illustration: _Photo: J. J. Ward, F.E.S._
+
+EARTHWORM
+
+Earthworms began the profitable habit of moving with one end of the body
+always in front, and from worms to man the great majority of animals
+have bilateral symmetry.]
+
+[Illustration: DIAGRAM ILLUSTRATING THE BEGINNING OF INDIVIDUAL LIFE
+
+1. An immature _sperm_-cell, with 4 chromosomes (nuclear bodies)
+represented as rods.
+
+2. A mature sperm-cell, with 2 chromosomes.
+
+3. An immature _egg_-cell, with 4 chromosomes represented as curved
+bodies.
+
+4. A mature egg-cell, with 2 chromosomes.
+
+5. The spermatozoon fertilises the ovum, introducing 2 chromosomes.
+
+6. The fertilised ovum, with 4 chromosomes, 2 of paternal origin and 2
+of maternal origin.
+
+7. The chromosomes lie at the equator, and each is split longitudinally.
+The centrosome introduced by the spermatozoon has divided into two
+centrosomes, one at each pole of the nucleus. These play an important
+part in the division or segmentation of the egg.
+
+8. The fertilised egg has divided into two cells. Each cell has 2
+paternal and 2 maternal chromosomes.]
+
+[Illustration: _Reproduced from the Smithsonian Report, 1917._
+
+GLASS MODEL OF A SEA-ANEMONE
+
+A long tubular sea-anemone, with a fine crown of tentacles around the
+mouth. The suggestion of a flower is very obvious. By means of stinging
+lassoes on the tentacles minute animals on which it feeds are paralysed
+and captured for food.]
+
+[Illustration: THIS DRAWING SHOWS THE EVOLUTION OF THE BRAIN FROM FISH
+TO MAN
+
+The Cerebrum, the seat of intelligence, increases in proportion to the
+other parts. In mammals it becomes more and more convoluted. The brain,
+which lies in one plane in fishes, becomes gradually curved on itself.
+In birds it is more curved than the drawing shows.]
+
+
+§ 6
+
+Great Acquisitions
+
+In animals like sea-anemones and jellyfishes the general symmetry of the
+body is radial; that is to say, there is no right or left, and the body
+might be halved along many planes. It is a kind of symmetry well suited
+for sedentary or for drifting life. But worms began the profitable habit
+of moving with one end of the body always in front, and from worms to
+man the great majority of animals have bilateral symmetry. They have a
+right and a left side, and there is only one cut that halves the body.
+This kind of symmetry is suited for a more strenuous life than radial
+animals show; it is suited for pursuing food, for avoiding enemies, for
+chasing mates. And _with the establishment of bilateral symmetry must be
+associated the establishment of head-brains_, the beginning of which is
+to be found in some simple worm-types.
+
+Among the other great acquisitions gradually evolved we may notice: a
+well-developed head with sense-organs, the establishment of large
+internal surfaces such as the digestive and absorptive wall of the
+food-canal, the origin of quickly contracting striped muscle and of
+muscular appendages, the formation of blood as a distributing medium
+throughout the body, from which all the parts take what they need and to
+which they also contribute.
+
+Another very important acquisition, almost confined (so far as is known)
+to backboned animals, was the evolution of what are called glands of
+internal secretion, such as the thyroid and the supra-renal. These
+manufacture subtle chemical substances which are distributed by the
+blood throughout the body, and have a manifold influence in regulating
+and harmonising the vital processes. Some of these chemical messengers
+are called hormones, which stimulate organs and tissues to greater
+activity; others are called chalones, which put on a brake. Some
+regulate growth and others rapidly alter the pressure and composition
+of the blood. Some of them call into active development certain parts of
+the body which have been, as it were, waiting for an appropriate
+trigger-pulling. Thus, at the proper time, the milk-glands of a
+mammalian mother are awakened from their dormancy. This very interesting
+outcome of evolution will be dealt with in another portion of this work.
+
+
+THE INCLINED PLANE OF ANIMAL BEHAVIOUR
+
+§ 1
+
+Before passing to a connected story of the gradual emergence of higher
+and higher forms of life in the course of the successive ages--the
+procession of life, as it may be called--it will be useful to consider
+the evolution of animal behaviour.
+
+
+Evolution of Mind
+
+A human being begins as a microscopic fertilised egg-cell, within which
+there is condensed the long result of time--Man's inheritance. The long
+period of nine months before birth, with its intimate partnership
+between mother and offspring, is passed as it were in sleep, and no one
+can make any statement in regard to the mind of the unborn child. Even
+after birth the dawn of mind is as slow as it is wonderful. To begin
+with, there is in the ovum and early embryo no nervous system at all,
+and it develops very gradually from simple beginnings. Yet as mentality
+cannot come in from outside, we seem bound to conclude that the
+potentiality of it--whatever that means--resides in the individual from
+the very first. The particular kind of activity known to us as thinking,
+feeling, and willing is the most intimate part of our experience, known
+to us directly apart from our senses, and the possibility of that must
+be implicit in the germ-cell just as the genius of Newton was implicit
+in a very miserable specimen of an infant. Now what is true of the
+individual is true also of the race--there is a gradual evolution of
+that aspect of the living creature's activity which we call mind. We
+cannot put our finger on any point and say: Before this stage there was
+no mind. Indeed, many facts suggest the conclusion that wherever there
+is life there is some degree of mind--even in the plants. Or it might be
+more accurate to put the conclusion in another way, that the activity we
+call life has always in some degree an inner or mental aspect.
+
+[Illustration: OKAPI AND GIRAFFE
+
+The Okapi is one of the great zoölogical discoveries. It gives a good
+idea of what the Giraffe's ancestors were like. The Okapi was unknown
+until discovered in 1900 by Sir Harry Johnston in Central Africa, where
+these strange animals have probably lived in dense forests from time
+immemorial.]
+
+In another part of this book there is an account of the dawn of mind in
+backboned animals; what we aim at here is an outline of what may be
+called the inclined plane of animal behaviour.
+
+A very simple animal accumulates a little store of potential energy, and
+it proceeds to expend this, like an explosive, by acting on its
+environment. It does so in a very characteristic self-preservative
+fashion, so that it burns without being consumed and explodes without
+being blown to bits. It is characteristic of the organism that it
+remains a going concern for a longer or shorter period--its length of
+life. Living creatures that expended their energy ineffectively or
+self-destructively would be eliminated in the struggle for existence.
+When a simple one-celled organism explores a corner of the field seen
+under a microscope, behaving to all appearance very like a dog scouring
+a field seen through a telescope, it seems permissible to think of
+something corresponding to mental endeavour associated with its
+activity. This impression is strengthened when an amoeba pursues
+another amoeba, overtakes it, engulfs it, loses it, pursues it again,
+recaptures it, and so on. What is quite certain is that the behaviour of
+the animalcule is not like that of a potassium pill fizzing about in a
+basin of water, nor like the lurching movements of a gun that has got
+loose and "taken charge" on board ship. Another feature is that the
+locomotor activity of an animalcule often shows a distinct
+individuality: it may swim, for instance, in a loose spiral.
+
+But there is another side to vital activity besides acting upon the
+surrounding world; the living creature is acted on by influences from
+without. The organism acts on its environment; that is the one side of
+the shield: the environment acts upon the organism; that is the other
+side. If we are to see life whole we must recognise these two sides of
+what we call living, and it is missing an important part of the history
+of animal life if we fail to see that evolution implies becoming more
+advantageously sensitive to the environment, making more of its
+influences, shutting out profitless stimuli, and opening more gateways
+to knowledge. The bird's world is a larger and finer world than an
+earthworm's; the world means more to the bird than to the worm.
+
+
+The Trial and Error Method
+
+Simple creatures act with a certain degree of spontaneity on their
+environment, and they likewise react effectively to surrounding stimuli.
+Animals come to have definite "answers back," sometimes several,
+sometimes only one, as in the case of the Slipper Animalcule, which
+reverses its cilia when it comes within the sphere of some disturbing
+influence, retreats, and, turning upon itself tentatively, sets off
+again in the same general direction as before, but at an angle to the
+previous line. If it misses the disturbing influence, well and good; if
+it strikes it again, the tactics are repeated until a satisfactory way
+out is discovered or the stimulation proves fatal.
+
+It may be said that the Slipper Animalcule has but one answer to every
+question, but there are many Protozoa which have several enregistered
+reactions. When there are alternative reactions which are tried one
+after another, the animal is pursuing what is called the trial-and-error
+method, and a higher note is struck.
+
+There is an endeavour after satisfaction, and a trial of answers. When
+the creature profits by experience to the extent of giving the right
+answer first, there is the beginning of learning.
+
+[Illustration: DIAGRAM OF A SIMPLE REFLEX ARC IN A BACKBONELESS ANIMAL
+LIKE AN EARTHWORM
+
+1. A sensory nerve-cell (S.C.) on the surface receives a stimulus.
+
+2. The stimulus travels along the sensatory nerve-fibre (S.F.)
+
+3. The sensory nerve-fibre branches in the nerve-cord.
+
+4. Its branches come into close contact (SY^{1}) with those of an
+associative or communicating nerve-cell (A.C.).
+
+5. Other branches of the associative cell come into close contact
+(SY^{2}) with the branches or dendrites of a motor nerve-cell (M.C.).
+
+6. An impulse or command travels along the motor nerve-fibre or axis
+cylinder of the motor nerve-cell.
+
+7. The motor nerve-fibre ends on a muscle-fibre (M.F.) near the surface.
+This moves and the reflex action is complete.]
+
+[Illustration: _Photo: British Museum_ (_Natural History_).
+
+THE YUCCA MOTH
+
+The Yucca Moth, emerging from her cocoon, flies at night to a Yucca
+flower and collects pollen from the stamens, holding a little ball of it
+in her mouth-parts. She then visits another flower and lays an egg in
+the seed-box. After this she applies the pollen to the tip of the
+pistil, thus securing the fertilisation of the flower and the growth of
+the ovules in the pod. Yucca flowers in Britain do not produce seeds
+because there are no Yucca Moths.]
+
+[Illustration: INCLINED PLANE OF ANIMAL BEHAVIOUR
+
+Diagram illustrating animal behaviour. The main line represents the
+general life of the creature. On the upper side are activities implying
+initiative; on the lower side actions which are almost automatic.
+
+_Upper Side._--I. Energetic actions. II. Simple tentatives. III.
+Trial-and-error methods. IV. Non-intelligent experiments. V.
+Experiential "learning." VI. Associative "learning." VII. Intelligent
+behaviour. VIII. Rational conduct (man).
+
+_Lower Side._--1. Reactions to environment. 2. Enregistered reactions.
+3. Simple reflex actions. 4. Compound reflex actions. 5. Tropisms. 6.
+Enregistered rhythms. 7. Simple instincts. 8. Chain instincts. 9.
+Instinctive activities influenced by intelligence. 10. Subconscious
+cerebration at a high level (man).]
+
+[Illustration: _Photo: J. J. Ward, F.E.S._
+
+VENUS' FLY-TRAP
+
+One of the most remarkable plants in the world, which captures its prey
+by means of a trap formed from part of its leaf. It has been induced to
+snap at and hold a bristle. If an insect lighting on the leaf touches
+one of six very sensitive hairs, which pull the trigger of the movement,
+the two halves of the leaf close rapidly and the fringing teeth on the
+margin interlock, preventing the insect's escape. Then follows an
+exudation of digestive juice.]
+
+[Illustration: _Reproduced by permission from "The Wonders of Instinct"
+by J. H. Fabre._
+
+A SPIDER SUNNING HER EGGS
+
+A kind of spider, called Lycosa, lying head downwards at the edge of her
+nest, and holding her silken cocoon--the bag containing the eggs--up
+towards the sun in her hindmost pair of legs. This extraordinary
+proceeding is believed to assist in the hatching.]
+
+
+Reflex Actions
+
+Among simple multicellular animals, such as sea-anemones, we find the
+beginnings of reflex actions, and a considerable part of the behaviour
+of the lower animals is reflex. That is to say, there are laid down in
+the animal in the course of its development certain pre-arrangements of
+nerve-cells and muscle-cells which secure that a fit and proper answer
+is given to a frequently recurrent stimulus. An earthworm half out of
+its burrow becomes aware of the light tread of a thrush's foot, and
+jerks itself back into its hole before anyone can say "reflex action."
+What is it that happens?
+
+Certain sensory nerve-cells in the earthworm's skin are stimulated by
+vibrations in the earth; the message travels down a sensory nerve-fibre
+from each of the stimulated cells and enters the nerve-cord. The sensory
+fibres come into vital connection with branches of intermediary,
+associative, or communicating cells, which are likewise connected with
+motor nerve-cells. To these the message is thus shunted. From the motor
+nerve-cells an impulse or command travels by motor nerve-fibres, one
+from each cell, to the muscles, which contract. If this took as long to
+happen as it takes to describe, even in outline, it would not be of much
+use to the earthworm. But the motor answer follows the sensory stimulus
+almost instantaneously. The great advantage of establishing or
+enregistering these reflex chains is that the answers are practically
+ready-made or inborn, not requiring to be learned. It is not necessary
+that the brain should be stimulated if there is a brain; nor does the
+animal will to act, though in certain cases it may by means of higher
+controlling nerve-centres keep the natural reflex response from being
+given, as happens, for instance, when we control a cough or a sneeze on
+some solemn occasion. The evolutionary method, if we may use the
+expression, has been to enregister ready-made responses; and as we
+ascend the animal kingdom, we find reflex actions becoming complicated
+and often linked together, so that the occurrence of one pulls the
+trigger of another, and so on in a chain. The behaviour of the
+insectivorous plant called Venus's fly-trap when it shuts on an insect
+is like a reflex action in an animal, but plants have no definite
+nervous system.
+
+
+What are Called Tropisms
+
+A somewhat higher level on the inclined plane is illustrated by what are
+called "tropisms," obligatory movements which the animal makes,
+adjusting its whole body so that physiological equilibrium results in
+relation to gravity, pressure, currents, moisture, heat, light,
+electricity, and surfaces of contact. A moth is flying past a candle;
+the eye next the light is more illumined than the other; a physiological
+inequilibrium results, affecting nerve-cells and muscle-cells; the
+outcome is that the moth automatically adjusts its flight so that both
+eyes become equally illumined; in doing this it often flies into the
+candle.
+
+It may seem bad business that the moth should fly into the candle, but
+the flame is an utterly artificial item in its environment to which no
+one can expect it to be adapted. These tropisms play an important rôle
+in animal behaviour.
+
+
+§ 2
+
+Instinctive Behaviour
+
+On a higher level is instinctive behaviour, which reaches such
+remarkable perfection in ants, bees, and wasps. In its typical
+expression instinctive behaviour depends on inborn capacities; it does
+not require to be learned; it is independent of practice or experience,
+though it may be improved by both; it is shared equally by all members
+of the species of the same sex (for the female's instincts are often
+different from the male's); it refers to particular conditions of life
+that are of vital importance, though they may occur only once in a
+lifetime. The female Yucca Moth emerges from the cocoon when the Yucca
+flower puts forth its bell-like blossoms. She flies to a flower,
+collects some pollen from the stamens, kneads it into a pill-like ball,
+and stows this away under her chin. She flies to an older Yucca flower
+and lays her eggs in some of the ovules within the seed-box, but before
+she does so she has to deposit on the stigma the ball of pollen. From
+this the pollen-tubes grow down and the pollen-nucleus of a tube
+fertilises the egg-cell in an ovule, so that the possible seeds become
+real seeds, for it is only a fraction of them that the Yucca Moth has
+destroyed by using them as cradles for her eggs. Now it is plain that
+the Yucca Moth has no individual experience of Yucca flowers, yet she
+secures the continuance of her race by a concatenation of actions which
+form part of her instinctive repertory.
+
+From a physiological point of view instinctive behaviour is like a chain
+of compound reflex actions, but in some cases, at least, there is reason
+to believe that the behaviour is suffused with awareness and backed by
+endeavour. This is suggested in exceptional cases where the stereotyped
+routine is departed from to meet exceptional conditions. It should also
+be noted that just as ants, hive bees, and wasps exhibit in most cases
+purely instinctive behaviour, but move on occasion on the main line of
+trial and error or of experimental initiative, so among birds and
+mammals the intelligent behaviour is sometimes replaced by instinctive
+routine. Perhaps there is no instinctive behaviour without a spice of
+intelligence, and no intelligent behaviour without an instinctive
+element. The old view that instinctive behaviour was originally
+intelligent, and that instinct is "lapsed intelligence," is a tempting
+one, and is suggested by the way in which habitual intelligent actions
+cease in the individual to require intelligent control, but it rests on
+the unproved hypothesis that the acquisitions of the individual can be
+entailed on the race. It is almost certain that instinct is on a line of
+evolution quite different from intelligence, and that it is nearer to
+the inborn inspirations of the calculating boy or the musical genius
+than to the plodding methods of intelligent learning.
+
+
+Animal Intelligence
+
+The higher reaches of the inclined plane of behaviour show intelligence
+in the strict sense. They include those kinds of behaviour which cannot
+be described without the suggestion that the animal makes some sort of
+perceptual inference, not only profiting by experience but learning by
+ideas. Such intelligent actions show great individual variability; they
+are plastic and adjustable in a manner rarely hinted at in connection
+with instincts where routine cannot be departed from without the
+creature being nonplussed; they are not bound up with particular
+circumstances as instinctive actions are, but imply an appreciative
+awareness of relations.
+
+When there is an experimenting with general ideas, when there is
+_conceptual_ as contrasted with _perceptual_ inference, we speak of
+Reason, but there is no evidence of this below the level of man. It is
+not, indeed, always that we can credit man with rational conduct, but he
+has the possibility of it ever within his reach.
+
+Animal instinct and intelligence will be illustrated in another part of
+this work. We are here concerned simply with the general question of the
+evolution of behaviour. There is a main line of tentative experimental
+behaviour both below and above the level of intelligence, and it has
+been part of the tactics of evolution to bring about the hereditary
+enregistration of capacities of effective response, the advantages being
+that the answers come more rapidly and that the creature is left free,
+if it chooses, for higher adventures.
+
+There is no doubt as to the big fact that in the course of evolution
+animals have shown an increasing complexity and masterfulness of
+behaviour, that they have become at once more controlled and more
+definitely free agents, and that the inner aspect of the
+behaviour--experimenting, learning, thinking, feeling, and willing--has
+come to count for more and more.
+
+
+§ 3
+
+Evolution of Parental Care
+
+Mammals furnish a crowning instance of a trend of evolution which
+expresses itself at many levels--the tendency to bring forth the young
+at a well-advanced stage and to an increase of parental care associated
+with a decrease in the number of offspring. There is a British starfish
+called _Luidia_ which has two hundred millions of eggs in a year, and
+there are said to be several millions of eggs in conger-eels and some
+other fishes. These illustrate the spawning method of solving the
+problem of survival. Some animals are naturally prolific, and the number
+of eggs which they sow broadcast in the waters allows for enormous
+infantile mortality and obviates any necessity for parental care.
+
+But some other creatures, by nature less prolific, have found an
+entirely different solution of the problem. They practise parental care
+and they secure survival with greatly economised reproduction. This is a
+trend of evolution particularly characteristic of the higher animals. So
+much so that Herbert Spencer formulated the generalisation that the size
+and frequency of the animal family is inverse ratio to the degree of
+evolution to which the animal has attained.
+
+Now there are many different methods of parental care which secure the
+safety of the young, and one of these is called viviparity. The young
+ones are not liberated from the parent until they are relatively well
+advanced and more or less able to look after themselves. This gives the
+young a good send-off in life, and their chances of death are greatly
+reduced. In other words, the animals that have varied in the direction
+of economised reproduction may keep their foothold in the struggle for
+existence if they have varied at the same time in the direction of
+parental care. In other cases it may have worked the other way round.
+
+In the interesting archaic animal called _Peripatus_, which has to face
+a modern world too severe for it, one of the methods of meeting the
+environing difficulties is the retention of the offspring for many
+months within the mother, so that it is born a fully-formed creature.
+There are only a few offspring at a time, and, although there are
+exceptional cases like the summer green-flies, which are very prolific
+though viviparous, the general rule is that viviparity is associated
+with a very small family. The case of flowering plants stands by itself,
+for although they illustrate a kind of viviparity, the seed being
+embryos, an individual plant may have a large number of flowers and
+therefore a huge family.
+
+Viviparity naturally finds its best illustrations among terrestrial
+animals, where the risks to the young life are many, and it finds its
+climax among mammals.
+
+Now it is an interesting fact that the three lowest mammals, the
+Duckmole and two Spiny Ant-eaters, lay eggs, i.e. are oviparous; that
+the Marsupials, on the next grade, bring forth their young, as it were,
+prematurely, and in most cases stow them away in an external pouch;
+while all the others--the Placentals--show a more prolonged ante-natal
+life and an intimate partnership between the mother and the unborn
+young.
+
+
+§ 4
+
+There is another way of looking at the sublime process of evolution. It
+has implied a mastery of all the possible haunts of life; it has been a
+progressive conquest of the environment.
+
+1. It is highly probable that living organisms found their foothold in
+the stimulating conditions of the shore of the sea--the shallow water,
+brightly illumined, seaweed-growing shelf fringing the Continents. This
+littoral zone was a propitious environment where sea and fresh water,
+earth and air all meet, where there is stimulating change, abundant
+oxygenation and a copious supply of nutritive material in what the
+streams bring down and in the rich seaweed vegetation.
+
+[Illustration: THE HOATZIN INHABITS BRITISH GUIANA
+
+The newly hatched bird has claws on its thumb and first finger and so is
+enabled to climb on the branches of trees with great dexterity until
+such time as the wings are strong enough to sustain it in flight.]
+
+[Illustration: _Photograph, from the British Museum (Natural History),
+of a drawing by Mr. E. Wilson._
+
+PERIPATUS
+
+A widely distributed old-fashioned type of animal, somewhat like a
+permanent caterpillar. It has affinities both with worms and with
+insects. It has a velvety skin, minute diamond-like eyes, and short
+stump-like legs. A defenceless, weaponless animal, it comes out at
+night, and is said to capture small insects by squirting jets of slime
+from its mouth.]
+
+[Illustration: _Photo: W. S. Berridge, F.Z.S._
+
+ROCK KANGAROO CARRYING ITS YOUNG IN A POUCH
+
+The young are born so helpless that they cannot even suck. The mother
+places them in the external pouch, and fitting their mouths on the teats
+injects the milk. After a time the young ones go out and in as they
+please.]
+
+It is not an easy haunt of life, but none the worse for that, and it is
+tenanted to-day by representatives of practically every class of animals
+from infusorians to seashore birds and mammals.
+
+
+The Cradle of the Open Sea
+
+2. The open-sea or pelagic haunt includes all the brightly illumined
+surface waters beyond the shallow water of the shore area.
+
+It is perhaps the easiest of all the haunts of life, for there is no
+crowding, there is considerable uniformity, and an abundance of food for
+animals is afforded by the inexhaustible floating "sea-meadows" of
+microscopic Algæ. These are reincarnated in minute animals like the
+open-sea crustaceans, which again are utilised by fishes, these in turn
+making life possible for higher forms like carnivorous turtles and
+toothed whales. It is quite possible that the open sea was the original
+cradle of life and perhaps Professor Church is right in picturing a long
+period of pelagic life before there was any sufficiently shallow water
+to allow the floating plants to anchor. It is rather in favour of this
+view that many shore animals such as crabs and starfishes, spend their
+youthful stages in the relatively safe cradle of the open sea, and only
+return to the more strenuous conditions of their birthplace after they
+have gained considerable strength of body. It is probably safe to say
+that the honour of being the original cradle of life lies between the
+shore of the sea and the open sea.
+
+
+The Great Deeps
+
+3. A third haunt of life is the floor of the Deep Sea, the abyssal area,
+which occupies more than a half of the surface of the globe. It is a
+region of extreme cold--an eternal winter; of utter darkness--an eternal
+night--relieved only by the fitful gleams of "phosphorescent" animals;
+of enormous pressure--2-1/2 tons on the square inch at a depth of 2,500
+fathoms; of profound calm, unbroken silence, immense monotony. And as
+there are no plants in the great abysses, the animals must live on one
+another, and, in the long run, on the rain of moribund animalcules which
+sink from the surface through the miles of water. It seems a very
+unpromising haunt of life, but it is abundantly tenanted, and it gives
+us a glimpse of the insurgent nature of the living creature that the
+difficulties of the Deep Sea should have been so effectively conquered.
+It is probable that the colonising of the great abysses took place in
+relatively recent times, for the fauna does not include many very
+antique types. It is practically certain that the colonisation was due
+to littoral animals which followed the food-débris, millennium after
+millennium, further and further down the long slope from the shore.
+
+
+The Freshwaters
+
+4. A fourth haunt of life is that of the freshwaters, including river
+and lake, pond and pool, swamp and marsh. It may have been colonised by
+gradual migration up estuaries and rivers, or by more direct passage
+from the seashore into the brackish swamp. Or it may have been in some
+cases that partially landlocked corners of ancient seas became gradually
+turned into freshwater basins. The animal population of the freshwaters
+is very representative, and is diversely adapted to meet the
+characteristic contingencies--the risk of being dried up, the risk of
+being frozen hard in winter, and the risk of being left high and dry
+after floods or of being swept down to the sea.
+
+
+Conquest of the Dry Land
+
+5. The terrestrial haunt has been invaded age after age by contingents
+from the sea or from the freshwaters. We must recognise the worm
+invasion, which led eventually to the making of the fertile soil, the
+invasion due to air-breathing Arthropods, which led eventually to the
+important linkage between flowers and their insect visitors, and the
+invasion due to air-breathing Amphibians, which led eventually to the
+higher terrestrial animals and to the development of intelligence and
+family affection. Besides these three great invasions, there were minor
+ones such as that leading to land-snails, for there has been a
+widespread and persistent tendency among aquatic animals to try to
+possess the dry land.
+
+Getting on to dry land had a manifold significance.
+
+It implied getting into a medium with a much larger supply of oxygen
+than there is dissolved in the water. But the oxygen of the air is more
+difficult to capture, especially when the skin becomes hard or well
+protected, as it is almost bound to become in animals living on dry
+ground. Thus this leads to the development of _internal surfaces_, such
+as those of lungs, where the oxygen taken into the body may be absorbed
+by the blood. In most animals the blood goes to the surface of
+oxygen-capture; but in insects and their relatives there is a different
+idea--of taking the air to the blood or in greater part to the area of
+oxygen-combustion, the living tissues. A system of branching air-tubes
+takes air into every hole and corner of the insect's body, and this
+thorough aeration is doubtless in part the secret of the insect's
+intense activity. The blood never becomes impure.
+
+The conquest of the dry land also implied a predominance of that kind of
+locomotion which may be compared to punting, when the body is pushed
+along by pressing a lever against a hard substratum. And it also
+followed that with few exceptions the body of the terrestrial animal
+tended to be compact, readily lifted off the ground by the limbs or
+adjusted in some other way so that there may not be too large a surface
+trailing on the ground. An animal like a jellyfish, easily supported in
+the water, would be impossible on land. Such apparent exceptions as
+earthworms, centipedes, and snakes are not difficult to explain, for the
+earthworm is a burrower which eats its way through the soil, the
+centipede's long body is supported by numerous hard legs, and the snake
+pushes itself along by means of the large ventral scales to which the
+lower ends of very numerous ribs are attached.
+
+
+Methods of Mastering the Difficulties of Terrestrial Life
+
+A great restriction attendant on the invasion of the dry land is that
+locomotion becomes limited to one plane, namely, the surface of the
+earth. This is in great contrast to what is true in the water, where the
+animal can move up or down, to right or to left, at any angle and in
+three dimensions. It surely follows from this that the movements of land
+animals must be rapid and precise, unless, indeed, safety is secured in
+some other way. Hence it is easy to understand why most land animals
+have very finely developed striped muscles, and why a beetle running on
+the ground has far more numerous muscles than a lobster swimming in the
+sea.
+
+Land animals were also handicapped by the risks of drought and of frost,
+but these were met by defences of the most diverse description, from the
+hairs of woolly caterpillars to the fur of mammals, from the carapace of
+tortoises to the armour of armadillos. In other cases, it is hardly
+necessary to say, the difficulties may be met in other ways, as frogs
+meet the winter by falling into a lethargic state in some secluded
+retreat.
+
+Another consequence of getting on to dry land is that the eggs or young
+can no longer be set free anyhow, as is possible when the animal is
+surrounded by water, which is in itself more or less of a cradle. If the
+eggs were laid or the young liberated on dry ground, the chances are
+many that they would be dried up or devoured. So there are numerous ways
+in which land animals secure the safety of their young, e.g. by burying
+them in the ground, or by hiding them in nests, or by carrying them
+about for a prolonged period either before or after birth. This may mean
+great safety for the young, this may make it possible to have only a
+small family, and this may tend to the evolution of parental care and
+the kindly emotions. Thus it may be understood that from the conquest of
+the land many far-reaching consequences have followed.
+
+[Illustration: _Photo: Rischgitz._
+
+PROFESSOR THOMAS HENRY HUXLEY (1825-95)
+
+One of the most distinguished of zoologists, with unsurpassed gifts as a
+teacher and expositor. He did great service in gaining a place for
+science in ordinary education and in popular estimation. No one
+championed Evolutionism with more courage and skill.]
+
+[Illustration: BARON CUVIER, 1769-1832
+
+One of the founders of modern Comparative Anatomy. A man of gigantic
+intellect, who came to Paris as a youth from the provinces, and became
+the director of the higher education of France and a peer of the Empire.
+He was opposed to Evolutionist ideas, but he had anatomical genius.]
+
+[Illustration: AN ILLUSTRATION SHOWING VARIOUS METHODS OF FLYING AND
+SWOOPING
+
+Gull, with a feather-wing, a true flier. Fox-bat, with a skin-wing, a
+true flier. Flying Squirrel, with a parachute of skin, able to swoop
+from tree to tree, but not to fly. Flying Fish, with pectoral fins used
+as volplanes in a great leap due to the tail. To some extent able to
+sail in albatros fashion.]
+
+Finally, it is worth dwelling on the risks of terrestrial life, because
+they enable us better to understand why so many land animals have become
+burrowers and others climbers of trees, why some have returned to the
+water and others have taken to the air. It may be asked, perhaps, why
+the land should have been colonised at all when the risks and
+difficulties are so great. The answer must be that necessity and
+curiosity are the mother and father of invention. Animals left the water
+because the pools dried up, or because they were overcrowded, or because
+of inveterate enemies, but also because of that curiosity and spirit of
+adventure which, from first to last, has been one of the spurs of
+progress.
+
+
+Conquering the Air
+
+6. The last great haunt of life is the air, a mastery of which must be
+placed to the credit of insects, Pterodactyls, birds, and bats. These
+have been the successes, but it should be noted that there have been
+many brilliant failures, which have not attained to much more than
+parachuting. These include the Flying Fishes, which take leaps from the
+water and are carried for many yards and to considerable heights,
+holding their enlarged pectoral fins taut or with little more than a
+slight fluttering. There is a so-called Flying Frog (_Rhacophorus_) that
+skims from branch to branch, and the much more effective Flying Dragon
+(_Draco volans_) of the Far East, which has been mentioned already.
+Among mammals there are Flying Phalangers, Flying Lemurs, and more
+besides, all attaining to great skill as parachutists, and illustrating
+the endeavour to master the air which man has realised in a way of his
+own.
+
+The power of flight brings obvious advantages. A bird feeding on the
+ground is able to evade the stalking carnivore by suddenly rising into
+the air; food and water can be followed rapidly and to great distances;
+the eggs or the young can be placed in safe situations; and birds in
+their migrations have made a brilliant conquest both of time and space.
+Many of them know no winter in their year, and the migratory flight of
+the Pacific Golden Plover from Hawaii to Alaska and back again does not
+stand alone.
+
+
+THE PROCESSION OF LIFE THROUGH THE AGES
+
+§ 1
+
+The Rock Record
+
+How do we know when the various classes of animals and plants were
+established on the earth? How do we know the order of their appearance
+and the succession of their advances? The answer is: by reading the Rock
+Record. In the course of time the crust of the earth has been elevated
+into continents and depressed into ocean-troughs, and the surface of the
+land has been buckled up into mountain ranges and folded in gentler
+hills and valleys. The high places of the land have been weathered by
+air and water in many forms, and the results of the weathering have been
+borne away by rivers and seas, to be laid down again elsewhere as
+deposits which eventually formed sandstones, mudstones, and similar
+sedimentary rocks. Much of the material of the original crust has thus
+been broken down and worked up again many times over, and if the total
+thickness of the sedimentary rocks is added up it amounts, according to
+some geologists, to a total of 67 miles. In most cases, however, only a
+small part of this thickness is to be seen in one place, for the
+deposits were usually formed in limited areas at any one time.
+
+
+The Use of Fossils
+
+When the sediments were accumulating age after age, it naturally came
+about that remains of the plants and animals living at the time were
+buried, and these formed the fossils by the aid of which it is possible
+to read the story of the past. By careful piecing together of evidence
+the geologist is able to determine the order in which the different
+sedimentary rocks were laid down, and thus to say, for instance, that
+the Devonian period was the time of the origin of Amphibians. In other
+cases the geologist utilises the fossils in his attempt to work out the
+order of the strata when these have been much disarranged. For the
+simpler fossil forms of any type must be older than those that are more
+complex. There is no vicious circle here, for the general succession of
+strata is clear, and it is quite certain that there were fishes before
+there were amphibians, and amphibians before there were reptiles, and
+reptiles before there were birds and mammals. In certain cases, e.g. of
+fossil horses and elephants, the actual historical succession has been
+clearly worked out.
+
+If the successive strata contained good samples of all the plants and
+animals living at the time when the beds were formed, then it would be
+easy to read the record of the rocks, but many animals were too soft to
+become satisfactory fossils, many were eaten or dissolved away, many
+were destroyed by heat and pressure, so that the rock record is like a
+library very much damaged by fire and looting and decay.
+
+
+§ 2
+
+The Geological Time-table
+
+The long history of the earth and its inhabitants is conveniently
+divided into eras. Thus, just as we speak of the ancient, mediæval, and
+modern history of mankind, so we may speak of Palæozoic, Mesozoic and
+Cenozoic eras in the history of the earth as a whole.
+
+Geologists cannot tell us except in an approximate way how long the
+process of evolution has taken. One of the methods is to estimate how
+long has been required for the accumulation of the salts of the sea,
+for all these have been dissolved out of the rocks since rain began to
+fall on the earth. Dividing the total amount of saline matter by what is
+contributed every year in modern times, we get about a hundred million
+years as the age of the sea. But as the present rate of
+salt-accumulation is probably much greater than it was during many of
+the geological periods, the prodigious age just mentioned is in all
+likelihood far below the mark. Another method is to calculate how long
+it would take to form the sedimentary rocks, like sandstones and
+mudstones, which have a _total_ thickness of over fifty miles, though
+the _local_ thickness is rarely over a mile. As most of the materials
+have come from the weathering of the earth's crust, and as the annual
+amount of weathering now going on can be estimated, the time required
+for the formation of the sedimentary rocks of the world can be
+approximately calculated. There are some other ways of trying to tell
+the earth's age and the length of the successive periods, but no
+certainty has been reached.
+
+The eras marked on the table (page 92) as _before the Cambrian_
+correspond to about thirty-two miles of thickness of strata; and all the
+subsequent eras with fossil-bearing rocks to a thickness of about
+twenty-one miles--in itself an astounding fact. Perhaps thirty million
+years must be allotted to the Pre-Cambrian eras, eighteen to the
+Palæozoic, nine to the Mesozoic, three to the Cenozoic, making a grand
+total of sixty millions.
+
+
+The Establishment of Invertebrate Stocks
+
+It is an astounding fact that at least half of geological time (the
+Archæozoic and Proterozoic eras) passed before there were living
+creatures with parts sufficiently hard to form fossils. In the latter
+part of the Proterozoic era there are traces of one-celled marine
+animals (Radiolarians) with shells of flint, and of worms that wallowed
+in the primal mud. It is plain that as regards the most primitive
+creatures the rock record tells us little.
+
+[Illustration: _From Knipe's "Nebula to Man."_
+
+ANIMALS OF THE CAMBRIAN PERIOD e.g. Sponges, Jellyfish, Starfish,
+Sea-lilies, Water-fleas, and Trilobites]
+
+[Illustration: _Photo: J. J. Ward, F.E.S._
+
+A TRILOBITE
+
+Trilobites were ancient seashore animals, abundant from the Upper
+Cambrian to the Carboniferous eras. They have no direct descendants
+to-day. They were jointed-footed animals, allied to Crustaceans and
+perhaps also to King-crabs. They were able to roll themselves up in
+their ring-armour.]
+
+[Illustration: _Photo: British Museum (Natural History)._
+
+THE GAMBIAN MUD-FISH, PROTOPTERUS
+
+It can breathe oxygen dissolved in water by its gills; it can also
+breathe dry air by means of its swim-bladder, which has become a lung.
+It is a _double-breather_, showing evolution in process. For seven
+months of the year, the dry season, it can remain inert in the mud,
+getting air through an open pipe to the surface. When water fills the
+pools it can use its gills again. Mud-nests or mud encasements with the
+lung-fish inside have often been brought to Britain and the fish when
+liberated were quite lively.]
+
+[Illustration: THE ARCHÆOPTERYX
+
+(_After William Leche of Stockholm._)
+
+A good restoration of the oldest known bird, Archæopteryx (Jurassic
+Era). It was about the size of a crow; it had teeth on both jaws; it had
+claws on the thumb and two fingers; and it had a long lizard-like tail.
+But it had feathers, proving itself a true bird.]
+
+[Illustration: WING OF A BIRD, SHOWING THE ARRANGEMENT OF THE FEATHERS
+
+The longest feathers or primaries (PR) are borne by the two fingers (2
+and 3), and their palm-bones (CMC); the second longest or secondaries
+are borne by the ulna bone (U) of the fore-arm; there is a separate tuft
+(AS) on the thumb (TH).]
+
+The rarity of direct traces of life in the oldest rocks is partly due to
+the fact that the primitive animals would be of delicate build, but it
+must also be remembered that the ancient rocks have been profoundly and
+repeatedly changed by pressure and heat, so that the traces which did
+exist would be very liable to obliteration. And if it be asked what
+right we have to suppose the presence of living creatures in the absence
+or extreme rarity of fossils, we must point to great accumulations of
+limestone which indicate the existence of calcareous algæ, and to
+deposits of iron which probably indicate the activity of iron-forming
+Bacteria. Ancient beds of graphite similarly suggest that green plants
+flourished in these ancient days.
+
+
+§ 3
+
+The Era of Ancient Life (Palæozoic)
+
+The _Cambrian_ period was the time of the establishment of the chief
+stocks of backboneless animals such as sponges, jellyfishes, worms,
+sea-cucumbers, lamp-shells, trilobites, crustaceans, and molluscs. There
+is something very eloquent in the broad fact that the peopling of the
+seas had definitely begun some thirty million years ago, for Professor
+H. F. Osborn points out that in the Cambrian period there was already a
+colonisation of the shore of the sea, the open sea, and the deep waters.
+
+The _Ordovician_ period was marked by abundant representation of the
+once very successful class of Trilobites--jointed-footed,
+antenna-bearing, segmented marine animals, with numerous appendages and
+a covering of chitin. They died away entirely with the end of the
+Palæozoic era. Also very notable was the abundance of predatory
+cuttlefishes, the bullies of the ancient seas. But it was in this period
+that the first backboned animals made their appearance--an epoch-making
+step in evolution. In other words, true fishes were evolved--destined in
+the course of ages to replace the cuttlefishes (which are mere molluscs)
+in dominating the seas.
+
+ _______________________________________________________________________
+
+ _RECENT TIMES_ Human civilisation.
+ _______________________________________________________________________
+
+ {PLEISTOCENE OR GLACIAL TIME Last great Ice Age.
+ _CENOZOIC ERA_ {MIOCENE AND PLIOCENE TIMES Emergence of Man.
+ {EOCENE AND OLIGOCENE TIMES Rise of higher mammals.
+ _______________________________________________________________________
+
+ {CRETACEOUS PERIOD Rise of primitive mammals,
+ { flowering plants,
+ { and higher insects.
+ _MESOZOIC ERA_ {JURASSIC PERIOD Rise of birds and flying
+ { reptiles.
+ {TRIASSIC PERIOD Rise of dinosaur reptiles.
+ _______________________________________________________________________
+
+ {PERMIAN PERIOD Rise of reptiles.
+ {CARBONIFEROUS PERIOD Rise of insects.
+ {DEVONIAN PERIOD First amphibians.
+ _PALÆOZOIC ERA_ {SILURIAN PERIOD Land animals began.
+ {ORDOVICIAN PERIOD First fishes.
+ {CAMBRIAN PERIOD Peopling of the sea.
+ _______________________________________________________________________
+
+ _PROTEROZOIC AGES_ Many of the Backboneless stocks began.
+ _ARCHÆOZOIC AGES_ Living creatures began to be upon the earth.
+ _______________________________________________________________________
+
+ {Making of continents and ocean-basins.
+ {Beginnings of atmosphere and hydrosphere.
+ _FORMATIVE TIMES_ {Cooling of the earth.
+ {Establishment of the solar system.
+ _______________________________________________________________________
+
+In the _Silurian_ period in which the peopling of the seas went on
+apace, there was the first known attempt at colonising the dry land. For
+in Silurian rocks there are fossil scorpions, and that implies ability
+to breathe dry air--by means of internal surfaces, in this case known as
+lungbooks. It was also towards the end of the Silurian, when a period of
+great aridity set in, that fishes appeared related to our mud-fishes or
+double-breathers (Dipnoi), which have lungs as well as gills. This,
+again, meant utilising dry air, just as the present-day mud-fishes do
+when the water disappears from the pools in hot weather. The lung-fishes
+or mud-fishes of to-day are but three in number, one in Queensland, one
+in South America, and one in Africa, but they are extremely
+interesting "living fossils," binding the class of fishes to that of
+amphibians. It is highly probable that the first invasion of the dry
+land should be put to the credit of some adventurous worms, but the
+second great invasion was certainly due to air-breathing Arthropods,
+like the pioneer scorpion we mentioned.
+
+[Illustration: PICTORIAL REPRESENTATION OF THE SUCCESSIVE STRATA OF THE
+EARTH'S CRUST, WITH SUGGESTIONS OF CHARACTERISTIC FOSSILS
+
+E.g. Fish and Trilobite in the Devonian (red), a large Amphibian in the
+Carboniferous (blue), Reptiles in Permian (light red), the first Mammal
+in the Triassic (blue), the first Bird in the Jurassic (yellow), Giant
+Reptiles in the Cretaceous (white), then follow the Tertiary strata with
+progressive mammals, and Quaternary at the top with man and mammoth.]
+
+The _Devonian_ period, including that of the Old Red Sandstone, was one
+of the most significant periods in the earth's history. For it was the
+time of the establishment of flowering plants upon the earth and of
+terrestrial backboned animals. One would like to have been the
+discoverer of the Devonian foot-print of _Thinopus_, the first known
+Amphibian foot-print--an eloquent vestige of the third great invasion of
+the dry land. It was probably from a stock of Devonian lung-fishes that
+the first Amphibians sprang, but it was not till the next period that
+they came to their own. While they were still feeling their way, there
+was a remarkable exuberance of shark-like and heavily armoured fishes in
+the Devonian seas.
+
+
+EVOLUTION OF LAND ANIMALS
+
+§ 1
+
+Giant Amphibians and Coal-measures
+
+The _Carboniferous_ period was marked by a mild moist climate and a
+luxuriant vegetation in the swampy low grounds. It was a much less
+strenuous time than the Devonian period; it was like a very long summer.
+There were no trees of the type we see now, but there were forests of
+club-mosses and horsetails which grew to a gigantic size compared with
+their pigmy representatives of to-day. In these forests the
+jointed-footed invaders of the dry land ran riot in the form of
+centipedes, spiders, scorpions, and insects, and on these the primeval
+Amphibians fed. The appearance of insects made possible a new linkage of
+far-reaching importance, namely, the cross-fertilisation of flowering
+plants by their insect visitors, and from this time onwards it may be
+said that flowers and their visitors have evolved hand in hand.
+Cross-fertilisation is much surer by insects than by the wind, and
+cross-fertilisation is more advantageous than self-fertilisation because
+it promotes both fertility and plasticity. It was probably in this
+period that _coloured_ flowers--attractive to insect-visitors--began to
+justify themselves as beauty became useful, and began to relieve the
+monotonous green of the horsetail and club-moss forests, which covered
+great tracts of the earth for millions of years. In the Carboniferous
+forests there were also land-snails, representing one of the minor
+invasions of the dry land, tending on the whole to check vegetation.
+They, too, were probably preyed upon by the Amphibians, some of which
+attained a large size. Each age has had its giants, and those of the
+Carboniferous were Amphibians called Labyrinthodonts, some of which were
+almost as big as donkeys. It need hardly be said that it was in this
+period that most of the Coal-measures were laid down by the immense
+accumulation of the spores and debris of the club-moss forests. Ages
+afterwards, it was given to man to tap this great source of
+energy--traceable back to the sunshine of millions of years ago. Even
+then it was true that no plant or animal lives or dies to itself!
+
+
+The Acquisitions of Amphibians.
+
+As Amphibians had their Golden Age in the Carboniferous period we may
+fitly use this opportunity of indicating the advances in evolution which
+the emergence of Amphibians implied. (1) In the first place the passage
+from water to dry land was the beginning of a higher and more promiseful
+life, taxed no doubt by increased difficulties. The natural question
+rises why animals should have migrated from water to dry land at all
+when great difficulties were involved in the transition. The answers
+must be: (_a_) that local drying up of water-basins or elevations of the
+land surface often made the old haunts untenable; (_b_) that there may
+have been great congestion and competition in the old quarters; and
+(_c_) that there has been an undeniable endeavour after well-being
+throughout the history of animal life. In the same way with mankind,
+migrations were prompted by the setting in of prolonged drought, by
+over-population, and by the spirit of adventure. (2) In Amphibians for
+the first time the non-digitate paired fins of fishes were replaced by
+limbs with fingers and toes. This implied an advantageous power of
+grasping, of holding firm, of putting food into the mouth, of feeling
+things in three dimensions. (3) We cannot be positive in regard to the
+soft parts of the ancient Amphibians known only as fossils, but if they
+were in a general way like the frogs and toads, newts and salamanders of
+the present day, we may say that they made among other acquisitions the
+following: true ventral lungs, a three-chambered heart, a movable
+tongue, a drum to the ear, and lids to the eyes. It is very interesting
+to find that though the tongue of the tadpole has some muscle-fibres in
+it, they are not strong enough to effect movement, recalling the tongue
+of fishes, which has not any muscles at all. Gradually, as the tadpole
+becomes a frog, the muscle-fibres grow in strength, and make it possible
+for the full-grown creature to shoot out its tongue upon insects. This
+is probably a recapitulation of what was accomplished in the course of
+millennia in the history of the Amphibian race. (4) Another acquisition
+made by Amphibians was a voice, due, as in ourselves, to the rapid
+passage of air over taut membranes (vocal cords) stretched in the
+larynx. It is an interesting fact that for millions of years there was
+upon the earth no sound of life at all, only the noise of wind and wave,
+thunder and avalanche. Apart from the instrumental music of some
+insects, perhaps beginning in the Carboniferous, the first vital sounds
+were due to Amphibians, and theirs certainly was the first voice--surely
+one of the great steps in organic evolution.
+
+[Illustration: _Photo: British Museum (Natural History)._
+
+FOSSIL OF A PTERODACTYL OR EXTINCT FLYING DRAGON
+
+The wing is made of a web of skin extended on the enormously elongated
+outermost finger. The long tail served for balancing and steering. The
+Pterodactyls varied from the size of sparrows to a wing-span of fifteen
+feet--the largest flying creatures.]
+
+[Illustration: _From Knipe's "Nebula to Man."_
+
+PARIASAURUS: AN EXTINCT VEGETARIAN TRIASSIC REPTILE
+
+Total length about 9 feet. (Remains found in Cape Colony, South
+Africa.)]
+
+[Illustration: _From Knipe's "Nebula to Man."_
+
+TRICERATOPS: A HUGE EXTINCT REPTILE
+
+(From remains found in Cretaceous strata of Wyoming, U.S.A.)
+
+This Dinosaur, about the size of a large rhinoceros, had a huge
+three-horned skull with a remarkable bony collar over the neck. But, as
+in many other cases, its brain was so small that it could have passed
+down the spinal canal in which the spinal cord lies. Perhaps this partly
+accounts for the extinction of giant reptiles.]
+
+[Illustration: _Photo: "Daily Mail."_
+
+THE DUCKMOLE OR DUCK-BILLED PLATYPUS OF AUSTRALIA
+
+The Duckmole or Duck-billed Platypus of Australia is a survivor of the
+most primitive mammals. It harks back to reptiles, e.g. in being an
+egg-layer, in having comparatively large eggs, and in being imperfectly
+warm-blooded. It swims well and feeds on small water-animals. It can
+also burrow.]
+
+
+Evolution of the Voice
+
+The first use of the voice was probably that indicated by our frogs and
+toads--it serves as a sex-call. That is the meaning of the trumpeting
+with which frogs herald the spring, and it is often only in the males
+that the voice is well developed. But if we look forward, past
+Amphibians altogether, we find the voice becoming a maternal call
+helping to secure the safety of the young--a use very obvious when young
+birds squat motionless at the sound of the parent's danger-note. Later
+on, probably, the voice became an infantile call, as when the unhatched
+crocodile pipes from within the deeply buried egg, signalling to the
+mother that it is time to be unearthed. Higher still the voice expresses
+emotion, as in the song of birds, often outside the limits of the
+breeding time. Later still, particular sounds become words, signifying
+particular things or feelings, such as "food," "danger," "home,"
+"anger," and "joy." Finally words become a medium of social intercourse
+and as symbols help to make it possible for man to reason.
+
+
+§ 2
+
+The Early Reptiles
+
+In the _Permian_ period reptiles appeared, or perhaps one should say,
+began to assert themselves. That is to say, there was an emergence of
+backboned animals which were free from water and relinquished the method
+of breathing by gills, which Amphibians retained in their young stages
+at least. The unhatched or unborn reptile breathes by means of a
+vascular hood spread underneath the egg-shell and absorbing dry air from
+without. It is an interesting point that this vascular hood, called the
+allantois, is represented in the Amphibians by an unimportant bladder
+growing out from the hind end of the food-canal. A great step in
+evolution was implied in the origin of this ante-natal hood or foetal
+membrane and another one--of protective significance--called the amnion,
+which forms a water-bag over the delicate embryo. The step meant total
+emancipation from the water and from gill-breathing, and the two
+foetal membranes, the amnion and the allantois, persist not only in
+all reptiles but in birds and mammals as well. These higher Vertebrates
+are therefore called Amniota in contrast to the Lower Vertebrates or
+Anamnia (the Amphibians, Fishes, and primitive types).
+
+It is a suggestive fact that the embryos of all reptiles, birds, and
+mammals show gill-clefts--_a tell-tale evidence of their distant aquatic
+ancestry_. But these embryonic gill-clefts are not used for respiration
+and show no trace of gills except in a few embryonic reptiles and birds
+where their dwindled vestiges have been recently discovered. As to the
+gill-clefts, they are of no use in higher Vertebrates except that the
+first becomes the Eustachian tube leading from the ear-passage to the
+back of the mouth. The reason why they persist when only one is of any
+use, and that in a transformed guise, would be difficult to interpret
+except in terms of the Evolution theory. They illustrate the lingering
+influence of a long pedigree, the living hand of the past, the tendency
+that individual development has to recapitulate racial evolution. In a
+condensed and telescoped manner, of course, for what took the race a
+million years may be recapitulated by the individual in a week!
+
+In the Permian period the warm moist climate of most of the
+Carboniferous period was replaced by severe conditions, culminating in
+an Ice Age which spread from the Southern Hemisphere throughout the
+world. With this was associated a waning of the Carboniferous flora, and
+the appearance of a new one, consisting of ferns, conifers, ginkgos, and
+cycads, which persisted until near the end of the Mesozoic era. The
+Permian Ice Age lasted for millions of years, and was most severe in the
+Far South. Of course, it was a very different world then, for North
+Europe was joined to North America, Africa to South America, and
+Australia to Asia. It was probably during the Permian Ice Age that many
+of the insects divided their life-history into two main chapters--the
+feeding, growing, moulting, immature, larval stages, e.g. caterpillars,
+and the more ascetic, non-growing, non-moulting, winged phase, adapted
+for reproduction. Between these there intervened the quiescent,
+well-protected pupa stage or chrysalis, probably adapted to begin with
+as a means of surviving the severe winter. For it is easier for an
+animal to survive when the vital processes are more or less in abeyance.
+
+
+Disappearance of many Ancient Types
+
+We cannot leave the last period of the Palæozoic era and its prolonged
+ice age without noticing that it meant the entire cessation of a large
+number of ancient types, especially among plants and backboneless
+animals, which now disappear for ever. It is necessary to understand
+that the animals of ancient days stand in three different relations to
+those of to-day. (_a_) There are ancient types that have living
+representatives, sometimes few and sometimes many, sometimes much
+changed and sometimes but slightly changed. The lamp-shell,
+_Lingulella_, of the Cambrian and Ordovician period has a very near
+relative in the _Lingula_ of to-day. There are a few extremely
+conservative animals. (_b_) There are ancient types which have no living
+representatives, except in the guise of transformed descendants, as the
+King-crab (_Limulus_) may be said to be a transformed descendant of the
+otherwise quite extinct race to which Eurypterids or Sea-scorpions
+belonged. (_c_) There are altogether extinct types--_lost races_--which
+have left not a wrack behind. For there is not any representation to-day
+of such races as Graptolites and Trilobites.
+
+Looking backwards over the many millions of years comprised in the
+Palæozoic era, what may we emphasise as the most salient features? There
+was in the _Cambrian_ the establishment of the chief classes of
+backboneless animals; in the _Ordovician_ the first fishes and perhaps
+the first terrestrial plants; in the _Silurian_ the emergence of
+air-breathing Invertebrates and mud-fishes; in the _Devonian_ the
+appearance of the first Amphibians, from which all higher land animals
+are descended, and the establishment of a land flora; in the
+_Carboniferous_ the great Club-moss forests and an exuberance of
+air-breathing insects and their allies; in the _Permian_ the first
+reptiles and a new flora.
+
+
+THE GEOLOGICAL MIDDLE AGES
+
+§ 1
+
+The Mesozoic Era
+
+In a broad way the Mesozoic era corresponds with the Golden Age of
+reptiles, and with the climax of the Conifer and Cycad flora, which was
+established in the Permian. But among the Conifers and Cycads our modern
+flowering plants were beginning to show face tentatively, just like
+birds and mammals among the great reptiles.
+
+In the _Triassic_ period the exuberance of reptilian life which marked
+the Permian was continued. Besides Turtles which still persist, there
+were Ichthyosaurs, Plesiosaurs, Dinosaurs, and Pterosaurs, none of which
+lasted beyond the Mesozoic era. Of great importance was the rise of the
+Dinosaurs in the Triassic, for it is highly probable that within the
+limits of this vigorous and plastic stock--some of them bipeds--we must
+look for the ancestors of both birds and mammals. Both land and water
+were dominated by reptiles, some of which attained to gigantic size. Had
+there been any zoologist in those days, he would have been very
+sagacious indeed if he had suspected that reptiles did not represent the
+climax of creation.
+
+
+The Flying Dragons
+
+The _Jurassic_ period showed a continuance of the reptilian splendour.
+They radiated in many directions, becoming adapted to many haunts. Thus
+there were many Fish Lizards paddling in the seas, many types of
+terrestrial dragons stalking about on land, many swiftly gliding
+alligator-like forms, and the Flying Dragons which began in the Triassic
+attained to remarkable success and variety. Their wing was formed by the
+extension of a great fold of skin on the enormously elongated outermost
+finger, and they varied from the size of a sparrow to a spread of over
+five feet. A soldering of the dorsal vertebræ as in our Flying Birds was
+an adaptation to striking the air with some force, but as there is not
+more than a slight keel, if any, on the breast-bone, it is unlikely that
+they could fly far. For we know from our modern birds that the power of
+flight may be to some extent gauged from the degree of development of
+the keel, which is simply a great ridge for the better insertion of the
+muscles of flight. It is absent, of course, in the Running Birds, like
+the ostrich, and it has degenerated in an interesting way in the
+burrowing parrot (_Stringops_) and a few other birds that have "gone
+back."
+
+
+The First Known Bird
+
+But the Jurassic is particularly memorable because its strata have
+yielded two fine specimens of the first known bird, _Archæopteryx_.
+These were entombed in the deposits which formed the fine-grained
+lithographic stones of Bavaria, and practically every bone in the body
+is preserved except the breast-bone. Even the feathers have left their
+marks with distinctness. This oldest known bird--too far advanced to be
+the first bird--was about the size of a crow and was probably of
+arboreal habits. Of great interest are its reptilian features, so
+pronounced that one cannot evade the evolutionist suggestion. It had
+teeth in both jaws, which no modern bird has; it had a long lizard-like
+tail, which no modern bird has; it had claws on three fingers, and a
+sort of half-made wing. That is to say, it does not show, what all
+modern birds show, a fusion of half the wrist-bones with the whole of
+the palm-bones, the well-known carpo-metacarpus bone which forms a basis
+for the longest pinions. In many reptiles, such as Crocodiles, there are
+peculiar bones running across the abdomen beneath the skin, the
+so-called "abdominal ribs," and it seems an eloquent detail to find
+these represented in _Archæopteryx_, the earliest known bird. No modern
+bird shows any trace of them. [Illustration: SKELETON OF AN EXTINCT
+FLIGHTLESS TOOTHED BIRD, HESPERORNIS
+
+(_After Marsh._)
+
+The bird was five or six feet high, something like a swimming ostrich,
+with a very powerful leg but only a vestige of a wing. There were sharp
+teeth in a groove. The modern divers come nearest to this ancient
+type.]
+
+[Illustration: SIX STAGES IN THE EVOLUTION OF THE HORSE, SHOWING GRADUAL
+INCREASE IN SIZE
+
+(_After Lull and Matthew._)
+
+1. Four-toed horse, Eohippus, about one foot high. Lower Eocene, N.
+America.
+
+2. Another four-toed horse, Orohippus, a little over a foot high. Middle
+Eocene, N. America.
+
+3. Three-toed horse, Mesohippus, about the size of a sheep. Middle
+Oligocene, N. America.
+
+4. Three-toed horse, Merychippus, Miocene, N. America. Only one toe
+reaches the ground on each foot, but the remains of two others are
+prominent.
+
+5. The first one-toed horse, Pliohippus, about forty inches high at the
+shoulder. Pliocene, N. America.
+
+6. The modern horse, running on the third digit of each foot.]
+
+There is no warrant for supposing that the flying reptiles or
+Pterodactyls gave rise to birds, for the two groups are on different
+lines, and the structure of the wings is entirely different. Thus the
+long-fingered Pterodactyl wing was a parachute wing, while the secret of
+the bird's wing has its centre in the feathers. It is highly probable
+that birds evolved from certain Dinosaurs which had become bipeds, and
+it is possible that they were for a time swift runners that took "flying
+jumps" along the ground. Thereafter, perhaps, came a period of arboreal
+apprenticeship during which there was much gliding from tree to tree
+before true flight was achieved. It is an interesting fact that the
+problem of flight has been solved four times among animals--by insects,
+by Pterodactyls, by birds, and by bats; and that the four solutions are
+on entirely different lines.
+
+In the _Cretaceous_ period the outstanding events included the waning of
+giant reptiles, the modernising of the flowering plants, and the
+multiplication of small mammals. Some of the Permian reptiles, such as
+the dog-toothed Cynodonts, were extraordinarily mammal-like, and it was
+probably from among them that definite mammals emerged in the Triassic.
+Comparatively little is known of the early Triassic mammals save that
+their back-teeth were marked by numerous tubercles on the crown, but
+they were gaining strength in the late Triassic when small arboreal
+insectivores, not very distant from the modern tree-shrews (_Tupaia_),
+began to branch out in many directions indicative of the great divisions
+of modern mammals, such as the clawed mammals, hoofed mammals, and the
+race of monkeys or Primates. In the Upper Cretaceous there was an
+exuberant "radiation" of mammals, adaptive to the conquest of all sorts
+of haunts, and this was vigorously continued in Tertiary times.
+
+There is no difficulty in the fact that the earliest remains of definite
+mammals in the Triassic precede the first-known bird in the Jurassic.
+For although we usually rank mammals as higher than birds (being mammals
+ourselves, how could we do otherwise?), there are many ways in which
+birds are pre-eminent, e.g. in skeleton, musculature, integumentary
+structures, and respiratory system. The fact is that birds and mammals
+are on two quite different tacks of evolution, not related to one
+another, save in having a common ancestry in extinct reptiles. Moreover,
+there is no reason to believe that the Jurassic _Archæopteryx_ was the
+first bird in any sense except that it is the first of which we have any
+record. In any case it is safe to say that birds came to their own
+before mammals did.
+
+Looking backwards, we may perhaps sum up what is most essential in the
+Mesozoic era in Professor Schuchert's sentence: "The Mesozoic is the Age
+of Reptiles, and yet the little mammals and the toothed birds are
+storing up intelligence and strength to replace the reptiles when the
+cycads and conifers shall give way to the higher flowering plants."
+
+
+§ 2
+
+The Cenozoic or Tertiary Era
+
+In the _Eocene_ period there was a replacement of the small-brained
+archaic mammals by big-brained modernised types, and with this must be
+associated the covering of the earth with a garment of grass and dry
+pasture. Marshes were replaced by meadows and browsing by grazing
+mammals. In the spreading meadows an opportunity was also offered for a
+richer evolution of insects and birds.
+
+During the _Oligocene_ the elevation of the land continued, the climate
+became much less moist, and the grazing herds extended their range.
+
+The _Miocene_ was the mammalian Golden Age and there were crowning
+examples of what Osborn calls "adaptive radiation." That is to say,
+mammals, like the reptiles before them, conquer every haunt of life.
+There are flying bats, volplaning parachutists, climbers in trees like
+sloths and squirrels, quickly moving hoofed mammals, burrowers like the
+moles, freshwater mammals, like duckmole and beaver, shore-frequenting
+seals and manatees, and open-sea cetaceans, some of which dive far more
+than full fathoms five. It is important to realise the perennial
+tendency of animals to conquer every corner and to fill every niche of
+opportunity, and to notice that this has been done by successive sets of
+animals in succeeding ages. _Most notably the mammals repeat all the
+experiments of reptiles on a higher turn of the spiral._ Thus arises
+what is called convergence, the superficial resemblance of unrelated
+types, like whales and fishes, the resemblance being due to the fact
+that the different types are similarly adapted to similar conditions of
+life. Professor H. F. Osborn points out that mammals may seek any one of
+the twelve different habitat-zones, and that in each of these there may
+be six quite different kinds of food. Living creatures penetrate
+everywhere like the overflowing waters of a great river in flood.
+
+
+§ 3
+
+The _Pliocene_ period was a more strenuous time, with less genial
+climatic conditions, and with more intense competition. Old land bridges
+were broken and new ones made, and the geographical distribution
+underwent great changes. Professor R. S. Lull describes the _Pliocene_
+as "a period of great unrest." "Many migrations occurred the world over,
+new competitions arose, and the weaker stocks began to show the effects
+of the strenuous life. One momentous event seems to have occurred in the
+Pliocene, and that was the transformation of the precursor of humanity
+into man--the culmination of the highest line of evolution."
+
+The _Pleistocene_ period was a time of sifting. There was a continued
+elevation of the continental masses, and Ice Ages set in, relieved by
+less severe interglacial times when the ice-sheets retreated northwards
+for a time. Many types, like the mammoth, the woolly rhinoceros, the
+sabre-toothed tiger, the cave-lion, and the cave-bear, became extinct.
+Others which formerly had a wide range became restricted to the Far
+North or were left isolated here and there on the high mountains, like
+the Snow Mouse, which now occurs on isolated Alpine heights above the
+snow-line. Perhaps it was during this period that many birds of the
+Northern Hemisphere learned to evade the winter by the sublime device of
+migration.
+
+Looking backwards we may quote Professor Schuchert again:
+
+ "The lands in the Cenozoic began to bloom with more and more
+ flowering plants and grand hardwood forests, the atmosphere is
+ scented with sweet odours, a vast crowd of new kinds of insects
+ appear, and the places of the once dominant reptiles of the lands
+ and seas are taken by the mammals. Out of these struggles there
+ rises a greater intelligence, seen in nearly all of the mammal
+ stocks, but particularly in one, the monkey-ape-man. Brute man
+ appears on the scene with the introduction of the last glacial
+ climate, a most trying time for all things endowed with life, and
+ finally there results the dominance of reasoning man over all his
+ brute associates."
+
+In man and human society the story of evolution has its climax.
+
+
+The Ascent of Man
+
+Man stands apart from animals in his power of building up general ideas
+and of using these in the guidance of his behaviour and the control of
+his conduct. This is essentially wrapped up with his development of
+language as an instrument of thought. Some animals have words, but man
+has language (Logos). Some animals show evidence of _perceptual_
+inference, but man often gets beyond this to _conceptual_ inference
+(Reason). Many animals are affectionate and brave, self-forgetful and
+industrious, but man "thinks the ought," definitely guiding his conduct
+in the light of ideals, which in turn are wrapped up with the fact that
+he is "a social person."
+
+Besides his big brain, which may be three times as heavy as that of a
+gorilla, man has various physical peculiarities. He walks erect, he
+plants the sole of his foot flat on the ground, he has a chin and a good
+heel, a big forehead and a non-protrusive face, a relatively uniform set
+of teeth without conspicuous canines, and a relatively naked body.
+
+[Illustration: DIAGRAM SHOWING SEVEN STAGES IN THE EVOLUTION OF THE
+FORE-LIMBS AND HIND-LIMBS OF THE ANCESTORS OF THE MODERN HORSE,
+BEGINNING WITH THE EARLIEST KNOWN PREDECESSORS OF THE HORSE AND
+CULMINATING WITH THE HORSE OF TO-DAY
+
+(_After Marsh and Lull._)
+
+1 and 1A, fore-limb and hind-limb of Eohippus; 2 and 2A, Orohippus; 3
+and 3A, Mesohippus; 4 and 4A, Hypohippus; 5 and 5A, Merychippus; 6 and
+6A, Hipparion; 7 and 7A, the modern horse. Note how the toes shorten and
+disappear.]
+
+[Illustration: A. Fore-limb of Monkey B. Fore-limb of Whale
+
+WHAT IS MEANT BY HOMOLOGY? ESSENTIAL SIMILARITY OF ARCHITECTURE, THOUGH
+THE APPEARANCES MAY BE VERY DIFFERENT
+
+This is seen in comparing these two fore-limbs, A, of Monkey, B, of
+Whale. They are as different as possible, yet they show the same bones,
+e.g. SC, the scapula or shoulder-blade; H, the humerus or upper arm; R
+and U, the radius and ulna of the fore-arm; CA, the wrist; MC, the palm;
+and then the fingers.]
+
+But in spite of man's undeniable apartness, there is no doubt as to his
+solidarity with the rest of creation. There is an "all-pervading
+similitude of structure," between man and the Anthropoid Apes, though it
+is certain that it is not from any living form that he took his origin.
+None of the anatomical distinctions, except the heavy brain, could be
+called momentous. Man's body is a veritable museum of relics (vestigial
+structures) inherited from pre-human ancestors. In his everyday bodily
+life and in some of its disturbances, man's pedigree is often revealed.
+Even his facial expression, as Darwin showed, is not always human. Some
+fossil remains bring modern man nearer the anthropoid type.
+
+It is difficult not to admit the ring of truth in the closing words of
+Darwin's _Descent of Man_:
+
+ "We must, however, acknowledge, as it seems to me, that man, with
+ all his noble qualities, with sympathy which feels for the most
+ debased, with benevolence which extends not only to other men but to
+ the humblest living creature, with his God-like intellect which has
+ penetrated into the movements and constitution of the solar
+ system--with all these exalted powers--man still bears in his bodily
+ frame the indelible stamp of his lowly origin."
+
+
+THE EVOLVING SYSTEM OF NATURE
+
+There is another side of evolution so obvious that it is often
+overlooked, the tendency to link lives together in vital
+inter-relations. Thus flowers and their insect visitors are often
+vitally interlinked in mutual dependence. Many birds feed on berries and
+distribute the seeds. The tiny freshwater snail is the host of the
+juvenile stages of the liver-fluke of the sheep. The mosquito is the
+vehicle of malaria from man to man, and the tse-tse fly spreads sleeping
+sickness. The freshwater mussel cannot continue its race without the
+unconscious co-operation of the minnow, and the freshwater fish called
+the bitterling cannot continue its race without the unconscious
+co-operation of the mussel. There are numerous mutually beneficial
+partnerships between different kinds of creatures, and other
+inter-relations where the benefit is one-sided, as in the case of
+insects that make galls on plants. There are also among kindred animals
+many forms of colonies, communities, and societies. Nutritive chains
+bind long series of animals together, the cod feeding on the whelk, the
+whelk on the worm, the worm on the organic dust of the sea. There is a
+system of successive incarnations and matter is continually passing from
+one embodiment to another. These instances must suffice to illustrate
+the central biological idea of the web of life, the interlinked System
+of Animate Nature. Linnæus spoke of the Systema Naturæ, meaning the
+orderly hierarchy of classes, orders, families, genera, and species; but
+we owe to Darwin in particular some knowledge of a more dynamic Systema
+Naturæ, the network of vital inter-relations. This has become more and
+more complex as evolution has continued, and man's web is most complex
+of all. It means making Animate Nature more of a unity; it means an
+external method of registering steps of progress; it means an evolving
+set of sieves by which new variations are sifted, and living creatures
+are kept from slipping down the steep ladder of evolution.
+
+
+Parasitism
+
+It sometimes happens that the inter-relation established between one
+living creature and another works in a retrograde direction. This is the
+case with many thoroughgoing internal parasites which have sunk into an
+easygoing kind of life, utterly dependent on their host for food,
+requiring no exertions, running no risks, and receiving no spur to
+effort. Thus we see that evolution is not necessarily progressive;
+everything depends on the conditions in reference to which the living
+creatures have been evolved. When the conditions are too easygoing, the
+animal may be thoroughly well adapted to them--as a tapeworm certainly
+is--but it slips down the rungs of the ladder of evolution.
+
+This is an interesting minor chapter in the story of evolution--the
+establishment of different kinds of parasites, casual and constant,
+temporary and lifelong, external hangers-on and internal unpaying
+boarders, those that live in the food-canal and depend on the host's
+food and those that inhabit the blood or the tissues and find their food
+there. It seems clear that ichneumon grubs and the like which hatch
+inside a caterpillar and eat it alive are not so much parasites as
+"beasts of prey" working from within.
+
+But there are two sides to this minor chapter: there is the evolution of
+the parasite, and there is also the evolution of counteractive measures
+on the part of the host. Thus there is the maintenance of a bodyguard of
+wandering amoeboid cells, which tackle the microbes invading the body
+and often succeed in overpowering and digesting them. Thus, again, there
+is the protective capacity the blood has of making antagonistic
+substances or "anti-bodies" which counteract poisons, including the
+poisons which the intruding parasites often make.
+
+
+THE EVIDENCES OF EVOLUTION--HOW IT CAME ABOUT
+
+§ 1
+
+Progress in Evolution
+
+There has often been slipping back and degeneracy in the course of
+evolution, but the big fact is that there has been progress. For
+millions of years Life has been slowly creeping upwards, and if we
+compare the highest animals--Birds and Mammals--with their predecessors,
+we must admit that they are more controlled, more masters of their
+fate, with more mentality. Evolution is on the whole _integrative_; that
+is to say, it makes against instability and disorder, and towards
+harmony and progress. Even in the rise of Birds and Mammals we can
+discern that the evolutionary process was making towards a fuller
+embodiment or expression of what Man values most--control, freedom,
+understanding, and love. The advance of animal life through the ages has
+been chequered, but on the whole it has been an advance towards
+increasing fullness, freedom, and fitness of life. In the study of this
+advance--the central fact of Organic Evolution--there is assuredly much
+for Man's instruction and much for his encouragement.
+
+
+Evidences of Evolution
+
+In all this, it may be said, the fact of evolution has been taken for
+granted, but what are the evidences? Perhaps it should be frankly
+answered that the idea of evolution, that the present is the child of
+the past and the parent of the future, cannot be _proved_ as one may
+prove the Law of Gravitation. All that can be done is to show that it is
+a key--a way of looking at things--that fits the facts. There is no lock
+that it does not open.
+
+But if the facts that the evolution theory vividly interprets be called
+the evidences of its validity, there is no lack of them. There is
+_historical_ evidence; and what is more eloquent than the general fact
+that fishes emerge before amphibians, and these before reptiles, and
+these before birds, and so on? There are wonderfully complete fossil
+series, e.g. among cuttlefishes, in which we can almost see evolution in
+process. The pedigree of horse and elephant and crocodile is in general
+very convincing, though it is to be confessed that there are other cases
+in regard to which we have no light. Who can tell, for instance, how
+Vertebrates arose or from what origin?
+
+There is _embryological_ evidence, for the individual development often
+reads like an abbreviated recapitulation of the presumed evolution of
+the race. The mammal's visceral clefts are tell-tale evidence of remote
+aquatic ancestors, breathing by gills. Something is known in regard to
+the historical evolution of antlers in bygone ages; the Red Deer of
+to-day recapitulates at least the general outlines of the history. The
+individual development of an asymmetrical flat-fish, like a plaice or
+sole, which rests and swims on one side, tells us plainly that its
+ancestors were symmetrical fishes.
+
+There is what might be called _physiological_ evidence, for many plants
+and animals are variable before our eyes, and evolution is going on
+around us to-day. This is familiarly seen among domesticated animals and
+cultivated plants, but there is abundant flux in Wild Nature. It need
+hardly be said that some organisms are very conservative, and that
+change need not be expected when a position of stable equilibrium has
+been secured.
+
+There is also _anatomical_ evidence of a most convincing quality. In the
+fore-limbs of backboned animals, say, the paddle of a turtle, the wing
+of a bird, the flipper of a whale, the fore-leg of a horse, and the arm
+of a man; the same essential bones and muscles are used to such diverse
+results! What could it mean save blood relationship? And as to the two
+sets of teeth in whalebone whales, which never even cut the gum, is
+there any alternative but to regard them as relics of useful teeth which
+ancestral forms possessed? In short, the evolution theory is justified
+by the way in which it works.
+
+
+§ 2
+
+Factors in Evolution
+
+If it be said "So much for the _fact_ of evolution, but what of the
+_factors_?" the answer is not easy. For not only is the problem the
+greatest of all scientific problems, but the inquiry is still very
+young. The scientific study of evolution practically dates from the
+publication of _The Origin of Species_ in 1859.
+
+Heritable novelties or variations often crop up in living creatures, and
+these form the raw material of evolution. These variations are the
+outcome of expression of changes in the germ-cells that develop into
+organisms. But why should there be changes in the constitution of the
+germ-cells? Perhaps because the living material is very complex and
+inherently liable to change; perhaps because it is the vehicle of a
+multitude of hereditary items among which there are very likely to be
+reshufflings or rearrangements; perhaps because the germ-cells have very
+changeful surroundings (the blood, the body-cavity fluid, the
+sea-water); perhaps because deeply saturating outside influences, such
+as change of climate and habitat, penetrate through the body to its
+germ-cells and provoke them to vary. But we must be patient with the
+wearisome reiteration of "perhaps." Moreover, every many-celled organism
+reproduced in the usual way, arises from an egg-cell fertilised by a
+sperm-cell, and the changes involved in and preparatory to this
+fertilisation may make new permutations and combinations of the living
+items and hereditary qualities not only possible but necessary. It is
+something like shuffling a pack of cards, but the cards are living. As
+to the changes wrought on the body during its lifetime by peculiarities
+in nurture, habits, and surroundings, these dents or modifications are
+often very important for the individual, but it does not follow that
+they are directly important for the race, since it is not certain that
+they are transmissible.
+
+Given a crop of variations or new departures or mutations, whatever the
+inborn novelties may be called, we have then to inquire how these are
+sifted. The sifting, which means the elimination of the relatively less
+fit variations and the selection of the relatively more fit, effected in
+many different ways in the course of the struggle for existence. The
+organism plays its new card in the game of life, and the consequences
+may determine survival. The relatively less fit to given conditions
+will tend to be eliminated, while the relatively more fit will tend to
+survive. If the variations are hereditary and reappear, perhaps
+increased in amount, generation after generation, and if the process of
+sifting continue consistently, the result will be the evolution of the
+species. The sifting process may be helped by various forms of
+"isolation" which lessen the range of free intercrossing between members
+of a species, e.g. by geographical barriers. Interbreeding of similar
+forms tends to make a stable stock; out-breeding among dissimilars tends
+to promote variability. But for an outline like this it is enough to
+suggest the general method of organic evolution: Throughout the ages
+organisms have been making tentatives--new departures of varying
+magnitude--and these tentatives have been tested. The method is that of
+testing all things and holding fast that which is good.
+
+
+BIBLIOGRAPHY
+
+(The following short list may be useful to readers who desire to have
+further books recommended to them.)
+
+ CLODD, _Story of Creation: A Plain Account of Evolution._
+ DARWIN, _Origin of Species, Descent of Man._
+ DEPERET, _Transformation of the Animal World_ (Internat. Sci. Series).
+ GEDDES AND THOMSON, _Evolution_ (Home University Library).
+ GOODRICH, _Evolution_ (The People's Books).
+ HEADLEY, _Life and Evolution._
+ HUTCHINSON, H. NEVILLE, _Extinct Monsters_ (1892).
+ LULL, _Organic Evolution._
+ MCCABE, _A B C of Evolution._
+ METCALF, _Outline of the Theory of Organic Evolution._
+ OSBORN, H. F., _The Evolution of Life_ (1921).
+ THOMSON, _Darwinism and Human Life._
+ WALLACE, _Darwinism._
+
+
+
+
+III
+
+ADAPTATIONS TO ENVIRONMENT
+
+
+
+
+ADAPTATIONS TO ENVIRONMENT
+
+
+We saw in a previous chapter how the process of evolution led to a
+mastery of all the haunts of life. But it is necessary to return to
+these haunts or homes of animals in some detail, so as to understand the
+peculiar circumstances of each, and to see how in the course of ages of
+struggle all sorts of self-preserving and race-continuing adaptations or
+fitnesses have been wrought out and firmly established. Living creatures
+have spread over all the earth and in the waters under the earth; some
+of them have conquered the underground world and others the air. It is
+possible, however, as has been indicated, to distinguish six great
+haunts of life, each tenanted by a distinctive fauna, namely, the shore
+of the sea, the open sea, the depths of the sea, the freshwaters, the
+dry land, and the air. In the deep sea there are no plants at all; in
+the air the only plants are floating bacteria, though there is a sense
+in which a tree is very aerial, and the orchid perched on its branches
+still more so; in the other four haunts there is a flora as well as a
+fauna--the two working into one another's hands in interesting and often
+subtle inter-relations--the subject of a separate study.
+
+
+I. THE SHORE OF THE SEA
+
+The Seaweed Area
+
+By the shore of the sea the zoologist means much more than the narrow
+zone between tide-marks; he means the whole of the relatively shallow,
+well-illumined, seaweed-growing shelf around the continents and
+continental islands. Technically, this is called the littoral area, and
+it is divisible into zones, each with its characteristic population. It
+may be noted that the green seaweeds are highest up on the shore; the
+brown ones come next; the beautiful red ones are lowest. All of them
+have got green chlorophyll, which enables them to utilise the sun's rays
+in photosynthesis (i.e. building up carbon compounds from air, water,
+and salts), but in the brown and red seaweeds the green pigment is
+masked by others. It is maintained by some botanists that these other
+pigments enable their possessors to make more of the scantier light in
+the deeper waters. However this may be, we must always think of the
+shore-haunt as the seaweed-growing area. Directly and indirectly the
+life of the shore animals is closely wrapped up with the seaweeds, which
+afford food and foothold, and temper the force of the waves. The minute
+fragments broken off from seaweeds and from the sea-grass (a flowering
+plant called Zostera) form a sort of nutritive sea-dust which is swept
+slowly down the slope from the shore, to form a very useful deposit in
+the quietness of deepish water. It is often found in the stomachs of
+marine animals living a long way offshore.
+
+
+Conditions of Shore Life
+
+The littoral area as defined is not a large haunt of life; it occupies
+only about 9 million square miles, a small fraction of the 197,000,000
+of the whole earth's surface. But it is a very long haunt, some 150,000
+miles, winding in and out by bay and fiord, estuary and creek. Where
+deep water comes close to cliffs there may be no shore at all; in other
+places the relatively shallow water, with seaweeds growing over the
+bottom, may extend outwards for miles. The nature of the shore varies
+greatly according to the nature of the rocks, according to what the
+streams bring down from inland, and according to the jetsam that is
+brought in by the tides. The shore is a changeful place; there is, in
+the upper reaches, a striking difference between "tide in" and "tide
+out"; there are vicissitudes due to storms, to freshwater floods, to
+wind-blown sand, and to slow changes of level, up and down. The shore is
+a very crowded haunt, for it is comparatively narrow, and every niche
+among the rocks may be precious.
+
+[Illustration: AN EIGHT-ARMED CUTTLEFISH OR OCTOPUS ATTACKING A SMALL
+CRAB
+
+These molluscs are particularly fond of crustaceans, which they crunch
+with their parrot's beak-like jaws. Their salivary juice has a
+paralysing effect on their prey. To one side, below the eye, may be seen
+the funnel through which water is very forcibly ejected in the process
+of locomotion.]
+
+[Illustration: A COMMON STARFISH, WHICH HAS LOST THREE ARMS AND IS
+REGROWING THEM
+
+The lowest arm is being regrown double.
+
+(_After Professor W. C. McIntosh._)]
+
+[Illustration: A PHOTOGRAPH SHOWING A STARFISH (_Asterias Forreri_)
+WHICH HAS CAPTURED A LARGE FISH
+
+The suctorial tube-feet are seen gripping the fish firmly. (After an
+observation on the Californian coast.)]
+
+[Illustration: _Photo: J. J. Ward, F.E.S._
+
+THE PAPER NAUTILUS (ARGONAUTA), AN ANIMAL OF THE OPEN SEA
+
+The delicate shell is made by the female only, and is used as a shelter
+for the eggs and young ones. It is secreted by two of the arms, not by
+the mantle as other mollusc shells are. It is a single-chambered shell,
+very different from that of the Pearly Nautilus.]
+
+
+Keen Struggle for Existence
+
+It follows that the shore must be the scene of a keen struggle for
+existence--which includes all the answers-back that living creatures
+make to environing difficulties and limitations. There is struggle for
+food, accentuated by the fact that small items tend to be swept away by
+the outgoing tide or to sink down the slope to deep water. Apart from
+direct competition, e.g. between hungry hermit-crabs, it often involves
+hard work to get a meal. This is true even of apparently sluggish
+creatures. Thus the Crumb-of-Bread Sponge, or any other seashore sponge,
+has to lash large quantities of water through the intricate canal system
+of its body before it can get a sufficient supply of the microscopic
+organisms and organic particles on which it feeds. An index of the
+intensity of the struggle for food is afforded by the nutritive chains
+which bind animals together. The shore is almost noisy with the
+conjugation of the verb to eat in its many tenses. One pound of rock-cod
+requires for its formation ten pounds of whelk; one pound of whelk
+requires ten pounds of sea-worms; and one pound of worms requires ten
+pounds of sea-dust. Such is the circulation of matter, ever passing from
+one embodiment or incarnation to another.
+
+Besides struggle for food there is struggle for foothold and for fresh
+air, struggle against the scouring tide and against the pounding
+breakers. The risk of dislodgment is often great and the fracture of
+limbs is a common accident. Of kinds of armour--the sea-urchin's
+hedgehog-like test, the crab's shard, the limpet's shell--there is great
+variety, surpassed only by that of weapons--the sea-anemone's
+stinging-cells, the sea-urchin's snapping-blades, the hermit-crab's
+forceps, the grappling tentacles and parrot's-beak jaws of the octopus.
+
+
+Shifts for a Living
+
+We get another glimpse of the intensity of the seashore struggle for
+existence in the frequency of "shifts for a living," adaptations of
+structure or of behaviour which meet frequently recurrent vicissitudes.
+The starfish is often in the dilemma of losing a limb or its life; by a
+reflex action it jettisons the captured arm and escapes. And what is
+lost is gradually regrown. The crab gets its leg broken past all
+mending; it casts off the leg across a weak breakage plane near the
+base, and within a preformed bandage which prevents bleeding a new leg
+is formed in miniature. Such is the adaptive device--more reflex than
+reflective--which is called self-mutilation or autotomy.
+
+In another part of this book there is a discussion of camouflaging and
+protective resemblance; how abundantly these are illustrated on the
+shore! But there are other "shifts for a living." Some of the
+sand-hoppers and their relatives illustrate the puzzling phenomenon of
+"feigning death," becoming suddenly so motionless that they escape the
+eyes of their enemies. Cuttlefishes, by discharging sepia from their
+ink-bags, are able to throw dust in the eyes of their enemies. Some
+undisguised shore-animals, e.g. crabs, are adepts in a hide-and-seek
+game; some fishes, like the butterfish or gunnel, escape between stones
+where there seemed no opening and are almost uncatchable in their
+slipperiness. Subtlest of all, perhaps, is the habit some hermit-crabs
+have of entering into mutually beneficial partnership (commensalism)
+with sea-anemones, which mask their bearers and also serve as mounted
+batteries, getting transport as their reward and likewise crumbs from
+the frequently spread table. But enough has been said to show that the
+shore-haunt exhibits an extraordinary variety of shifts for a living.
+
+
+Parental Care on the Shore
+
+According to Darwin, the struggle for existence, as a big fact in the
+economy of Animate Nature, includes not only competition but all the
+endeavours which secure the welfare of the offspring, and give them a
+good send-off in life. So it is without a jolt that we pass from
+struggle for food and foothold to parental care. The marine leech called
+Pontobdella, an interesting greenish warty creature fond of fixing
+itself to skate, places its egg-cocoons in the empty shell of a bivalve
+mollusc, and guards them for weeks, removing any mud that might injure
+their development. We have seen a British starfish with its fully-formed
+young ones creeping about on its body, though the usual mode of
+development for shore starfishes is that the young ones pass through a
+free-swimming larval period in the open water. The father sea-spider
+carries about the eggs attached to two of his limbs; the father
+sea-horse puts his mate's eggs into his breast pocket and carries them
+there in safety until they are hatched; the father stickleback of the
+shore-pools makes a seaweed nest and guards the eggs which his wives are
+induced to lay there; the father lumpsucker mounts guard over the bunch
+of pinkish eggs which his mate has laid in a nook of a rocky shore-pool,
+and drives off intruders with zest. He also aerates the developing eggs
+by frequent paddling with his pectoral fins and tail, as the Scots name
+Cock-paidle probably suggests. It is interesting that the salient
+examples of parental care in the shore-haunt are mostly on the male
+parent's side. But there is maternal virtue as well.
+
+[Illustration: TEN-ARMED CUTTLEFISH OR SQUID IN THE ACT OF CAPTURING A
+FISH
+
+The arms bear numerous prehensile suckers, which grip the prey. In the
+mouth there are strong jaws shaped like a parrot's beak. The
+cuttlefishes are molluscs and may be regarded as the highest of the
+backboneless or Invertebrate animals. Many occur near shore, others in
+the open sea, and others in the great depths.]
+
+[Illustration: GREENLAND WHALE
+
+Showing the double blowhole or nostrils on the top of the head and the
+whalebone plates hanging down from the roof of the mouth.]
+
+[Illustration: MINUTE TRANSPARENT EARLY STAGE OF A SEA-CUCUMBER
+
+It swims in the open sea by means of girdles of microscopic cilia shown
+in the figure. After a period of free swimming and a remarkable
+metamorphosis, the animal settles down on the floor of the sea in
+relatively shallow water.]
+
+[Illustration: _Photo: British Museum (Natural History)_
+
+AN INTRICATE COLONY OF OPEN-SEA ANIMALS (_Physophora Hydrostatica_)
+RELATED TO THE PORTUGUESE MAN-OF-WAR
+
+There is great division of labor in the colony. At the top are floating
+and swimming "persons"; the long ones below are offensive "persons"
+bearing batteries of stinging cells; in the middle zone there are
+nutritive, reproductive, and other "persons." The color of the colony is
+a fine translucent blue. Swimmers and bathers are often badly stung by
+this strange animal and its relatives.]
+
+[Illustration: A SCENE IN THE GREAT DEPTHS
+
+Showing a deep-sea fish of large gape, two feather-stars on the end of
+long stalks, a "sea-spider" (or Pycnogon) walking on lanky legs on the
+treacherous ooze, likewise a brittle-star, and some deep-sea corals.]
+
+The fauna of the shore is remarkably _representative_--from unicellular
+Protozoa to birds like the oyster-catcher and mammals like the seals.
+Almost all the great groups of animals have apparently served an
+apprenticeship in the shore-haunt, and since lessons learned for
+millions of years sink in and become organically enregistered, it is
+justifiable to look to the shore as a great school in which were gained
+racial qualities of endurance, patience, and alertness.
+
+
+II. THE OPEN SEA
+
+In great contrast to the narrow, crowded, difficult conditions of the
+shore-haunt (littoral area) are the spacious, bountiful, and relatively
+easygoing conditions of the open sea (pelagic area), which means the
+well-lighted surface waters quite away from land. Many small organisms
+have their maximum abundance at about fifty fathoms, so that the word
+"surface" is to be taken generously. The light becomes very dim at 250
+fathoms, and the open sea, as a zoological haunt, stops with the light.
+It is hardly necessary to say that the pelagic plants are more abundant
+near the surface, and that below a certain depth the population consists
+almost exclusively of animals. Not a few of the animals sink and rise in
+the water periodically; there are some that come near the surface by
+day, and others that come near the surface by night. Of great interest
+is the habit of the extremely delicate Ctenophores or
+"sea-gooseberries," which the splash of a wave would tear into shreds.
+Whenever there is any hint of a storm they sink beyond its reach, and
+the ocean's surface must have remained flat as a mirror for many hours
+before they can be lured upwards from the calm of their deep retreat.
+
+
+The Floating Sea-meadows
+
+To understand the vital economy of the open sea, we must recognise the
+incalculable abundance of minute unicellular plants, for they form the
+fundamental food-supply. Along with these must also be included numerous
+microscopic animals which have got possession of chlorophyll, or have
+entered into internal partnership with unicellular Algæ (symbiosis).
+These green or greenish plants and animals are the _producers_, using
+the energy of the sunlight to help them in building up carbon compounds
+out of air, water, and salts. The animals which feed on the producers,
+or on other animals, are the _consumers_. Between the two come those
+open-sea bacteria that convert nitrogenous material, e.g. from dead
+plants or animals that other bacteria have rotted, into forms, e.g.
+nitrates, which plants can re-utilise. The importance of these
+_middlemen_ is great in keeping "the circulation of matter" agoing.
+
+[Illustration: 1. SEA-HORSE IN SARGASSO WEED. In its frond-like tags of
+skin and in its colouring this kind of sea-horse is well concealed among
+the floating seaweed of the so-called Sargasso Sea.
+
+2. THE LARGE MARINE LAMPREYS (_PETROMYZON MARINUS_), WHICH MAY BE AS
+LONG AS ONE'S ARM, SPAWN IN FRESH WATER. Stones and pebbles, gripped in
+the suctorial mouth, are removed from a selected spot and piled around
+the circumference, so that the eggs, which are laid within the circle,
+are not easily washed away.
+
+3. THE DEEP-SEA FISH _CHIASMODON NIGER_ IS FAMOUS FOR ITS VORACITY. It
+sometimes manages to swallow a fish larger than itself, which causes an
+extraordinary protrusion of the stomach.
+
+4. DEEP-SEA FISHES. Two of them--_Melanocetus murrayi_ and _Melanocetus
+indicus_--are related to the Angler of British coasts, but adapted to
+life in the great abysses. They are very dark in colour, and delicately
+built; they possess well-developed luminous organs. The third form is
+called Chauliodus, a predatory animal with large gape and formidable
+teeth.]
+
+[Illustration: FLINTY SKELETON OF VENUS FLOWER BASKET (EUPLECTELLA), A
+JAPANESE DEEP-SEA SPONGE]
+
+[Illustration: EGG DEPOSITORY OF _Semotilus Atromaculatus_
+
+In the building of this egg depository, the male fish takes stones from
+the bottom of the stream, gripping them in his mouth, and heaps them up
+into the dam. In the egg depository he arranges the stones so that when
+the eggs are deposited in the interstices they are thoroughly protected,
+and cannot be washed down-stream.
+
+1, dam of stones; 2, egg depository; 3, hillock of sand. The arrow shows
+the direction of the stream. Upper fish, male; lower, female.]
+
+The "floating sea-meadows," as Sir John Murray called them, are always
+receiving contributions from inshore waters, where the conditions are
+favourable for the prolific multiplication of unicellular Algæ, and
+there is also a certain amount of non-living sea-dust always being swept
+out from the seaweed and sea-grass area.
+
+
+Swimmers and Drifters
+
+The animals of the open sea are conveniently divided into the active
+swimmers (Nekton) and the more passive drifters (Plankton). The swimmers
+include whales great and small, such birds as the storm petrel, the
+fish-eating turtles and sea-snakes, such fishes as mackerel and herring,
+the winged snails or sea-butterflies on which whalebone whales largely
+feed, some of the active cuttles or squids, various open-sea prawns and
+their relatives, some worms like the transparent arrow-worm, and such
+active Protozoa as Noctiluca, whose luminescence makes the waves sparkle
+in the short summer darkness. Very striking as an instance of the
+insurgence of life are the sea-skimmers (Halobatidæ), wingless insects
+related to the water-measurers in the ditch. They are found hundreds of
+miles from land, skimming on the surface of the open sea, and diving in
+stormy weather. They feed on floating dead animals.
+
+The drifters or easygoing swimmers--for there is no hard and fast
+line--are represented, for instance, by the flinty-shelled Radiolarians
+and certain of the chalk-forming animals (Globigerinid Foraminifera); by
+jellyfishes, swimming-bells, and Portuguese men-of-war; by the
+comb-bearers or Ctenophores; by legions of minute Crustaceans; by
+strange animals called Salps, related to the sedentary sea-squirts; and
+by some sluggish fishes like globe-fishes, which often float idly on the
+surface.
+
+Open-sea animals tend to be delicately built, with a specific gravity
+near that of the sea-water, with adaptations, such as projecting
+filaments, which help flotation, and with capacities of rising and
+sinking according to the surrounding conditions. Many of them are
+luminescent, and many of them are very inconspicuous in the water owing
+to their transparency or their bluish colour. In both cases the
+significance is obscure.
+
+
+Hunger and Love
+
+Hunger is often very much in evidence in the open sea, especially in
+areas where the Plankton is poor. For there is great diversity in this
+respect, most of the Mediterranean, for instance, having a scanty
+Plankton as compared with the North Sea. In the South Pacific, west of
+Patagonia, there is said to be an immense "sea desert" where there is
+little Plankton, and therefore little in the way of fishes. The success
+of fisheries in the North, e.g. on the Atlantic cod-banks, is due to the
+richness of the floating sea-meadows and the abundance of the smaller
+constituents of the animal Plankton.
+
+Hunger is plain enough when the Baleen Whale rushes through the water
+with open jaws, engulfing in the huge cavern of its mouth, where the
+pendent whalebone plates form a huge sieve, incalculable millions of
+small fry.
+
+But there is love as well as hunger in the open sea. The maternal care
+exhibited by the whale reaches a very high level, and the delicate shell
+of the female Paper Nautilus or Argonaut, in which the eggs and the
+young ones are sheltered, may well be described as "the most beautiful
+cradle in the world."
+
+Besides the permanent inhabitants of the open sea, there are the larval
+stages of many shore-animals which are there only for a short time. For
+there is an interesting give and take between the shore-haunt and the
+open sea. From the shore come nutritive contributions and minute
+organisms which multiply quickly in the open waters. But not less
+important is the fact that the open waters afford a safe cradle or
+nursery for many a delicate larva, e.g. of crab and starfish,
+acorn-shell and sea-urchin, which could not survive for a day in the
+rough-and-tumble conditions of the shore and the shallow water. After
+undergoing radical changes and gaining strength, the young creatures
+return to the shore in various ways.
+
+
+III. THE DEEP SEA
+
+Very different from all the other haunts are the depths of the sea,
+including the floor of the abysses and the zones of water near the
+bottom. This haunt, forever unseen, occupies more than a third of the
+earth's surface, and it is thickly peopled. It came into emphatic notice
+in connection with the mending of telegraph cables, but the results of
+the _Challenger_ expedition (1873-6) gave the first impressive picture
+of what was practically a new world.
+
+
+Physical Conditions
+
+The average depth of the ocean is about two and a half miles; therefore,
+since many parts are relatively shallow, there must be enormous depths.
+A few of these, technically called "deeps," are about six miles deep, in
+which Mount Everest would be engulfed. There is enormous pressure in
+such depths; even at 2,500 fathoms it is two and a half tons on the
+square inch. The temperature is on and off the freezing-point of fresh
+water (28°-34° Fahr.), due to the continual sinking down of cold water
+from the Poles, especially from the South. Apart from the fitful gleams
+of luminescent animals, there is utter darkness in the deep waters. The
+rays of sunlight are practically extinguished at 250 fathoms, though
+very sensitive bromogelatine plates exposed at 500 fathoms have shown
+faint indications even at that depth. It is a world of absolute calm and
+silence, and there is no scenery on the floor. A deep, cold, dark,
+silent, monotonous world!
+
+
+Biological Conditions
+
+While some parts of the floor of the abysses are more thickly peopled
+than others, there is no depth limit to the distribution of life.
+Wherever the long arm of the dredge has reached, animals have been
+found, e.g. Protozoa, sponges, corals, worms, starfishes, sea-urchins,
+sea-lilies, crustaceans, lamp-shells, molluscs, ascidians, and fishes--a
+very representative fauna. In the absence of light there can be no
+chlorophyll-possessing plants, and as the animals cannot all be eating
+one another there must be an extraneous source of food-supply. This is
+found in the sinking down of minute organisms which are killed on the
+surface by changes of temperature and other causes. What is left of
+them, before or after being swallowed, and of sea-dust and mineral
+particles of various kinds forms the diversified "ooze" of the
+sea-floor, a soft muddy precipitate, which is said to have in places the
+consistence of butter in summer weather.
+
+There seems to be no bacteria in the abysses, so there can be no
+rotting. Everything that sinks down, even the huge carcase of a whale,
+must be nibbled away by hungry animals and digested, or else, in the
+case of most bones, slowly dissolved away. Of the whale there are left
+only the ear-bones, of the shark his teeth.
+
+
+Adaptations to Deep-sea Life
+
+In adaptation to the great pressure the bodies of deep-sea animals are
+usually very permeable, so that the water gets through and through them,
+as in the case of Venus' Flower Basket, a flinty sponge which a child's
+finger would shiver. But when the pressure inside is the same as that
+outside nothing happens. In adaptation to the treacherous ooze, so apt
+to smother, many of the active deep-sea animals have very long,
+stilt-like legs, and many of the sedentary types are lifted into safety
+on the end of long stalks which have their bases embedded in the mud. In
+adaptation to the darkness, in which there is only luminescence that
+eyes could use, there is a great development of tactility. The
+interesting problem of luminescence will be discussed elsewhere.
+
+As to the origin of the deep-sea fauna, there seems no doubt that it
+has arisen by many contributions from the various shore-haunts.
+Following the down-drifting food, many shore-animals have in the course
+of many generations reached the world of eternal night and winter, and
+become adapted to its strange conditions. For the animals of the
+deep-sea are as fit, beautiful, and vigorous as those elsewhere. There
+are no slums in Nature.
+
+[Illustration: THE BITTERLING (_Rhodeus Amarus_)
+
+A Continental fish which lays its eggs by means of a long ovipositor
+inside the freshwater mussel. The eggs develop inside the mollusc's
+gill-plates.]
+
+[Illustration: _Photo: W. S. Berridge._
+
+WOOLLY OPOSSUM CARRYING HER FAMILY
+
+One of the young ones is clinging to its mother and has its long
+prehensile tail coiled round hers.]
+
+[Illustration: SURINAM TOAD (_Pipa Americana_) WITH YOUNG ONES HATCHING
+OUT OF LITTLE POCKETS ON HER BACK]
+
+[Illustration: STORM PETREL OR MOTHER CAREY'S CHICKEN
+
+(_Procellaria Pelagica_)
+
+This characteristic bird of the open sea does not come to land at all
+except to nest. It is the smallest web-footed bird, about four inches
+long. The legs are long and often touch the water as the bird flies. The
+storm petrel is at home in the Atlantic, and often nests on islands off
+the west coast of Britain.]
+
+
+IV. THE FRESH WATERS
+
+Of the whole earth's surface the freshwaters form a very small fraction,
+about a hundredth, but they make up for their smallness by their
+variety. We think of deep lake and shallow pond, of the great river and
+the purling brook, of lagoon and swamp, and more besides. There is a
+striking resemblance in the animal population of widely separated
+freshwater basins: and this is partly because birds carry many small
+creatures on their muddy feet from one water-shed to another; partly
+because some of the freshwater animals are descended from types which
+make their way from the sea and the seashore through estuaries and
+marshes, and only certain kinds of constitution could survive the
+migration; and partly because some lakes are landlocked dwindling relics
+of ancient seas, and similar forms again would survive the change.
+
+A typical assemblage of freshwater animals would include many Protozoa,
+like Amoebæ and the Bell-Animalcules, a representative of one family
+of sponges (Spongillidæ), the common Hydra, many unsegmented worms
+(notably Planarians and Nematodes), many Annelids related to the
+earthworms, many crustaceans, insects, and mites, many bivalves and
+snails, various fishes, a newt or two, perhaps a little mud-turtle or in
+warm countries a huge Crocodilian, various interesting birds like the
+water-ouzel or dipper, and mammals like the water-vole and the
+water-shrew.
+
+Freshwater animals have to face certain difficulties, the greatest of
+which are drought, frost, and being washed away in times of flood.
+There is no more interesting study in the world than an inquiry into the
+adaptations by which freshwater animals overcome the difficulties of the
+situation. We cannot give more than a few illustrations.
+
+(1) Drought is circumvented by the capacity that many freshwater animals
+have of lying low and saying nothing. Thus the African mudfish may spend
+half the year encased in the mud, and many minute crustaceans can
+survive being dried up for years. (2) Escape from the danger of being
+frozen hard in the pool is largely due to the almost unique property of
+water that it expands as it approaches the freezing-point. Thus the
+colder water rises to the surface and forms or adds to the protecting
+blanket of ice. The warmer water remains unfrozen at the bottom, and the
+animals live on. (3) The risk of being washed away, e.g. to the sea, is
+lessened by all sorts of gripping, grappling, and anchoring structures,
+and by shortening the juvenile stages when the risks are greatest.
+
+
+V. THE DRY LAND
+
+Over and over again in the history of animal life there have been
+attempts to get out of the water on to terra firma, and many of these
+have been successful, notably those made (1) by worms, (2) by
+air-breathing Arthropods, and (3) by amphibians.
+
+In thinking of the conquest of the dry land by animals, we must
+recognise the indispensable rôle of plants in preparing the way. The dry
+ground would have proved too inhospitable had not terrestrial plants
+begun to establish themselves, affording food, shelter, and humidity.
+There had to be plants before there could be earthworms, which feed on
+decaying leaves and the like, but how soon was the debt repaid when the
+earthworms began their worldwide task of forming vegetable mould,
+opening up the earth with their burrows, circulating the soil by means
+of their castings, and bruising the particles in their
+gizzard--certainly the most important mill in the world.
+
+Another important idea is that littoral haunts, both on the seashore and
+in the freshwaters, afforded the necessary apprenticeship and
+transitional experience for the more strenuous life on dry land. Much
+that was perfected on land had its beginnings on the shore. Let us
+inquire, however, what the passage from water to dry land actually
+implied. This has been briefly discussed in a previous article (on
+Evolution), but the subject is one of great interest and importance.
+
+
+Difficulties and Results of the Transition from Water to Land
+
+Leaving the water for dry land implied a loss in freedom of movement,
+for the terrestrial animal is primarily restricted to the surface of the
+earth. Thus it became essential that movements should be very rapid and
+very precise, needs with which we may associate the acquisition of fine
+cross-striped, quickly contracting muscles, and also, in time, their
+multiplication into very numerous separate engines. We exercise
+fifty-four muscles in the half-second that elapses between raising the
+heel of our foot in walking and planting it firmly on the ground again.
+Moreover, the need for rapid precisely controlled movements implied an
+improved nervous system, for the brain was a movement-controlling organ
+for ages before it did much in the way of thinking. The transition to
+terra firma also involved a greater compactness of body, so that there
+should not be too great friction on the surface. An animal like the
+jellyfish is unthinkable on land, and the elongated bodies of some land
+animals like centipedes and snakes are specially adapted so that they do
+not "sprawl." They are exceptions that prove the rule.
+
+Getting on to dry land meant entering a kingdom where the differences
+between day and night, between summer and winter are more felt than in
+the sea. This made it advantageous to have protections against
+evaporation and loss of heat and other such dangers. Hence a variety of
+ways in which the surface of the body acquired a thickened skin, or a
+dead cuticle, or a shell, or a growth of hair, and so forth. In many
+cases there is an increase of the protection before the winter sets in,
+e.g. by growing thicker fur or by accumulating a layer of fat below the
+skin.
+
+But the thickening or protection of the skin involved a partial or total
+loss of the skin as a respiratory surface. There is more oxygen
+available on dry land than in the water, but it is not so readily
+captured. Thus we see the importance of moist internal surfaces for
+capturing the oxygen which has been drawn into the interior of the body
+into some sort of lung. A unique solution was offered by Tracheate
+Arthropods, such as Peripatus, Centipedes, Millipedes, and Insects,
+where the air is carried to every hole and corner of the body by a
+ramifying system of air-tubes or tracheæ. In most animals the blood goes
+to the air, in insects the air goes to the blood. In the Robber-Crab,
+which has migrated from the shore inland, the dry air is absorbed by
+vascular tufts growing under the shelter of the gill-cover.
+
+The problem of disposing of eggs or young ones is obviously much more
+difficult on land than in the water. For the water offers an immediate
+cradle, whereas on the dry land there were many dangers, e.g. of
+drought, extremes of temperature, and hungry sharp-eyed enemies, which
+had to be circumvented. So we find all manner of ways in which land
+animals hide their eggs or their young ones in holes and nests, on herbs
+and on trees. Some carry their young ones about after they are born,
+like the Surinam toad and the kangaroo, while others have prolonged the
+period of ante-natal life during which the young ones develop in safety
+within their mother, and in very intimate partnership with her in the
+case of the placental mammals. It is very interesting to find that the
+pioneer animal called Peripatus, which bridges the gap between worms and
+insects, carries its young for almost a year before birth.
+
+Enough has been said to show that the successive conquests of the dry
+land had great evolutionary results. It is hardly too much to say that
+the invasion which the Amphibians led was the beginning of better
+brains, more controlled activities, and higher expressions of family
+life.
+
+[Illustration: ALBATROSS: A CHARACTERISTIC PELAGIC BIRD OF THE SOUTHERN
+SEA
+
+It may have a spread of wing of over 11 feet from tip to tip. It is
+famous for its extraordinary power of "sailing" round the ship without
+any apparent strokes of its wings.]
+
+
+VI. THE AIR
+
+There are no animals thoroughly aerial, but many insects spend much of
+their adult life in the free air, and the swift hardly pauses in its
+flight from dawn to dusk of the long summer day, alighting only for
+brief moments at the nest to deliver insects to the young. All the
+active life of bats certainly deserves to be called aerial.
+
+The air was the last haunt of life to be conquered, and it is
+interesting to inquire what the conquest implied. (1) It meant
+transcending the radical difficulty of terrestrial life which confines
+the creatures of the dry land to moving on one plane, the surface of the
+earth. But the power of flight brought its possessors back to the
+universal freedom of movement which water animals enjoy. When we watch a
+sparrow rise into the air just as the cat has completed her stealthy
+stalking, we see that flight implies an enormous increase of safety. (2)
+The power of flight also opened up new possibilities of following the
+prey, of exploring new territories, of prospecting for water. (3) Of
+great importance too was the practicability of placing the eggs and the
+young, perhaps in a nest, in some place inaccessible to most enemies.
+When one thinks of it, the rooks' nests swaying on the tree-tops express
+the climax of a brilliant experiment. (4) The crowning advantage was the
+possibility of migrating, of conquering time (by circumventing the arid
+summer and the severe winter) and of conquering space (by passing
+quickly from one country to another and sometimes almost girdling the
+globe). There are not many acquisitions that have meant more to their
+possessors than the power of flight. It was a key opening the doors of a
+new freedom.
+
+The problem of flight, as has been said in a previous chapter, has been
+solved four times, and the solution has been different in each case. The
+four solutions are those offered by insects, extinct Pterodactyls,
+birds, and bats. Moreover, as has been pointed out, there have been
+numerous attempts at flight which remain glorious failures, notably the
+flying fishes, which take a great leap and hold their pectoral fins
+taut; the Flying Tree-Toad, whose webbed fingers and toes form a
+parachute; the Flying Lizard (_Draco volans_), which has its skin pushed
+out on five or six greatly elongated mobile ribs; and various "flying"
+mammals, e.g. Flying Phalangers and Flying Squirrels, which take great
+swooping leaps from tree to tree.
+
+The wings of an insect are hollow flattened sacs which grow out from the
+upper parts of the sides of the second and third rings of the region
+called the thorax. They are worked by powerful muscles, and are
+supported, like a fan, by ribs of chitin, which may be accompanied by
+air-tubes, blood-channels, and nerves. The insect's body is lightly
+built and very perfectly aerated, and the principle of the insect's
+flight is the extremely rapid striking of the air by means of the
+lightly built elastic wings. Many an insect has over two hundred strokes
+of its wings in one _second_. Hence, in many cases, the familiar hum,
+comparable on a small scale to that produced by the rapidly revolving
+blades of an aeroplane's propeller. For a short distance a bee can
+outfly a pigeon, but few insects can fly far, and they are easily blown
+away or blown back by the wind. Dragon-flies and bees may be cited as
+examples of insects that often fly for two or three miles. But this is
+exceptional, and the usual shortness of insect flight is an important
+fact for man since it limits the range of insects like house-flies and
+mosquitoes which are vehicles of typhoid fever and malaria respectively.
+The most primitive insects (spring-tails and bristle-tails) show no
+trace of wings, while fleas and lice have become secondarily wingless.
+It is interesting to notice that some insects only fly once in their
+lifetime, namely, in connection with mating. The evolution of the
+insect's wing remains quite obscure, but it is probable that insects
+could run, leap, and parachute before they could actually fly.
+
+The extinct Flying Dragons or Pterodactyls had their golden age in the
+Cretaceous era, after which they disappeared, leaving no descendants. A
+fold of skin was spread out from the sides of the body by the enormously
+elongated outermost finger (usually regarded as corresponding to our
+little finger); it was continued to the hind-legs and thence to the
+tail.
+
+It is unlikely that the Pterodactyls could fly far, for they have at
+most a weak keel on their breast-bone; on the other hand, some of them
+show a marked fusion of dorsal vertebræ, which, as in flying birds, must
+have served as a firm fulcrum for the stroke of the wings. The quaint
+creatures varied from the size of a sparrow up to a magnificent spread
+of 15-20 feet from tip to tip of the wings. They were the largest of all
+flying creatures.
+
+The bird's solution of the problem of flight, which will be discussed
+separately, is centred in the feather, which forms a coherent vane for
+striking the air. In Pterodactyl and bat the wing is a web-wing or
+patagium, and a small web is to be seen on the front side of the bird's
+wing. But the bird's patagium is unimportant, and the bird's wing is on
+an evolutionary tack of its own--a fore-limb transformed for bearing the
+feathers of flight. Feathers are in a general way comparable to the
+scales of reptiles, but only in a general way, and no transition stage
+is known between the two. Birds evolved from a bipedal Dinosaur stock,
+as has been noticed already, and it is highly probable that they began
+their ascent by taking running leaps along the ground, flapping their
+scaly fore-limbs, and balancing themselves in kangaroo-like fashion with
+an extended tail. A second chapter was probably an arboreal
+apprenticeship, during which they made a fine art of parachuting--a
+persistence of which is to be seen in the pigeon "gliding" from the
+dovecot to the ground. It is in birds that the mastery of the air
+reaches its climax, and the mysterious "sailing" of the albatross and
+the vulture is surely the most remarkable locomotor triumph that has
+ever been achieved. Without any apparent stroke of the wings, the bird
+sails for half an hour at a time with the wind and against the wind,
+around the ship and in majestic spirals in the sky, probably taking
+advantage of currents of air of different velocities, and continually
+changing energy of position into energy of motion as it sinks, and
+energy of motion into energy of position as it rises. It is interesting
+to know that some dragon-flies are also able to "sail."
+
+The web-wing of bats involves much more than the fore-arm. The double
+fold of skin begins on the side of the neck, passes along the front of
+the arm, skips the thumb, and is continued over the elongated palm-bones
+and fingers to the sides of the body again, and to the hind-legs, and to
+the tail if there is a tail. It is interesting to find that the bones of
+the bat's skeleton tend to be lightly built as in birds, that the
+breast-bone has likewise a keel for the better insertion of the pectoral
+muscles, and that there is a solidifying of the vertebræ of the back,
+affording as in birds a firm basis for the wing action. Such similar
+adaptations to similar needs, occurring in animals not nearly related to
+one another, are called "convergences," and form a very interesting
+study. In addition to adaptations which the bat shares with the flying
+bird, it has many of its own. There are so many nerve-endings on the
+wing, and often also on special skin-leaves about the ears and nose,
+that the bat flying in the dusk does not knock against branches or other
+obstacles. Some say that it is helped by the echoes of its high-pitched
+voice, but there is no doubt as to its exquisite tactility. That it
+usually produces only a single young one at a time is a clear adaptation
+to flight, and similarly the sharp, mountain-top-like cusps on the back
+teeth are adapted in insectivorous bats for crunching insects.
+
+Whether we think of the triumphant flight of birds, reaching a climax in
+migration, or of the marvel that a creature of the earth--as a mammal
+essentially is--should evolve such a mastery of the air as we see in
+bats, or even of the repeated but splendid failures which parachuting
+animals illustrate, we gain an impression of the insurgence of living
+creatures in their characteristic endeavour after fuller well-being.
+
+We have said enough to show how well adapted many animals are to meet
+the particular difficulties of the haunt which they tenant. But
+difficulties and limitations are ever arising afresh, and so one fitness
+follows on another. It is natural, therefore, to pass to the frequent
+occurrence of protective resemblance, camouflage, and mimicry--the
+subject of the next article.
+
+
+BIBLIOGRAPHY
+
+ ELMHIRST, R., _Animals of the Shore_.
+ FLATTELY AND WALTON, _The Biology of the Shore_ (1921).
+ FURNEAUX, _Life of Ponds and Streams_.
+ HICKSON, S. J., _Story of Life in the Seas_ and _Fauna of the Deep Sea_.
+ JOHNSTONE, J., _Life in the Sea_ (Cambridge Manual of Science).
+ MIALL, L. C., _Aquatic Insects_.
+ MURRAY, SIR JOHN, _The Ocean_ (Home University Library).
+ MURRAY, SIR JOHN AND HJORT, DR. J., _The Depths of the Ocean_.
+ NEWBIGIN, M. I., _Life by the Sea Shore_.
+ PYCRAFT, W. P., _History of Birds_.
+ SCHARFF, R. F., _History of the European Fauna_ (Contemp. Sci. Series).
+ THOMSON, J. ARTHUR, _The Wonder of Life_ (1914) and
+ _The Haunts of Life_ (1921).
+
+
+
+
+IV
+
+THE STRUGGLE FOR EXISTENCE
+
+
+
+
+ANIMAL AND BIRD MIMICRY AND DISGUISE
+
+
+§ 1
+
+For every animal one discovers when observing carefully, there must be
+ten unseen. This is partly because many animals burrow in the ground or
+get in underneath things and into dark corners, being what is called
+cryptozoic or elusive. But it is partly because many animals put on
+disguise or have in some way acquired a garment of invisibility. This is
+very common among animals, and it occurs in many forms and degrees. The
+reason why it is so common is because the struggle for existence is
+often very keen, and the reasons why the struggle for existence is keen
+are four. First, there is the tendency to over-population in many
+animals, especially those of low degree. Second, there is the fact that
+the scheme of nature involves nutritive chains or successive
+incarnations, one animal depending upon another for food, and all in the
+long run on plants; thirdly, every vigorous animal is a bit of a
+hustler, given to insurgence and sticking out his elbows. There is a
+fourth great reason for the struggle for existence, namely, the frequent
+changefulness of the physical environment, which forces animals to
+answer back or die; but the first three reasons have most to do with the
+very common assumption of some sort of disguise. Even when an animal is
+in no sense a weakling, it may be very advantageous for it to be
+inconspicuous when it is resting or when it is taking care of its young.
+Our problem is the evolution of elusiveness, so far at least as that
+depends on likeness to surroundings, on protective resemblance to other
+objects, and in its highest reaches on true mimicry.
+
+
+Colour Permanently Like That of Surroundings
+
+Many animals living on sandy places have a light-brown colour, as is
+seen in some lizards and snakes. The green lizard is like the grass and
+the green tree-snake is inconspicuous among the branches. The spotted
+leopard is suited to the interrupted light of the forest, and it is
+sometimes hard to tell where the jungle ends and the striped tiger
+begins. There is no better case than the hare or the partridge sitting a
+few yards off on the ploughed field. Even a donkey grazing in the dusk
+is much more readily heard than seen.
+
+The experiment has been made of tethering the green variety of Praying
+Mantis on green herbage, fastening them with silk threads. They escape
+the notice of birds. The same is true when the brown variety is tethered
+on withered herbage. But if the green ones are put on brown plants, or
+the brown ones on green plants, the birds pick them off. Similarly, out
+of 300 chickens in a field, 240 white or black and therefore
+conspicuous, 60 spotted and inconspicuous, 24 were soon picked off by
+crows, but only one of these was spotted. This was not the proportion
+that there should have been if the mortality had been fortuitous. There
+is no doubt that it often pays an animal to be like its habitual
+surroundings, like a little piece of scenery if the animal is not
+moving. It is safe to say that in process of time wide departures from
+the safest coloration will be wiped out in the course of Nature's
+ceaseless sifting.
+
+But we must not be credulous, and there are three cautions to be borne
+in mind. (1) An animal may be very like its surroundings without there
+being any protection implied. The arrow-worms in the sea are as clear as
+glass, and so are many open-sea animals. But this is because their
+tissues are so watery, with a specific gravity near that of the salt
+water. And the invisibility does not save them, always or often, from
+being swallowed by larger animals that gather the harvest of the sea.
+(2) Among the cleverer animals it looks as if the creature sometimes
+sought out a spot where it was most inconspicuous. A spider may place
+itself in the middle of a little patch of lichen, where its
+self-effacement is complete. Perhaps it is more comfortable as well as
+safer to rest in surroundings the general colour of which is like that
+of the animal's body. (3) The fishes that live among the coral-reefs are
+startling in their brilliant coloration, and there are many different
+patterns. To explain this it has been suggested that these fishes are so
+safe among the mazy passages and endless nooks of the reefs, that they
+can well afford to wear any colour that suits their constitution. In
+some cases this may be true, but naturalists who have put on a diving
+suit and walked about among the coral have told us that each kind of
+fish is particularly suited to some particular place, and that some are
+suited for midday work and others for evening work. Sometimes there is a
+sort of Box and Cox arrangement by which two different fishes utilise
+the same corner at different times.
+
+[Illustration: THE PRAYING MANTIS (_Mantis Religiosa_)
+
+A very voracious insect with a quiet, unobtrusive appearance. It holds
+its formidable forelegs as if in the attitude of prayer; its movements
+are very slow and stealthy; and there is a suggestion of a leaf in the
+forewing. But there is no reason to credit the creature with conscious
+guile!]
+
+[Illustration: PROTECTIVE COLORATION: A WINTER SCENE IN NORTH
+SCANDINAVIA
+
+Showing Variable Hare, Willow Grouse, and Arctic Fox, all white in
+winter and inconspicuous against the snow. But the white dress is also
+the dress that is physiologically best, for it loses least of the animal
+heat.]
+
+[Illustration: THE VARIABLE MONITOR (_Varanus_)
+
+The monitors are the largest of existing lizards, the Australian species
+represented in the photograph attaining a length of four feet. It has a
+brown colour with yellow spots, and in spite of its size it is not
+conspicuous against certain backgrounds, such as the bark of a tree.]
+
+
+§ 2
+
+Gradual Change of Colour
+
+The common shore-crab shows many different colours and mottlings,
+especially when it is young. It may be green or grey, red or brown, and
+so forth, and it is often in admirable adjustment to the colour of the
+rock-pool where it is living. Experiments, which require extension, have
+shown that when the crab has moulted, which it has to do very often when
+it is young, the colour of the new shell tends to harmonise with the
+general colour of the rocks and seaweed. How this is brought about, we
+do not know. The colour does not seem to change till the next moult, and
+not then unless there is some reason for it. A full-grown shore-crab is
+well able to look after itself, and it is of interest to notice,
+therefore, that the variety of coloration is mainly among the small
+individuals, who have, of course, a much less secure position. It is
+possible, moreover, that the resemblance to the surroundings admits of
+more successful hunting, enabling the small crab to take its victim
+unawares.
+
+Professor Poulton's experiments with the caterpillars of the small
+tortoise-shell butterfly showed that in black surroundings the pupæ tend
+to be darker, in white surroundings lighter, in gilded boxes golden; and
+the same is true in other cases. It appears that the surrounding colour
+affects the caterpillars through the skin during a sensitive period--the
+twenty hours immediately preceding the last twelve hours of the larval
+state. The result will tend to make the quiescent pupæ less conspicuous
+during the critical time of metamorphosis. The physiology of this
+sympathetic colouring remains obscure.
+
+
+Seasonal Change of Colouring
+
+The ptarmigan moults three times in the year. Its summer plumage is
+rather grouselike above, with a good deal of rufous brown; the back
+becomes much more grey in autumn; almost all the feathers of the winter
+plumage are white. That is to say, they develop without any pigment and
+with numerous gas-bubbles in their cells. Now there can be no doubt that
+this white winter plumage makes the ptarmigan very inconspicuous amidst
+the snow. Sometimes one comes within a few feet of the crouching bird
+without seeing it, and this garment of invisibility may save it from the
+hungry eyes of golden eagles.
+
+Similarly the brown stoat becomes the white ermine, mainly by the
+growth, of a new suit of white fur, and the same is true of the mountain
+hare. The ermine is all white except the black tip of its tail; the
+mountain hare in its winter dress is all white save the black tips of
+its ears. In some cases, especially in the mountain hare, it seems that
+individual hairs may turn white, by a loss of pigment, as may occur in
+man. According to Metchnikoff, the wandering amoeboid cells of the
+body, called phagocytes, may creep up into the hairs and come back again
+with microscopic burdens of pigment. The place of the pigment is taken
+by gas-bubbles, and that is what causes the whiteness. In no animals is
+there any white _pigment_; the white _colour_ is like that of snow or
+foam, it is due to the complete reflection of the light from innumerable
+minute surfaces of crystals or bubbles.
+
+[Illustration: _Photo: W. S. Berridge, F.Z.S._
+
+BANDED KRAIT: A VERY POISONOUS SNAKE WITH ALTERNATING YELLOW AND DARK
+BANDS
+
+It is very conspicuous and may serve as an illustration of warning
+coloration. Perhaps, that is to say, its striking coloration serves as
+an advertisement, impressing other creatures with the fact that the
+Banded Krait should be left alone. It is very unprofitable for a snake
+to waste its venom on creatures it does not want.]
+
+[Illustration: _Photos: W. S. Berridge, F.Z.S._
+
+THE WARTY CHAMELEON
+
+The upper photograph shows the Warty Chameleon inflated and conspicuous.
+At another time, however, with compressed body and adjusted coloration,
+the animal is very inconspicuous. The lower photograph shows the sudden
+protrusion of the very long tongue on a fly.]
+
+[Illustration: SEASONAL COLOUR-CHANGE: A SUMMER SCENE IN NORTH
+SCANDINAVIA
+
+Showing a brown Variable Hare, Willow Grouse, and Arctic Fox, all
+inconspicuous in their coloration when seen in their natural
+surroundings.]
+
+The mountain hare may escape the fox the more readily because its
+whiteness makes it so inconspicuous against a background of snow; and
+yet, at other times, we have seen the creature standing out like a
+target on the dark moorland. So it cuts both ways. The ermine has almost
+no enemies except the gamekeeper, but its winter whiteness may help it
+to sneak upon its victims, such as grouse or rabbit, when there is snow
+upon the ground. In both cases, however, the probability is that the
+constitutional rhythm which leads to white hair in winter has been
+fostered and fixed for a reason quite apart from protection. The fact is
+that for a warm-blooded creature, whether bird or mammal, the
+physiologically best dress is a white one, for there is less radiation
+of the precious animal heat from white plumage or white pelage than from
+any other colour. The quality of warm-bloodedness is a prerogative of
+birds and mammals, and it means that the body keeps an almost constant
+temperature, day and night, year in and year out. This is effected by
+automatic internal adjustments which regulate the supply of heat,
+chiefly from the muscles, to the loss of heat, chiefly through the skin
+and from the lungs. The chief importance of this internal heat is that
+it facilitates the smooth continuance of the chemical processes on which
+life depends. If the temperature falls, as in hibernating mammals (whose
+warm-bloodedness is imperfect), the rate of the vital process is slowed
+down--sometimes dangerously. Thus we see how the white coat helps the
+life of the creature.
+
+
+§ 3
+
+Rapid Colour-change
+
+Bony flat-fishes, like plaice and sole, have a remarkable power of
+adjusting their hue and pattern to the surrounding gravel and sand, so
+that it is difficult to find them even when we know that they are there.
+It must be admitted that they are also very quick to get a sprinkling
+of sand over their upturned side, so that only the eyes are left
+showing. But there is no doubt as to the exactness with which they often
+adjust themselves to be like a little piece of the substratum on which
+they lie; they will do this within limits in experimental conditions
+when they are placed on a quite artificial floor. As these fishes are
+very palatable and are much sought after by such enemies as cormorants
+and otters, it is highly probably that their power of self-effacement
+often saves their life. And it may be effected within a few minutes, in
+some cases within a minute.
+
+In these self-effacing flat-fishes we know with some precision what
+happens. The adjustment of colour and pattern is due to changes in the
+size, shape, and position of mobile pigment-cells (chromatophores) and
+the skin. But what makes the pigment-cells change? The fact that a blind
+flat-fish does not change its colour gives us the first part of the
+answer. The colour and the pattern of the surroundings must affect the
+eye. The message travels by the optic nerve to the brain; from the
+brain, instead of passing down the spinal cord, the message travels down
+the chain of sympathetic ganglia. From these it passes along the nerves
+which comes out of the spinal cord and control the skin. Thus the
+message reaches the colour-cells in the skin, and before you have
+carefully read these lines the flat-fish has slipped on its Gyges ring
+and become invisible.
+
+The same power of rapid colour-change is seen in cuttlefishes, where it
+is often an expression of nervous excitement, though it sometimes helps
+to conceal. It occurs with much subtlety in the Æsop prawn, Hippolyte,
+which may be brown on a brown seaweed, green on sea-lettuce or
+sea-grass, red on red seaweed, and so on through an extensive repertory.
+
+ According to the nature of the background, [Professor Gamble writes]
+ so is the mixture of the pigments compounded so as to form a close
+ reproduction both of its colour and its pattern. A sweep of the
+ shrimp net detaches a battalion of these sleeping prawns, and if
+ we turn the motley into a dish and give a choice of seaweed, each
+ variety after its kind will select the one with which it agrees in
+ colour, and vanish. Both when young and when full-grown, the Æsop
+ prawn takes on the colour of its immediate surroundings. At
+ nightfall Hippolyte, of whatever colour, changes to a transparent
+ azure blue: its stolidity gives place to a nervous restlessness; at
+ the least tremor it leaps violently, and often swims actively from
+ one food-plant to another. This blue fit lasts till daybreak, and is
+ then succeeded by the prawn's diurnal tint.
+
+Thus, Professor Gamble continues, the colour of an animal may express a
+nervous rhythm.
+
+[Illustration: _Photo: J. J. Ward, F.E.S._
+
+PROTECTIVE RESEMBLANCE
+
+Hawk Moth, settled down on a branch, and very difficult to detect as
+long as it remains stationary. Note its remarkable sucking tongue, which
+is about twice the length of its body. The tongue can be quickly coiled
+up and put safely away beneath the lower part of the head.]
+
+[Illustration: WHEN ONLY A FEW DAYS OLD, YOUNG BITTERN BEGIN TO STRIKE
+THE SAME ATTITUDE AS THEIR PARENTS THRUSTING THEIR BILLS UPWARDS AND
+DRAWING THEIR BODIES UP SO THAT THEY RESEMBLE A BUNCH OF REEDS
+
+The soft browns and blue-greens harmonise with the dull sheaths of the
+young reeds; the nestling bittern is thus completely camouflaged.]
+
+
+The Case of Chameleons
+
+The highest level at which rapid colour-change occurs is among lizards,
+and the finest exhibition of it is among the chameleons. These quaint
+creatures are characteristic of Africa; but they occur also in
+Andalusia, Arabia, Ceylon, and Southern India. They are adapted for life
+on trees, where they hunt insects with great deliberateness and success.
+The protrusible tongue, ending in a sticky club, can be shot out for
+about seven inches in the common chameleon. Their hands and feet are
+split so that they grip the branches firmly, and the prehensile tail
+rivals a monkey's. When they wish they can make themselves very slim,
+contracting the body from side to side, so that they are not very
+readily seen. In other circumstances, however, they do not practise
+self-effacement, but the very reverse. They inflate their bodies, having
+not only large lungs, but air-sacs in connection with them. The throat
+bulges; the body sways from side to side; and the creature expresses its
+sentiments in a hiss. The power of colour-change is very remarkable, and
+depends partly on the contraction and expansion of the colour-cells
+(chromatophores) in the under-skin (or dermis) and partly on
+close-packed refractive granules and crystals of a waste-product called
+guanin. The repertory of possible colours in the common chameleon is
+greater than in any other animal except the Æsop prawn. There is a
+legend of a chameleon which was brown in a brown box, green in a green
+box, and blue in a blue box, and died when put into one lined with
+tartan; and there is no doubt that one and the same animal has a wide
+range of colours. The so-called "chameleon" (_Anolis_) of North America
+is so sensitive that a passing cloud makes it change its emerald hue.
+
+There is no doubt that a chameleon may make itself more inconspicuous by
+changing its colour, being affected by the play of light on its eyes. A
+bright-green hue is often seen on those that are sitting among strongly
+illumined green leaves. But the colour also changes with the time of day
+and with the animal's moods. A sudden irritation may bring about a rapid
+change; in other cases the transformation comes about very gradually.
+When the colour-change expresses the chameleon's feelings it might be
+compared to blushing, but that is due to an expansion of the arteries of
+the face, allowing more blood to get into the capillaries of the
+under-skin. The case of the chameleon is peculiarly interesting because
+the animal has two kinds of tactics--self-effacement on the one hand and
+bluffing on the other. There can be little doubt that the power of
+colour-change sometimes justifies itself by driving off intruders. Dr.
+Cyril Crossland observed that a chameleon attacked by a fox-terrier
+"turned round and opened its great pink mouth in the face of the
+advancing dog, at the same time rapidly changing colour, becoming almost
+black. This ruse succeeded every time, the dog turning off at once." In
+natural leafy surroundings the startling effect would be much greater--a
+sudden throwing off of the mantle of invisibility and the exposure of a
+conspicuous black body with a large red mouth.
+
+
+§ 4
+
+Likeness to Other Things
+
+Dr. H. O. Forbes tells of a flat spider which presents a striking
+resemblance to a bird's dropping on a leaf. Years after he first
+found it he was watching in a forest in the Far East when his eye fell
+on a leaf before him which had been blotched by a bird. He wondered idly
+why he had not seen for so long another specimen of the bird-dropping
+spider (_Ornithoscatoides decipiens_), and drew the leaf towards him.
+Instantaneously he got a characteristic sharp nip; it was the spider
+after all! Here the colour-resemblance was enhanced by a
+form-resemblance.
+
+[Illustration: A. PROTECTIVE COLORATION OR CAMOUFLAGING, GIVING ANIMALS
+A GARMENT OF INVISIBILITY
+
+At the foot of the plate is a Nightjar, with plumage like bark and
+withering leaves; to the right, resting on a branch, is shown a
+Chameleon in a green phase amid green surroundings; the insects on the
+reeds are Locusts; while a green Frog, merged into its surroundings,
+rests on a leaf near the centre at the top of the picture.
+
+B. ANOTHER EXAMPLE OF PROTECTIVE COLORATION OR CAMOUFLAGE
+
+A shore scene showing Trout in the pool almost invisible against their
+background. The Stone Curlews, both adult and young, are very
+inconspicuous among the stones on the beach.]
+
+But why should it profit a spider to be like a bird-dropping? Perhaps
+because it thereby escapes attention; but there is another possibility.
+It seems that some butterflies, allied to our Blues, are often attracted
+to excrementitious material, and the spider Dr. Forbes observed had
+actually caught its victim. This is borne out by a recent observation by
+Dr. D. G. H. Carpenter, who found a Uganda bug closely resembling a
+bird-dropping on sand. The bug actually settled down on a bird-dropping
+on sand, and caught a blue butterfly which came to feed there!
+
+Some of the walking-stick insects, belonging to the order of crickets
+and grasshoppers (Orthoptera), have their body elongated and narrow,
+like a thin dry branch, and they have a way of sticking out their limbs
+at abrupt and diverse angles, which makes the resemblance to twigs very
+close indeed. Some of these quaint insects rest through the day and have
+the remarkable habit of putting themselves into a sort of kataleptic
+state. Many creatures turn stiff when they get a shock, or pass suddenly
+into new surroundings, like some of the sand-hoppers when we lay them on
+the palm of our hand; but these twig-insects put themselves into this
+strange state. The body is rocked from side to side for a short time,
+and then it stiffens. An advantage may be that even if they were
+surprised by a bird or a lizard, they will not be able to betray
+themselves by even a tremor. Disguise is perfected by a remarkable
+habit, a habit which leads us to think of a whole series of different
+ways of lying low and saying nothing which are often of life-preserving
+value. The top end of the series is seen when a fox plays 'possum.
+
+The leaf-butterfly _Kallima_, conspicuously coloured on its upper
+surface, is like a withered leaf when it settles down and shows the
+under side of its wings. Here, again, there is precise form-resemblance,
+for the nervures on the wings are like the mid-rib and side veins on a
+leaf, and the touch of perfection is given in the presence of whitish
+spots which look exactly like the discolorations produced by lichens on
+leaves. An old entomologist, Mr. Jenner Weir, confessed that he
+repeatedly pruned off a caterpillar on a bush in mistake for a
+superfluous twig, for many brownish caterpillars fasten themselves by
+their posterior claspers and by an invisible thread of silk from their
+mouth, and project from the branch at a twig-like angle. An insect may
+be the very image of a sharp prickle or a piece of soft moss; a spider
+may look precisely like a tiny knob on a branch or a fragment of lichen;
+one of the sea-horses (_Phyllopteryx_) has frond-like tassels on various
+parts of its body, so that it looks extraordinarily like the seaweeds
+among which it lives. In a few cases, e.g. among spiders, it has been
+shown that animals with a special protective resemblance to something
+else seek out a position where this resemblance tells, and there is
+urgent need for observations bearing on this selection of environment.
+
+
+§ 5
+
+Mimicry in the True Sense
+
+It sometimes happens that in one and the same place there are two groups
+of animals not very nearly related which are "doubles" of one another.
+Investigation shows that the members of the one group, _always in the
+majority_, are in some way specially protected, e.g. by being
+unpalatable. They are the "mimicked." The members of the other group,
+_always in the minority_, have not got the special protection possessed
+by the others. They are the "mimickers," though the resemblance is not,
+of course, associated with any conscious imitation. The theory is that
+the mimickers live on the reputation of the mimicked. If the mimicked
+are left alone by birds because they have a reputation for
+unpalatability, or because they are able to sting, the mimickers
+survive--although they are palatable and stingless. They succeed, not
+through any virtue of their own, but because of their resemblance to the
+mimicked, for whom they are mistaken. There are many cases of mimetic
+resemblance so striking and so subtle that it seems impossible to doubt
+that the thing works; there are other cases which are rather
+far-fetched, and may be somewhat of the nature of coincidences. Thus
+although Mr. Bates tells us that he repeatedly shot humming-bird moths
+in mistake for humming-birds, we cannot think that this is a good
+illustration of mimicry. What is needed for many cases is what is
+forthcoming for some, namely, experimental evidence, e.g. that the
+unpalatable mimicked butterflies are left in relative peace while
+similar palatable butterflies are persecuted. It is also necessary to
+show that the mimickers do actually consort with the mimicked. Some
+beetles and moths are curiously wasplike, which may be a great
+advantage; the common drone-fly is superficially like a small bee; some
+harmless snakes are very like poisonous species; and Mr. Wallace
+maintained that the powerful "friar-birds" of the Far East are mimicked
+by the weak and timid orioles. When the model is unpalatable or
+repulsive or dangerous, and the mimic the reverse, the mimicry is called
+"Batesian" (after Mr. Bates), but there is another kind of mimicry
+called Müllerian (after Fritz Müller) where the mimic is also
+unpalatable. The theory in this case is that the mimicry serves as
+mutual assurance, the members of the ring getting on better by
+consistently presenting the same appearance, which has come to mean to
+possible enemies a signal, _Noli me tangere_ ("Leave me alone"). There
+is nothing out of the question in this theory, but it requires to be
+taken in a critical spirit. It leads us to think of "warning colours,"
+which are the very opposite of the disguises which we are now studying.
+Some creatures like skunks, magpies, coral-snakes, cobras, brightly
+coloured tree-frogs are obtrusive rather than elusive, and the theory
+of Alfred Russel Wallace was that the flaunting conspicuousness serves
+as a useful advertisement, impressing itself on the memories of
+inexperienced enemies, who soon learn to leave creatures with "warning
+colours" alone. In any case it is plain that an animal which is as safe
+as a wasp or a coral-snake can afford to wear any suit of clothes it
+likes.
+
+[Illustration: DEAD-LEAF BUTTERFLY (_Kallima Inachis_) FROM INDIA
+
+It is conspicuous on its upper surface, but when it settles down on a
+twig and shows the underside of its wings it is practically invisible.
+The colouring of the under surface of the wings is like that of the
+withering leaf; there are spots like fungas spots; and the venation of
+the wings suggests the mid-rib and veins of the leaf. A, showing upper
+surface; B, showing under surface; C, a leaf.]
+
+[Illustration: PROTECTIVE RESEMBLANCE BETWEEN A SMALL SPIDER (_to the
+left_) AND AN ANT (_to the right_)
+
+As ants are much dreaded, it is probably profitable to the spider to be
+like an ant. It will be noted that the spider has four pairs of legs and
+no feelers, whereas the ant has three pairs of legs and a pair of
+feelers.]
+
+[Illustration: _Photo: J. J. Ward, F.E.S._
+
+THE WASP BEETLE, WHICH, WHEN MOVING AMONGST THE BRANCHES GIVES A
+WASP-LIKE IMPRESSION]
+
+[Illustration: HERMIT-CRAB WITH PARTNER SEA-ANEMONES
+
+Hermit-crabs hide their soft tail in the shell of a whelk or some other
+sea-snail. But some hermit-crabs place sea-anemones on the back of their
+borrowed shell. The sea-anemones mask the hermit-crab and their
+tentacles can sting. As for the sea-anemones, they are carried about by
+the hermit-crab and they get crumbs from its table. This kind of
+mutually beneficial external partnership is called commensalism, i.e.
+eating at the same table.]
+
+[Illustration: _Photo: G. P. Duffus._
+
+CUCKOO-SPIT
+
+The white mass in the centre of the picture is a soapy froth which the
+young frog-hopper makes, and within which it lies safe both from the
+heat of the sun and almost all enemies. After sojourning for a time in
+the cuckoo-spit, the frog-hopper becomes a winged insect.]
+
+
+Masking
+
+The episode in Scottish history called "The Walking Wood of Birnam,"
+when the advancing troop masked their approach by cutting down branches
+of the trees, has had its counterpart in many countries. But it is also
+enacted on the seashore. There are many kinds of crabs that put on
+disguise with what looks like deliberateness. The sand-crab takes a
+piece of seaweed, nibbles at the end of it, and then rubs it on the back
+of the carapace or on the legs so that it fixes to the bristles. As the
+seaweed continues to live, the crab soon has a little garden on its back
+which masks the crab's real nature. It is most effective camouflaging,
+but if the crab continues to grow it has to moult, and that means losing
+the disguise. It is then necessary to make a new one. The crab must have
+on the shore something corresponding to a reputation; that is to say,
+other animals are clearly or dimly aware that the crab is a voracious
+and combative creature. How useful to the crab, then, to have its
+appearance cloaked by a growth of innocent seaweed, or sponge, or
+zoophyte. It will enable the creature to sneak upon its victims or to
+escape the attention of its own enemies.
+
+If a narrow-beaked crab is cleaned artificially it will proceed to
+clothe itself again, the habit has become instinctive; and it must be
+admitted that while a particular crab prefers a particular kind of
+seaweed for its dress, it will cover itself with unsuitable and even
+conspicuous material, such as pieces of coloured cloth, if nothing
+better is available. The disguise differs greatly, for one crab is
+masked by a brightly coloured and unpalatable sponge densely packed
+with flinty needles; another cuts off the tunic of a sea-squirt and
+throws it over its shoulders; another trundles about a bivalve shell.
+The facts recall the familiar case of the hermit-crab, which protects
+its soft tail by tucking it into the empty shell of a periwinkle or a
+whelk or some other sea-snail, and that case leads on to the elaboration
+known as commensalism, where the hermit-crab fixes sea-anemones on the
+back of its borrowed house. The advantage here is beyond that of
+masking, for the sea-anemone can sting, which is a useful quality in a
+partner. That this second advantage may become the main one is evident
+in several cases where the sea-anemone is borne, just like a weapon, on
+each of the crustacean's great claws. Moreover, as the term commensalism
+(eating at the same table) suggests, the partnership is _mutually_
+beneficial. For the sea-anemone is carried about by the hermit-crab, and
+it doubtless gets its share of crumbs from its partner's frequent meals.
+There is a very interesting sidelight on the mutual benefit in the case
+of a dislodged sea-anemone which sulked for a while and then waited in a
+state of preparedness until a hermit-crab passed by and touched it.
+Whereupon the sea-anemone gripped and slowly worked itself up on to the
+back of the shell.
+
+
+§ 6
+
+Other Kinds of Elusiveness
+
+There are various kinds of disguise which are not readily classified. A
+troop of cuttlefish swimming in the sea is a beautiful sight. They keep
+time with one another in their movements and they show the same change
+of colour almost at the same moment. They are suddenly attacked,
+however, by a small shark, and then comes a simultaneous discharge of
+sepia from their ink-bags. There are clouds of ink in the clear water,
+for, as Professor Hickson puts it, the cuttlefishes have thrown dust in
+the eyes of their enemies. One can see a newborn cuttlefish do this a
+minute after it escapes from the egg.
+
+Very beautiful is the way in which many birds, like our common
+chaffinch, disguise the outside of their nest with moss and lichen and
+other trifles felted together, so that the cradle is as inconspicuous as
+possible. There seems to be a touch of art in fastening pieces of
+spider's web on the outside of a nest!
+
+How curious is the case of the tree-sloth of South American forests,
+that walks slowly, back downwards, along the undersides of the branches,
+hanging on by its long, curved fingers and toes. It is a nocturnal
+animal, and therefore not in special danger, but when resting during the
+day it is almost invisible because its shaggy hair is so like certain
+lichens and other growths on the branches. But the protective
+resemblance is enhanced by the presence of a green alga, which actually
+lives on the surface of the sloth's hairs--an alga like the one that
+makes tree-stems and gate-posts green in damp weather.
+
+There is no commoner sight in the early summer than the cuckoo-spit on
+the grasses and herbage by the wayside. It is conspicuous and yet it is
+said to be left severely alone by almost all creatures. In some way it
+must be a disguise. It is a sort of soap made by the activity of small
+frog-hoppers while they are still in the wingless larval stage, before
+they begin to hop. The insect pierces with its sharp mouth-parts the
+skin of the plant and sucks in sweet sap which by and by overflows over
+its body. It works its body up and down many times, whipping in air,
+which mixes with the sugary sap, reminding one of how "whipped egg" is
+made. But along with the sugary sap and the air, there is a little
+ferment from the food-canal and a little wax from glands on the skin,
+and the four things mixed together make a kind of soap which lasts
+through the heat of the day.
+
+There are many other modes of disguise besides those which we have been
+able to illustrate. Indeed, the biggest fact is that there are so many,
+for it brings us back to the idea that life is not an easy business. It
+is true, as Walt Whitman says, that animals do not sweat and whine about
+their condition; perhaps it is true, as he says, that not one is
+unhappy over the whole earth. But there is another truth, that this
+world is not a place for the unlit lamp and the ungirt loin, and that
+when a creature has not armour or weapons or cleverness it must find
+some path of safety or go back. One of these paths of safety is
+disguise, and we have illustrated its evolution.
+
+
+
+
+V
+
+THE ASCENT OF MAN
+
+
+
+
+THE ASCENT OF MAN
+
+
+§ 1
+
+No one thinks less of Sir Isaac Newton because he was born as a very
+puny infant, and no one should think less of the human race because it
+sprang from a stock of arboreal mammals. There is no doubt as to man's
+apartness from the rest of creation when he is seen at his best--"a
+little lower than the angels, crowned with glory and honour." "What a
+piece of work is a man! How noble in reason! How infinite in faculty! in
+form and moving how express and admirable! in action how like an angel!
+in apprehension so like a God." Nevertheless, all the facts point to his
+affiliation to the stock to which monkeys and apes also belong. Not,
+indeed, that man is descended from any living ape or monkey; it is
+rather that he and they have sprung from a common ancestry--are branches
+of the same stem. This conclusion is so momentous that the reasons for
+accepting it must be carefully considered. They were expounded with
+masterly skill in Darwin's _Descent of Man_ in 1871--a book which was
+but an expansion of a chapter in _The Origin of Species_ (1859).
+
+
+Anatomical Proof of Man's Relationship with a Simian Stock
+
+The anatomical structure of man is closely similar to that of the
+anthropoid apes--the gorilla, the orang, the chimpanzee, and the gibbon.
+Bone for bone, muscle for muscle, blood-vessel for blood-vessel, nerve
+for nerve, man and ape agree. As the conservative anatomist, Sir
+Richard Owen, said, there is between them "an all-pervading similitude
+of structure." Differences, of course, there are, but they are not
+momentous except man's big brain, which may be three times as heavy as
+that of a gorilla. The average human brain weighs about 48 ounces; the
+gorilla brain does not exceed 20 ounces at its best. The capacity of the
+human skull is never less than 55 cubic inches; in the orang and the
+chimpanzee the figures are 26 and 27-1/2 respectively. We are not
+suggesting that the most distinctive features of man are such as can be
+measured and weighed, but it is important to notice that the main seat
+of his mental powers is physically far ahead of that of the highest of
+the anthropoid apes.
+
+Man alone is thoroughly erect after his infancy is past; his head
+weighted with the heavy brain does not droop forward as the ape's does;
+with his erect attitude there is perhaps to be associated his more
+highly developed vocal organs. Compared with an anthropoid ape, man has
+a bigger and more upright forehead, a less protrusive face region,
+smaller cheek-bones and eyebrow ridges, and more uniform teeth. He is
+almost unique in having a chin. Man plants the sole of his foot flat on
+the ground, his big toe is usually in a line with the other toes, and he
+has a better heel than any monkey has. The change in the shape of the
+head is to be thought of in connection with the enlargement of the
+brain, and also in connection with the natural reduction of the muzzle
+region when the hand was freed from being an organ of support and became
+suited for grasping the food and conveying it to the mouth.
+
+Everyone is familiar in man's clothing with traces of the past
+persisting in the present, though their use has long since disappeared.
+There are buttons on the back of the waist of the morning coat to which
+the tails of the coat used to be fastened up, and there are buttons,
+occasionally with buttonholes, at the wrist which were once useful in
+turning up the sleeve. The same is true of man's body, which is a
+veritable museum of relics. Some anatomists have made out a list of
+over a hundred of these _vestigial_ structures, and though this number
+is perhaps too high, there is no doubt that the list is long. In the
+inner upper corner of the eye there is a minute tag--but larger in some
+races than in others--which is the last dwindling relic of the third
+eyelid, used in cleaning the front of the eye, which most mammals
+possess in a large and well-developed form. It can be easily seen, for
+instance, in ox and rabbit. In man and in monkeys it has become a
+useless vestige, and the dwindling must be associated with the fact that
+the upper eyelid is much more mobile in man and monkeys than in the
+other mammals. The vestigial third eyelid in man is enough of itself to
+prove his relationship with the mammals, but it is only one example out
+of many. Some of these are discussed in the article dealing with the
+human body, but we may mention the vestigial muscles going to the
+ear-trumpet, man's dwindling counterpart of the skin-twitching muscle
+which we see a horse use when he jerks a fly off his flanks, and the
+short tail which in the seven-weeks-old human embryo is actually longer
+than the leg. Without committing ourselves to a belief in the entire
+uselessness of the vermiform appendix, which grows out as a blind alley
+at the junction of the small intestine with the large, we are safe in
+saying that it is a dwindling structure--the remains of a blind gut
+which must have been capacious and useful in ancestral forms. In some
+mammals, like the rabbit, the blind gut is the bulkiest structure in the
+body, and bears the vermiform appendix at its far end. In man the
+appendix alone is left, and it tells its tale. It is interesting to
+notice that it is usually longer in the orang than in man, and that it
+is very variable, as dwindling structures tend to be. One of the
+unpleasant expressions of this variability is the liability to go wrong:
+hence appendicitis. Now these vestigial structures are, as Darwin said,
+like the unsounded, i.e. functionless, letters in words, such as the _o_
+in "leopard," the _b_ in "doubt," the _g_ in "reign." They are of no
+use, but they tell us something of the history of the words. So do man's
+vestigial structures reveal his pedigree. They must have an historical
+or evolutionary significance. No other interpretation is possible.
+
+[Illustration: _Photo: New York Zoological Park._
+
+CHIMPANZEE, SITTING
+
+The head shows certain facial characteristics, e.g. the beetling eyebrow
+ridges, which were marked in the Neanderthal race of men. Note the
+shortening of the thumb and the enlargement of the big toe.]
+
+[Illustration: _Photo: New York Zoological Park._
+
+CHIMPANZEE, ILLUSTRATING WALKING POWERS
+
+Note the great length of the arms and the relative shortness of the
+legs.]
+
+[Illustration: SURFACE VIEW OF THE BRAINS OF MAN (1) AND CHIMPANZEE (2)
+
+The human brain is much larger and heavier, more dome-like, and with
+much more numerous and complicated convolutions.]
+
+[Illustration: _Photo: New York Zoological Park._
+
+SIDE-VIEW OF CHIMPANZEE'S HEAD.
+
+(Compare with opposite picture.)]
+
+[Illustration: _After a model by J. H. McGregor._
+
+PROFILE VIEW OF HEAD OF PITHECANTHROPUS, THE JAVA APE MAN, RECONSTRUCTED
+FROM THE SKULL-CAP.]
+
+[Illustration: THE FLIPPER OF A WHALE AND THE HAND OF A MAN
+
+In the bones and in their arrangement there is a close resemblance in
+the two cases, yet the outcome is very different. The multiplication of
+finger joints in the whale is a striking feature.]
+
+Some men, oftener than women, show on the inturned margin of the
+ear-trumpet or pinna, a little conical projection of great interest. It
+is a vestige of the tip of the pointed ear of lower mammals, and it is
+well named _Darwin's point_. It was he who described it as a "surviving
+symbol of the stirring times and dangerous days of man's animal youth."
+
+
+§ 2
+
+Physiological Proof of Man's Relationship with a Simian Stock
+
+The everyday functions of the human body are practically the same as
+those of the anthropoid ape, and similar disorders are common to both.
+Monkeys may be infected with certain microbes to which man is peculiarly
+liable, such as the bacillus of tuberculosis. Darwin showed that various
+human gestures and facial expressions have their counterparts in
+monkeys. The sneering curl of the upper lip, which tends to expose the
+canine tooth, is a case in point, though it may be seen in many other
+mammals besides monkeys--in dogs, for instance, which are at some
+considerable distance from the simian branch to which man's ancestors
+belonged.
+
+When human blood is transfused into a dog or even a monkey, it behaves
+in a hostile way to the other blood, bringing about a destruction of the
+red blood corpuscles. But when it is transfused into a chimpanzee there
+is an harmonious mingling of the two. This is a very literal
+demonstration of man's blood-relationship with the higher apes. But
+there is a finer form of the same experiment. When the blood-fluid (or
+serum) of a rabbit, which has had human blood injected into it, is
+mingled with human blood, it forms a cloudy precipitate. It forms almost
+as marked a precipitate when it is mingled with the blood of an
+anthropoid ape. But when it is mingled with the blood of an American
+monkey there is only a slight clouding after a considerable time and
+no actual precipitate. When it is added to the blood of one of the
+distantly related "half-monkeys" or lemurs there is no reaction or only
+a very weak one. With the blood of mammals off the simian line
+altogether there is no reaction at all. Thus, as a distinguished
+anthropologist, Professor Schwalbe, has said: "We have in this not only
+a proof of the literal blood-relationship between man and apes, but the
+degree of relationship with the different main groups of apes can be
+determined beyond possibility of mistake." We can imagine how this
+modern line of experiment would have delighted Darwin.
+
+[Illustration: THE GORILLA, INHABITING THE FOREST TRACT OF THE GABOON IN
+AFRICA
+
+A full-grown individual stands about 5 feet high. The gait is shuffling,
+the strength enormous, the diet mainly vegetarian, the temper rather
+ferocious.]
+
+
+Embryological Proof of Man's Relationship with a Simian Stock
+
+In his individual development, man does in some measure climb up his own
+genealogical tree. Stages in the development of the body during its nine
+months of ante-natal life are closely similar to stages in the
+development of the anthropoid embryo. Babies born in times of famine or
+siege are sometimes, as it were, imperfectly finished, and sometimes
+have what may be described as monkeyish features and ways. A visit to an
+institution for the care of children who show arrested, defective, or
+disturbed development leaves one sadly impressed with the risk of
+slipping down the rungs of the steep ladder of evolution; and even in
+adults the occurrence of serious nervous disturbance, such as
+"shell-shock," is sometimes marked by relapses to animal ways. It is a
+familiar fact that a normal baby reveals the past in its surprising
+power of grip, and the careful experiments of Dr. Louis Robinson showed
+that an infant three weeks old could support its own weight for over two
+minutes, holding on to a horizontal bar. "In many cases no sign of
+distress is evinced and no cry uttered, until the grasp begins to give
+way." This persistent grasp probably points back to the time when the
+baby had to cling to its arboreal mother. The human tail is represented
+in the adult by a fusion of four or five vertebræ forming the "coccyx"
+at the end of the backbone, and is normally concealed beneath the
+flesh, but in the embryo the tail projects freely and is movable. Up to
+the sixth month of the ante-natal sleep the body is covered, all but the
+palms and soles, with longish hair (the lanugo), which usually
+disappears before birth. This is a stage in the normal development,
+which is reasonably interpreted as a recapitulation of a stage in the
+racial evolution. We draw this inference when we find that the unborn
+offspring of an almost hairless whale has an abundant representation of
+hairs; we must draw a similar inference in the case of man.
+
+It must be noticed that there are two serious errors in the careless
+statement often made that man in his development is at one time like a
+little fish, at a later stage like a little reptile, at a later stage
+like a little primitive mammal, and eventually like a little monkey. The
+first error here is that the comparison should be made with
+_embryo_-fish, _embryo_-reptile, _embryo_-mammal, and so on. It is in
+the making of the embryos that the great resemblance lies. When the
+human embryo shows the laying down of the essential vertebrate
+characters, such as brain and spinal cord, then it is closely comparable
+to the embryo of a lower vertebrate at a similar stage. When, at a
+subsequent stage, its heart, for instance, is about to become a
+four-chambered mammalian heart, it is closely comparable to the heart
+of, let us say, a turtle, which never becomes more than three-chambered.
+The point is that in the making of the organs of the body, say brain and
+kidneys, the embryo of man pursues a path closely corresponding to the
+path followed by the embryos of other backboned animals lower in the
+scale, but at successive stages it parts company with these, with the
+lowest first and so on in succession. A human embryo is never like a
+little reptile, but the developing organs pass through stages which very
+closely resemble the corresponding stages in lower types which are in a
+general way ancestral.
+
+The second error is that every kind of animal, man included, has from
+the first a certain individuality, with peculiar characteristics which
+are all its own. This is expressed by the somewhat difficult word
+_specificity_, which just means that every species is itself and no
+other. So in the development of the human embryo, while there are close
+resemblances to the embryos of apes, monkeys, other mammals, and even,
+at earlier stages still, to the embryos of reptile and fish, it has to
+be admitted that we are dealing from first to last with a human embryo
+with peculiarities of its own.
+
+[Illustration: "DARWIN'S POINT" ON HUMAN EAR (MARKED D.P.)
+
+It corresponds to the tip (T) of the ear of an ordinary mammal, as shown
+in the hare's ear below. In the young orang the part corresponding to
+Darwin's point is still at the tip of the ear.]
+
+[Illustration: _Photo: J. Russell & Sons._
+
+PROFESSOR SIR ARTHUR KEITH, M.D., LL.D., F.R.S.
+
+Conservator of the Museum and Hunterian Professor, Royal College of
+Surgeons of England. One of the foremost living anthropologists and a
+leading authority on the antiquity of man.]
+
+[Illustration: _After T. H. Huxley (by permission of Messrs.
+Macmillan)._
+
+SKELETONS OF THE GIBBON, ORANG, CHIMPANZEE, GORILLA, MAN
+
+Photographically reduced from diagrams of the natural size (except that
+of the gibbon, which was twice as large as nature) drawn by Mr.
+Waterhouse Hawkins from specimens in the Museum of the Royal College of
+Surgeons.]
+
+Every human being begins his or her life as a single cell--a fertilised
+egg-cell, a treasure-house of all the ages. For in this living
+microcosm, only a small fraction (1/125) of an inch in diameter, there
+is condensed--who can imagine how?--all the natural inheritance of man,
+all the legacy of his parentage, of his ancestry, of his long pre-human
+pedigree. Darwin called the pinhead brain of the ant the most marvellous
+atom of matter in the world, but the human ovum is more marvellous
+still. It has more possibilities in it than any other thing, yet without
+fertilisation it will die. The fertilised ovum divides and redivides;
+there results a ball of cells and a sack of cells; gradually division of
+labour becomes the rule; there is a laying down of nervous system and
+food-canal, muscular system and skeleton, and so proceeds what is
+learnedly called differentiation. Out of the apparently simple there
+emerges the obviously complex. As Aristotle observed more than two
+thousand years ago, in the developing egg of the hen there soon appears
+the beating heart! There is nothing like this in the non-living world.
+But to return to the developing human embryo, there is formed from and
+above the embryonic food-canal a skeletal rod, which is called the
+notochord. It thrills the imagination to learn that this is the only
+supporting axis that the lower orders of the backboned race possess. The
+curious thing is that it does not become the backbone, which is
+certainly one of the essential features of the vertebrate race. The
+notochord is the supporting axis of the pioneer backboned animals,
+namely the Lancelets and the Round-mouths (Cyclostomes), such as the
+Lamprey. They have no backbone in the strict sense, but they have this
+notochord. It can easily be dissected out in the lamprey--a long gristly
+rod. It is surrounded by a sheath which becomes the backbone of most
+fishes and of all higher animals. The interesting point is that although
+the notochord is only a vestige in the adults of these types, it is
+never absent from the embryo. It occurs even in man, a short-lived relic
+of the primeval supporting axis of the body. It comes and then it goes,
+leaving only minute traces in the adult. We cannot say that it is of any
+use, unless it serves as a stimulus to the development of its
+substitute, the backbone. It is only a piece of preliminary scaffolding,
+but there is no more eloquent instance of the living hand of the past.
+
+One other instance must suffice of what Professor Lull calls the
+wonderful changes wrought in the dark of the ante-natal period, which
+recapitulate in rapid abbreviation the great evolutionary steps which
+were taken by man's ancestors "during the long night of the geological
+past." On the sides of the neck of the human embryo there are four pairs
+of slits, the "visceral clefts," openings from the beginning of the
+food-canals to the surface. There is no doubt as to their significance.
+They correspond to the gill-slits of fishes and tadpoles. Yet in
+reptiles, birds, and mammals they have no connection with breathing,
+which is their function in fishes and amphibians. Indeed, they are not
+of any use at all, except that the first becomes the Eustachian tube
+bringing the ear-passage into connection with the back of the mouth, and
+that the second and third have to do with the development of a curious
+organ called the thymus gland. Persistent, nevertheless, these
+gill-slits are, recalling even in man an aquatic ancestry of many
+millions of years ago.
+
+When all these lines of evidence are considered, they are seen to
+converge in the conclusion that man is derived from a simian stock of
+mammals. He is solidary with the rest of creation. To quote the closing
+words of Darwin's _Descent of Man_:
+
+ We must, however, acknowledge, as it seems to me, that man with all
+ his noble qualities, with sympathy which feels for the most debased,
+ with benevolence which extends not only to other men but to the
+ humblest living creature, with his God-like intellect, which has
+ penetrated into the movements and constitution of the solar
+ system--with all these exalted powers--man still bears in his bodily
+ frame the indelible stamp of his lowly origin.
+
+We should be clear that this view does not say more than that man sprang
+from a stock common to him and to the higher apes. Those who are
+repelled by the idea of man's derivation from a simian type should
+remember that the theory implies rather more than this, namely, that man
+is the outcome of a genealogy which has implied many millions of years
+of experimenting and sifting--the groaning and travailing of a whole
+creation. Speaking of man's mental qualities, Sir Ray Lankester says:
+"They justify the view that man forms a new departure in the gradual
+unfolding of Nature's predestined plan." In any case, we have to try to
+square our views with the facts, not the facts with our views, and while
+one of the facts is that man stands unique and apart, the other is that
+man is a scion of a progressive simian stock. Naturalists have exposed
+the pit whence man has been digged and the rock whence he has been hewn,
+but it is surely a heartening encouragement to know that it is an
+ascent, not a descent, that we have behind us. There is wisdom in
+Pascal's maxim:
+
+ It is dangerous to show man too plainly how like he is to the
+ animals, without, at the same time, reminding him of his greatness.
+ It is equally unwise to impress him with his greatness and not with
+ his lowliness. It is worse to leave him in ignorance of both. But it
+ is very profitable to recognise the two facts.
+
+
+§ 3
+
+Man's Pedigree
+
+The facts of anatomy, physiology, and embryology, of which we have given
+illustrations, all point to man's affiliation with the order of monkeys
+and apes. To this order is given the name Primates, and our first and
+second question must be when and whence the Primates began. The rock
+record answers the first question: the Primates emerged about the dawn
+of the Eocene era, when grass was beginning to cover the earth with a
+garment. Their ancestral home was in the north in both hemispheres, and
+then they migrated to Africa, India, Malay, and South America. In North
+America the Primates soon became extinct, and the same thing happened
+later on in Europe. In this case, however, there was a repeopling from
+the South (in the Lower Miocene) and then a second extinction (in the
+Upper Pliocene) before man appeared. There is considerable evidence in
+support of Professor R. S. Lull's conclusion, that in Southern Asia,
+Africa, and South America the evolution of Primates was continuous since
+the first great southward migration, and there is, of course, an
+abundant modern representation of Primates in these regions to-day.
+
+As to the second question: Whence the Primates sprang, the answer must
+be more conjectural. But it is a reasonable view that Carnivores and
+Primates sprang from a common Insectivore stock, the one order diverging
+towards flesh-eating and hunting on the ground, the other order
+diverging towards fruit-eating and arboreal habits. There is no doubt
+that the Insectivores (including shrews, tree-shrews, hedgehog, mole,
+and the like) were very plastic and progressive mammals.
+
+What followed in the course of ages was the divergence of branch after
+branch from the main Primate stem. First there diverged the South
+American monkeys on a line of their own, and then the Old World monkeys,
+such as the macaques and baboons. Ages passed and the main stems gave
+off (in the Oligocene period) the branch now represented by the small
+anthropoid apes--the gibbon and the siamang. Distinctly later there
+diverged the branch of the large anthropoid apes--the gorilla, the
+chimpanzee, and the orang. That left a generalised humanoid stock
+separated off from all monkeys and apes, and including the immediate
+precursors of man. When this sifting out of a generalised humanoid stock
+took place remains very uncertain, some authorities referring it to the
+Miocene, others to the early Pliocene. Some would estimate its date at
+half a million years ago, others at two millions! The fact is that
+questions of chronology do not as yet admit of scientific statement.
+
+[Illustration: SIDE-VIEW OF SKULL OF MAN (M) AND GORILLA (G)
+
+Notice in the gorilla's skull the protrusive face region, the big
+eyebrow ridges, the much less domed cranial cavity, the massive lower
+jaw, the big canine teeth. Notice in man's skull the well-developed
+forehead, the domed and spacious cranial cavity, the absence of any
+snout, the chin process, and many other marked differences separating
+the human skull from the ape's.]
+
+[Illustration: THE SKULL AND BRAIN-CASE OF PITHECANTHROPUS, THE JAVA
+APE-MAN, AS RESTORED. BY J. H. McGREGOR FROM THE SCANTY REMAINS
+
+The restoration shows the low, retreating forehead and the prominent
+eyebrow ridges.]
+
+[Illustration: SUGGESTED GENEALOGICAL TREE OF MAN AND ANTHROPOID APES
+
+From Sir Arthur Keith; the lettering to the right has been slightly
+simplified.]
+
+We are on firmer, though still uncertain, ground when we state the
+probability that it was in Asia that the precursors of man were
+separated off from monkeys and apes, and began to be terrestrial rather
+than arboreal. Professor Lull points out that Asia is nearest to the
+oldest known human remains (in Java), and that Asia was the seat of the
+most ancient civilisations and the original home of many domesticated
+animals and cultivated plants. The probability is that the cradle of the
+human race was in Asia.
+
+
+Man's Arboreal Apprenticeship
+
+At this point it will be useful to consider man's arboreal
+apprenticeship and how he became a terrestrial journeyman. Professor
+Wood Jones has worked out very convincingly the thesis that man had no
+direct four-footed ancestry, but that the Primate stock to which he
+belongs was from its first divergence arboreal. He maintains that the
+leading peculiarities of the immediate precursors of man were wrought
+out during a long arboreal apprenticeship. The first great gain of
+arboreal life on bipedal erect lines (not after the quadrupedal fashion
+of tree-sloths, for instance) was the emancipation of the hand. The
+foot became the supporting and branch-gripping member, and the hand was
+set free to reach upward, to hang on by, to seize the fruit, to lift it
+and hold it to the mouth, and to hug the young one close to the breast.
+The hand thus set free has remained plastic--a generalised, not a
+specialised member. Much has followed from man's "handiness."
+
+The arboreal life had many other consequences. It led to an increased
+freedom of movement of the thigh on the hip joint, to muscular
+arrangements for balancing the body on the leg, to making the backbone a
+supple yet stable curved pillar, to a strongly developed collar-bone
+which is only found well-formed when the fore-limb is used for more than
+support, and to a power of "opposing" the thumb and the big toe to the
+other digits of the hand and foot--an obvious advantage for
+branch-gripping. But the evolution of a free hand made it possible to
+dispense with protrusive lips and gripping teeth. Thus began the
+recession of the snout region, the associated enlargement of the
+brain-box, and the bringing of the eyes to the front. The overcrowding
+of the teeth that followed the shortening of the snout was one of the
+taxes on progress of which modern man is often reminded in his dental
+troubles.
+
+Another acquisition associated with arboreal life was a greatly
+increased power of turning the head from side to side--a mobility very
+important in locating sounds and in exploring with the eyes.
+Furthermore, there came about a flattening of the chest and of the back,
+and the movements of the midriff (or diaphragm) came to count for more
+in respiration than the movements of the ribs. The sense of touch came
+to be of more importance and the sense of smell of less; the part of the
+brain receiving tidings from hand and eye and ear came to predominate
+over the part for receiving olfactory messages. Finally, the need for
+carrying the infant about among the branches must surely have implied an
+intensification of family relations, and favoured the evolution of
+gentleness.
+
+[Illustration: _Photo: New York Zoological Park._
+
+THE GIBBON IS LOWER THAN THE OTHER APES AS REGARDS ITS SKULL AND
+DENTITION, BUT IT IS HIGHLY SPECIALIZED IN THE ADAPTATION OF ITS LIMBS
+TO ARBOREAL LIFE]
+
+[Illustration: _Photo: New York Zoological Park._
+
+THE ORANG HAS A HIGH ROUNDED SKULL AND A LONG FACE]
+
+[Illustration: _Photo: British Museum (Natural History)._
+
+COMPARISONS OF THE SKELETONS OF HORSE AND MAN
+
+Bone for bone, the two skeletons are like one another, though man is a
+biped and the horse a quadruped. The backbone in man is mainly vertical;
+the backbone in the horse is horizontal except in the neck and the tail.
+Man's skull is mainly in a line with the backbone; the horse's at an
+angle to it. Both man and horse have seven neck vertebræ. Man has five
+digits on each limb; the horse has only one digit well developed on each
+limb.]
+
+It may be urged that we are attaching too much importance to the
+arboreal apprenticeship, since many tree-loving animals remain to-day
+very innocent creatures. To this reasonable objection there are two
+answers, first that in its many acquisitions the arboreal evolution of
+the _humanoid_ precursors of man prepared the way for the survival of a
+_human_ type marked by a great step in brain-development; and second
+that the passage from the humanoid to the human was probably associated
+with _a return to mother earth_.
+
+According to Professor Lull, to whose fine textbook, _Organic Evolution_
+(1917), we are much indebted, "climatic conditions in Asia in the
+Miocene or early Pliocene were such as to compel the descent of the
+pre-human ancestor from the trees, a step which was absolutely essential
+to further human development." Continental elevation and consequent
+aridity led to a dwindling of the forests, and forced the ape-man to
+come to earth. "And at the last arose the man."
+
+According to Lull, the descent from the trees was associated with the
+assumption of a more erect posture, with increased liberation and
+plasticity of the hand, with becoming a hunter, with experiments towards
+clothing and shelter, with an exploring habit, and with the beginning of
+communal life.
+
+It is a plausible view that the transition from the humanoid to the
+human was effected by a discontinuous variation of considerable
+magnitude, what is nowadays called a _mutation_, and that it had mainly
+to do with the brain and the vocal organs. But given the gains of the
+arboreal apprenticeship, the stimulus of an enforced descent to terra
+firma, and an evolving brain and voice, we can recognise accessory
+factors which helped success to succeed. Perhaps the absence of great
+physical strength prompted reliance on wits; the prolongation of infancy
+would help to educate the parents in gentleness; the strengthening of
+the feeling of kinship would favour the evolution of family and social
+life--of which there are many anticipations at lower levels. There is
+much truth in the saying: "Man did not make society, society made man."
+
+A continuation of the story will deal with the emergence of the
+primitive types of man and the gradual ascent of the modern species.
+
+
+§ 4
+
+Tentative Men
+
+So far the story has been that of the sifting out of a humanoid stock
+and of the transition to human kind, from the ancestors of apes and men
+to the man-ape, and from the man-ape to man. It looks as if the
+sifting-out process had proceeded further, for there were several human
+branches that did not lead on to the modern type of man.
+
+1. The first of these is represented by the scanty fossil remains known
+as _Pithecanthropus erectus_, found in Java in fossiliferous beds which
+date from the end of the Pliocene or the beginning of the Pleistocene
+era. Perhaps this means half a million years ago, and the remains
+occurred along with those of some mammals which are now extinct.
+Unfortunately the remains of Pithecanthropus the Erect consisted only of
+a skull-cap, a thigh-bone, and two back teeth, so it is not surprising
+that experts should differ considerably in their interpretation of what
+was found. Some have regarded the remains as those of a large gibbon,
+others as those of a pre-human ape-man, and others as those of a
+primitive man off the main line of ascent. According to Sir Arthur
+Keith, Pithecanthropus was "a being human in stature, human in gait,
+human in all its parts, save its brain." The thigh-bone indicates a
+height of about 5 feet 7 inches, one inch less than the average height
+of the men of to-day. The skull-cap indicates a low, flat forehead,
+beetling brows, and a capacity about two-thirds of the modern size. The
+remains were found by Dubois, in 1894, in Trinil in Central Java.
+
+2. The next offshoot is represented by the Heidelberg man (_Homo
+heidelbergensis_), discovered near Heidelberg in 1907 by Dr.
+Schoetensack. But the remains consisted only of a lower jaw and its
+teeth. Along with this relic were bones of various mammals, including
+some long since extinct in Europe, such as elephant, rhinoceros, bison,
+and lion. The circumstances indicate an age of perhaps 300,000 years
+ago. There were also very crude flint implements (or eoliths). But the
+teeth are human teeth, and the jaw seems transitional between that of an
+anthropoid ape and that of man. Thus there was no chin. According to
+most authorities the lower jaw from the Heidelberg sand-pit must be
+regarded as a relic of a primitive type off the main line of human
+ascent.
+
+[Illustration: A RECONSTRUCTION OF THE JAVA MAN
+
+(_Pithecanthropus erectus._)]
+
+3. It was in all probability in the Pliocene that there took origin the
+Neanderthal species of man, _Homo neanderthalensis_, first known from
+remains found in 1856 in the Neanderthal ravine near Düsseldorf.
+According to some authorities Neanderthal man was living in Europe a
+quarter of a million years ago. Other specimens were afterwards found
+elsewhere, e.g. in Belgium ("the men of Spy"), in France, in Croatia,
+and at Gibraltar, so that a good deal is known of Neanderthal man. He
+was a loose-limbed fellow, short of stature and of slouching gait, but a
+skilful artificer, fashioning beautifully worked flints with a
+characteristic style. He used fire; he buried his dead reverently and
+furnished them with an outfit for a long journey; and he had a big
+brain. But he had great beetling, ape-like eyebrow ridges and massive
+jaws, and he showed "simian characters swarming in the details of his
+structure." In most of the points in which he differs from modern man he
+approaches the anthropoid apes, and he must be regarded as a low type of
+man off the main line. Huxley regarded the Neanderthal man as a low form
+of the modern type, but expert opinion seems to agree rather with the
+view maintained in 1864 by Professor William King of Galway, that the
+Neanderthal man represents a distinct species off the main line of
+ascent. He disappeared with apparent suddenness (like some aboriginal
+races to-day) about the end of the Fourth Great Ice Age; but there is
+evidence that before he ceased to be there had emerged a successor
+rather than a descendant--the modern man.
+
+4. Another offshoot from the main line is probably represented by the
+Piltdown man, found in Sussex in 1912. The remains consisted of the
+walls of the skull, which indicate a large brain, and a high forehead
+without the beetling eyebrows of the Neanderthal man and
+Pithecanthropus. The "find" included a tooth and part of a lower jaw,
+but these perhaps belong to some ape, for they are very discrepant. The
+Piltdown skull represents the most ancient human remains as yet found in
+Britain, and Dr. Smith Woodward's establishment of a separate genus
+Eoanthropus expresses his conviction that the Piltdown man was off the
+line of the evolution of the modern type. If the tooth and piece of
+lower jaw belong to the Piltdown skull, then there was a remarkable
+combination of ape-like and human characters. As regards the brain,
+_inferred_ from the skull-walls, Sir Arthur Keith says:
+
+ All the essential features of the brain of modern man are to be seen
+ in the brain cast. There are some which must be regarded as
+ primitive. There can be no doubt that it is built on exactly the
+ same lines as our modern brains. A few minor alterations would make
+ it in all respects a modern brain.... Although our knowledge of the
+ human brain is limited--there are large areas to which we can assign
+ no definite function--we may rest assured that a brain which was
+ shaped in a mould so similar to our own was one which responded to
+ the outside world as ours does. Piltdown man saw, heard, felt,
+ thought, and dreamt much as we do still.
+
+And this was 150,000 years ago at a modern estimate, and some would say
+half a million.
+
+There is neither agreement nor certainty as to the antiquity of man,
+except that the modern type was distinguishable from its collaterals
+hundreds of thousands of years ago. The general impression left is very
+grand. In remote antiquity the Primate stem diverged from the other
+orders of mammals; it sent forth its tentative branches, and the result
+was a tangle of monkeys; ages passed and the monkeys were left behind,
+while the main stem, still probing its way, gave off the Anthropoid
+apes, both small and large. But they too were left behind, and the main
+line gave off other experiments--indications of which we know in Java,
+at Heidelberg, in the Neanderthal, and at Piltdown. None of these lasted
+or was made perfect. They represent _tentative_ men who had their day
+and ceased to be, our predecessors rather than our ancestors. Still, the
+main stem goes on evolving, and who will be bold enough to say what
+fruit it has yet to bear!
+
+[Illustration: _After a model by J. H. McGregor._
+
+PROFILE VIEW OF THE HEAD OF PITHECANTHROPUS, THE JAVA APE-MAN--AN EARLY
+OFFSHOOT FROM THE MAIN LINE OF MAN'S ASCENT
+
+The animal remains found along with the skull-cap, thigh-bone, and two
+teeth of Pithecanthropus seem to indicate the lowest Pleistocene period,
+perhaps 500,000 years ago.]
+
+[Illustration: _From the reconstruction by J. H. McGregor._
+
+PILTDOWN SKULL. THE DARK PARTS ONLY ARE PRESERVED, NAMELY PORTIONS OF
+THE CRANIAL WALLS AND THE NASAL BONES
+
+Some authorities include a canine tooth and part of the lower jaw which
+were found close by. The remains were found in 1912 in Thames gravels in
+Sussex, and are usually regarded as vastly more ancient than those of
+Neanderthal Man. It has been suggested that Piltdown Man lived 100,000
+to 150,000 years ago, in the Third Interglacial period.]
+
+[Illustration: _Reproduced by permission from Osborn's "Men of the Old
+Stone Age."_
+
+SAND-PIT AT MAUER, NEAR HEIDELBERG: DISCOVERY SITE OF THE JAW OF
+HEIDELBERG MAN
+
+ _a-b._ "Newer loess," either of Third Interglacial or of Postglacial
+ times.
+ _b-c._ "Older loess" (sandy loess), of the close of Second Interglacial
+ times.
+ _c-f._ The "sands of Mauer."
+ _d-e._ An intermediate layer of clay.
+
+The white cross (X) indicates the spot at the base of the "sands of
+Mauer" at which the jaw of Heidelberg was discovered.]
+
+
+Primitive Men
+
+Ancient skeletons of men of the modern type have been found in many
+places, e.g. Combe Capelle in Dordogne, Galley Hill in Kent, Cro-Magnon
+in Périgord, Mentone on the Riviera; and they are often referred to as
+"Cave-men" or "men of the Early Stone Age." They had large skulls, high
+foreheads, well-marked chins, and other features such as modern man
+possesses. They were true men at last--that is to say, like ourselves!
+The spirited pictures they made on the walls of caves in France and
+Spain show artistic sense and skill. Well-finished statuettes
+representing nude female figures are also known. The elaborate burial
+customs point to a belief in life after death. They made stone
+implements--knives, scrapers, gravers, and the like, of the type known
+as Palæolithic, and these show interesting gradations of skill and
+peculiarities of style. The "Cave-men" lived between the third and
+fourth Ice Ages, along with cave-bear, cave-lion, cave-hyæna, mammoth,
+woolly rhinoceros, Irish elk, and other mammals now extinct--taking us
+back to 30,000-50,000 years ago, and many would say much more. Some of
+the big-brained skulls of these Palæolithic cave-men show not a single
+feature that could be called primitive. They show teeth which in size
+and form are exactly the same as those of a thousand generations
+afterwards--and suffering from gumboil too! There seems little doubt
+that these vigorous Palæolithic Cave-men of Europe were living for a
+while contemporaneously with the men of Neanderthal, and it is possible
+that they directly or indirectly hastened the disappearance of their
+more primitive collaterals. Curiously enough, however, they had not
+themselves adequate lasting power in Europe, for they seem for the most
+part to have dwindled away, leaving perhaps stray present-day survivors
+in isolated districts. The probability is that after their decline
+Europe was repeopled by immigrants from Asia. It cannot be said that
+there is any inherent biological necessity for the decline of a vigorous
+race--many animal races go back for millions of years--but in mankind
+the historical fact is that a period of great racial vigour and success
+is often followed by a period of decline, sometimes leading to practical
+disappearance as a definite race. The causes of this waning remain very
+obscure--sometimes environmental, sometimes constitutional, sometimes
+competitive. Sometimes the introduction of a new parasite, like the
+malaria organism, may have been to blame.
+
+After the Ice Ages had passed, perhaps 25,000 years ago, the Palæolithic
+culture gave place to the Neolithic. The men who made rudely dressed but
+often beautiful stone implements were succeeded or replaced by men who
+made polished stone implements. The earliest inhabitants of Scotland
+were of this Neolithic culture, migrating from the Continent when the
+ice-fields of the Great Glaciation had disappeared. Their remains are
+often associated with the "Fifty-foot Beach" which, though now high and
+dry, was the seashore in early Neolithic days. Much is known about these
+men of the polished stones. They were hunters, fowlers, and fishermen;
+without domesticated animals or agriculture; short folk, two or three
+inches below the present standard; living an active strenuous life.
+Similarly, for the south, Sir Arthur Keith pictures for us a Neolithic
+community at Coldrum in Kent, dating from about 4,000 years ago--a few
+ticks of the geological clock. It consisted, in this case, of
+agricultural pioneers, men with large heads and big brains, about two
+inches shorter in stature than the modern British average (5 ft. 8 in.),
+with better teeth and broader palates than men have in these days of
+soft food, with beliefs concerning life and death similar to those that
+swayed their contemporaries in Western and Southern Europe. Very
+interesting is the manipulative skill they showed on a large scale in
+erecting standing stones (probably connected with calendar-keeping and
+with worship), and on a small scale in making daring operations on the
+skull. Four thousand years ago is given as a probable date for that
+early community in Kent, but evidences of Neolithic man occur in
+situations which demand a much greater antiquity--perhaps 30,000 years.
+And man was not young then!
+
+[Illustration: PAINTINGS ON THE ROOF OF THE ALTAMIRA CAVE IN NORTHERN
+SPAIN, SHOWING A BISON ABOVE AND A GALLOPING BOAR BELOW
+
+The artistic drawings, over 2 feet in length, were made by the Reindeer
+Men or "Cromagnards" in the time of the Upper or Post-Glacial
+Pleistocene, before the appearance of the Neolithic men.]
+
+We must open one more chapter in the thrilling story of the Ascent of
+Man--the Metal Ages, which are in a sense still continuing. Metals began
+to be used in the late Polished Stone (Neolithic) times, for there were
+always overlappings. Copper came first, Bronze second, and Iron last.
+The working of copper in the East has been traced back to the fourth
+millennium B.C., and there was also a very ancient Copper Age in the New
+World. It need hardly be said that where copper is scarce, as in
+Britain, we cannot expect to find much trace of a Copper Age.
+
+The ores of different metals seem to have been smelted together in an
+experimental way by many prehistoric metallurgists, and bronze was the
+alloy that rewarded the combination of tin with copper. There is
+evidence of a more or less definite Bronze Age in Egypt and Babylonia,
+Greece and Europe.
+
+It is not clear why iron should not have been the earliest metal to be
+used by man, but the Iron Age dates from about the middle of the second
+millennium B.C. From Egypt the usage spread through the Mediterranean
+region to North Europe, or it may have been that discoveries made in
+Central Europe, so rich in iron-mines, saturated southwards, following
+for instance, the route of the amber trade from the Baltic. Compared
+with stone, the metals afforded much greater possibilities of
+implements, instruments, and weapons, and their discovery and usage had
+undoubtedly great influence on the Ascent of Man. Occasionally, however,
+on his descent.
+
+
+Retrospect
+
+Looking backwards, we discern the following stages: (1) The setting
+apart of a Primate stock, marked off from other mammals by a tendency to
+big brains, a free hand, gregariousness, and good-humoured
+talkativeness. (2) The divergence of marmosets and New World monkeys and
+Old World monkeys, leaving a stock--an anthropoid stock--common to the
+present-day and extinct apes and to mankind. (3) From this common stock
+the Anthropoid apes diverged, far from ignoble creatures, and a humanoid
+stock was set apart. (4) From the latter (we follow Sir Arthur Keith and
+other authorities) there arose what may be called, without
+disparagement, tentative or experimental men, indicated by
+Pithecanthropus "the Erect," the Heidelberg man, the Neanderthalers,
+and, best of all, the early men of the Sussex Weald--hinted at by the
+Piltdown skull. It matters little whether particular items are
+corroborated or disproved--e.g. whether the Heidelberg man came before
+or after the Neanderthalers--the general trend of evolution remains
+clear. (5) In any case, the result was the evolution of _Homo sapiens,
+the man we are_--a quite different fellow from the Neanderthaler. (6)
+Then arose various stocks of primitive men, proving everything and
+holding fast to that which is good. There were the Palæolithic peoples,
+with rude stone implements, a strong vigorous race, but probably, in
+most cases, supplanted by fresh experiments. These may have arisen as
+shoots from the growing point of the old race, or as a fresh offshoot
+from more generalised members at a lower level. This is the eternal
+possible victory alike of aristocracy and democracy. (7) Palæolithic men
+were involved in the succession of four Great Ice Ages or
+Glaciations, and it may be that the human race owes much to the
+alternation of hard times and easy times--glacial and interglacial. When
+the ice-fields cleared off Neolithic man had his innings. (8) And we
+have closed the story, in the meantime, with the Metal Ages.
+
+[Illustration: _After the restoration modelled by J. H. McGregor._
+
+PILTDOWN MAN, PRECEDING NEANDERTHAL MAN, PERHAPS 100,000 TO 150,000
+YEARS AGO]
+
+[Illustration: _After the restoration modelled by J. H. McGregor._
+
+THE NEANDERTHAL MAN OF LA CHAPELLE-AUX-SAINTS
+
+The men of this race lived in Europe from the Third Interglacial period
+through the Fourth Glacial. They disappeared somewhat suddenly, being
+replaced by the Modern Man type, such as the Cromagnards. Many regard
+the Neanderthal Men as a distinct species.]
+
+It seems not unfitting that we should at this point sound another
+note--that of the man of feeling. It is clear in William James's words:
+
+ Bone of our bone, and flesh of our flesh, are these half-brutish
+ prehistoric brothers. Girdled about with the immense darkness of
+ this mysterious universe even as we are, they were born and died,
+ suffered and struggled. Given over to fearful crime and passion,
+ plunged in the blackest ignorance, preyed upon by hideous and
+ grotesque delusions, yet steadfastly serving the profoundest of
+ ideals in their fixed faith that existence in any form is better
+ than non-existence, they ever rescued triumphantly from the jaws of
+ ever imminent destruction the torch of life which, thanks to them,
+ now lights the world for us.
+
+
+Races of Mankind
+
+Given a variable stock spreading over diverse territory, we expect to
+find it splitting up into varieties which may become steadied into races
+or incipient species. Thus we have races of hive-bees, "Italians,"
+"Punics," and so forth; and thus there arose races of men. Certain types
+suited certain areas, and periods of in-breeding tended to make the
+distinctive peculiarities of each incipient race well-defined and
+stable. When the original peculiarities, say, of negro and Mongol,
+Australian and Caucasian, arose as brusque variations or "mutations,"
+then they would have great staying power from generation to generation.
+They would not be readily swamped by intercrossing or averaged off.
+Peculiarities and changes of climate and surroundings, not to speak of
+other change-producing factors, would provoke new departures from age to
+age, and so fresh racial ventures were made. Moreover, the occurrence
+of out-breeding when two races met, in peace or in war, would certainly
+serve to induce fresh starts. Very important in the evolution of human
+races must have been the alternating occurrence of periods of
+in-breeding (endogamy), tending to stability and sameness, and periods
+of out-breeding (exogamy), tending to changefulness and diversity.
+
+Thus we may distinguish several more or less clearly defined primitive
+races of mankind--notably the African, the Australian, the Mongolian,
+and the Caucasian. The woolly-haired African race includes the negroes
+and the very primitive bushmen. The wavy-to curly-haired Australian race
+includes the Jungle Tribes of the Deccan, the Vedda of Ceylon, the
+Jungle Folk or Semang, and the natives of unsettled parts of
+Australia--all sometimes slumped together as "Pre-Dravidians." The
+straight-haired Mongols include those of Tibet, Indo-China, China, and
+Formosa, those of many oceanic islands, and of the north from Japan to
+Lapland. The Caucasians include Mediterraneans, Semites, Nordics,
+Afghans, Alpines, and many more.
+
+There are very few corners of knowledge more difficult than that of the
+Races of Men, the chief reason being that there has been so much
+movement and migration in the course of the ages. One physical type has
+mingled with another, inducing strange amalgams and novelties. If we
+start with what might be called "zoological" races or strains differing,
+for instance, in their hair (woolly-haired Africans, straight-haired
+Mongols, curly-or wavy-haired Pre-Dravidians and Caucasians), we find
+these replaced by _peoples_ who are mixtures of various races, "brethren
+by civilisation more than by blood." As Professor Flinders Petrie has
+said, the only meaning the term "race" now can have is that of a group
+of human beings whose type has been unified by their rate of
+assimilation exceeding the rate of change produced by the infiltration
+of foreign elements. It is probable, however, that the progress of
+precise anthropology will make it possible to distinguish the various
+racial "strains" that make up any people. For the human sense of race
+is so strong that it convinces us of reality even when scientific
+definition is impossible. It was this the British sailor expressed in
+his answer to the question "What is a Dago?" "Dagoes," he replied, "is
+anything wot isn't our sort of chaps."
+
+[Illustration: RESTORATION BY A. FORESTIER OF THE RHODESIAN MAN WHOSE
+SKULL WAS DISCOVERED IN 1921
+
+Attention may be drawn to the beetling eyebrow ridges, the projecting
+upper lip, the large eye-sockets, the well-poised head, the strong
+shoulders.
+
+The squatting figure is crushing seeds with a stone, and a crusher is
+lying on the rock to his right.]
+
+[Illustration: RESTORATION BY A. FORESTIER OF THE RHODESIAN MAN WHOSE
+SKULL WAS DISCOVERED IN 1921
+
+The figure in the foreground, holding a staff, shows the erect attitude
+and the straight legs. His left hand holds a flint implement.
+
+On the left, behind the sitting figure, is seen the entrance to the
+cave. This new Rhodesian cave-man may be regarded as a southern
+representative of a Neanderthal race, or as an extinct type intermediate
+between the Neanderthal Men and the Modern Man type.]
+
+
+Steps in Human Evolution
+
+Real men arose, we believe, by variational uplifts of considerable
+magnitude which led to big and complex brains and to the power of
+reasoned discourse. In some other lines of mammalian evolution there
+were from time to time great advances in the size and complexity of the
+brain, as is clear, for instance, in the case of horses and elephants.
+The same is true of birds as compared with reptiles, and everyone
+recognises the high level of excellence that has been attained by their
+vocal powers. How these great cerebral advances came about we do not
+know, but it has been one of the main trends of animal evolution to
+improve the nervous system. Two suggestions may be made. First, the
+prolongation of the period of ante-natal life, in intimate physiological
+partnership with the mother, may have made it practicable to start the
+higher mammal with a much better brain than in the lower orders, like
+Insectivores and Rodents, and still more Marsupials, where the period
+before birth (gestation) is short. Second, we know that the individual
+development of the brain is profoundly influenced by the internal
+secretions of certain ductless glands notably the thyroid. When this
+organ is not functioning properly the child's brain development is
+arrested. It may be that increased production of certain
+hormones--itself, of course, to be accounted for--may have stimulated
+brain development in man's remote ancestors.
+
+Given variability along the line of better brains and given a process of
+discriminate sifting which would consistently offer rewards to alertness
+and foresight, to kin-sympathy and parental care, there seems no great
+difficulty in imagining how Man would evolve. We must not think of an
+Aristotle or a Newton except as fine results which justify all the
+groaning and travailing; we must think of average men, of primitive
+peoples to-day, and of our forbears long ago. We must remember how much
+of man's advance is dependent on the external registration of the social
+heritage, not on the slowly changing natural inheritance.
+
+Looking backwards it is impossible, we think, to fail to recognise
+progress. There is a ring of truth in the fine description Æschylus gave
+of primitive men that--
+
+ first, beholding they beheld in vain, and, hearing, heard not, but,
+ like shapes in dreams, mixed all things wildly down the tedious
+ time, nor knew to build a house against the sun with wicketed sides,
+ nor any woodwork knew, but lived like silly ants, beneath the
+ ground, in hollow caves unsunned. There came to them no steadfast
+ sign of winter, nor of spring flower-perfumed, nor of summer full of
+ fruit, but blindly and lawlessly they did all things.
+
+Contrast this picture with the position of man to-day. He has mastered
+the forces of Nature and is learning to use their resources more and
+more economically; he has harnessed electricity to his chariot and he
+has made the ether carry his messages. He tapped supplies of material
+which seemed for centuries unavailable, having learned, for instance,
+how to capture and utilise the free nitrogen of the air. With his
+telegraph and "wireless" he has annihilated distance, and he has added
+to his navigable kingdom the depths of the sea and the heights of the
+air. He has conquered one disease after another, and the young science
+of heredity is showing him how to control in his domesticated animals
+and cultivated plants the nature of the generations yet unborn. With all
+his faults he has his ethical face set in the right direction. The main
+line of movement is towards the fuller embodiment of the true, the
+beautiful, and the good in healthy lives which are increasingly a
+satisfaction in themselves.
+
+[Illustration: _Photo: British Museum (Natural History)._
+
+SIDE-VIEW OF A PREHISTORIC HUMAN SKULL DISCOVERED IN 1921 IN BROKEN HILL
+CAVE, NORTHERN RHODESIA
+
+Very striking are the prominent eyebrow ridges and the broad massive
+face. The skull looks less domed than that of modern man, but its
+cranial capacity is far above the lowest human limit. The teeth are
+interesting in showing marked rotting or "caries," hitherto unknown in
+prehistoric skulls. In all probability the Rhodesian man was an African
+representative of the extinct Neanderthal species hitherto known only
+from Europe.]
+
+[Illustration: _After the restoration modelled by J. H. McGregor._
+
+A CROMAGNON MAN OR CROMAGNARD, REPRESENTATIVE OF A STRONG ARTISTIC RACE
+LIVING IN THE SOUTH OF FRANCE IN THE UPPER PLEISTOCENE, PERHAPS 25,000
+YEARS AGO
+
+They seemed to have lived for a while contemporaneously with the
+Neanderthal Men, and there may have been interbreeding. Some Cromagnards
+probably survive, but the race as a whole declined, and there was
+repopulation of Europe from the East.]
+
+[Illustration: _Reproduced by permission from Osborn's "Men of the Old
+Stone Age."_
+
+PHOTOGRAPH SHOWING A NARROW PASSAGE IN THE CAVERN OF FONT-DE-GAUME ON
+THE BEUNE
+
+Throughout the cavern the walls are crowded with engravings; on the left
+wall, shown in the photograph, are two painted bison. In the great
+gallery there may be found not less than eighty figures--bison,
+reindeer, and mammoths. A specimen of the last is reproduced below.]
+
+[Illustration: A MAMMOTH DRAWN ON THE WALL OF THE FONT-DE-GAUME CAVERN
+
+The mammoth age was in the Middle Pleistocene, while Neanderthal Men
+still flourished, probably far over 30,000 years ago.]
+
+[Illustration: A GRAZING BISON, DELICATELY AND CAREFULLY DRAWN, ENGRAVED
+ON A WALL OF THE ALTAMIRA CAVE, NORTHERN SPAIN
+
+This was the work of a Reindeer Man or Cromagnard, in the Upper or
+Post-Glacial Pleistocene, perhaps 25,000 years ago. Firelight must have
+been used in making these cave drawings and engravings.]
+
+
+Factors in Human Progress
+
+Many, we believe, were the gains that rewarded the arboreal
+apprenticeship of man's ancestors. Many, likewise, were the results of
+leaving the trees and coming down to the solid earth--a transition which
+marked the emergence of more than tentative men. What great steps
+followed?
+
+Some of the greatest were--the working out of a spoken language and of
+external methods of registration; the invention of tools; the discovery
+of the use of fire; the utilisation of iron and other metals; the taming
+of wild animals such as dog and sheep, horses and cattle; the
+cultivation of wild plants such as wheat and rice; and the irrigation of
+fields. All through the ages necessity has been the mother of invention
+and curiosity its father; but perhaps we miss the heart of the matter if
+we forget the importance of some leisure time--wherein to observe and
+think. If our earth had been so clouded that the stars were hidden from
+men's eyes the whole history of our race would have been different. For
+it was through his leisure-time observations of the stars that early man
+discovered the regularity of the year and got his fundamental
+impressions of the order of Nature--on which all his science is founded.
+
+If we are to think clearly of the factors of human progress we must
+recall the three great biological ideas--the living organism, its
+environment, and its functioning. For man these mean (1) the living
+creature, the outcome of parents and ancestors, a fresh expression of a
+bodily and mental inheritance; (2) the surroundings, including climate
+and soil, the plants and animals these allow; and (3) the activities of
+all sorts, occupations and habits, all the actions and reactions between
+man and his milieu. In short, we have to deal with FOLK, PLACE, WORK;
+the _Famille_, _Lieu_, _Travail_ of the LePlay school.
+
+As to FOLK, human progress depends on intrinsic racial
+qualities--notably health and vigour of body, clearness and alertness of
+mind, and an indispensable sociality. The most powerful factors in the
+world are clear ideas in the minds of energetic men of good will. The
+differences in bodily and mental health which mark races, and stocks
+within a people, just as they mark individuals, are themselves traceable
+back to germinal variations or mutations, and to the kind of sifting to
+which the race or stock has been subjected. Easygoing conditions are not
+only without stimulus to new departures, they are without the sifting
+which progress demands.
+
+As to PLACE, it is plain that different areas differ greatly in their
+material resources and in the availability of these. Moreover, even when
+abundant material resources are present, they will not make for much
+progress unless the climate is such that they can be readily utilised.
+Indeed, climate has been one of the great factors in civilisation, here
+stimulating and there depressing energy, in one place favouring certain
+plants and animals important to man, in another place preventing their
+presence. Moreover, climate has slowly changed from age to age.
+
+As to WORK, the form of a civilisation is in some measure dependent on
+the primary occupations, whether hunting or fishing, farming or
+shepherding; and on the industries of later ages which have a profound
+moulding effect on the individual at least. We cannot, however, say more
+than that the factors of human progress have always had these three
+aspects, Folk, Place, Work, and that if progress is to continue on
+stable lines it must always recognise the essential correlation of
+fitter folk in body and mind: improved habits and functions, alike in
+work and leisure; and bettered surroundings in the widest and deepest
+sense.
+
+
+BIBLIOGRAPHY
+
+ DARWIN, CHARLES, _Descent of Man_.
+ HADDON, A. C., _Races of Men_.
+ HADDON, A. C., _History of Anthropology_.
+ KEANE, A. H., _Man Past and Present_.
+ KEITH, ARTHUR, _Antiquity of Man_.
+ LULL, R. S., _Organic Evolution_.
+ MCCABE, JOSEPH, _Evolution of Civilization_.
+ MARETT, R. R., _Anthropology_ (Home University Library).
+ OSBORN, H. F., _Men of the Early Stone Age_.
+ SOLLAS, W. J., _Ancient Hunters and their Modern Representatives_.
+ TYLOR, E. B., _Anthropology and Primitive Culture_.
+
+
+
+
+VI
+
+EVOLUTION GOING ON
+
+
+
+
+EVOLUTION GOING ON
+
+
+Evolution, as we have seen in a previous chapter, is another word for
+race-history. It means the ceaseless process of Becoming, linking
+generation to generation of living creatures. The Doctrine of Evolution
+states the fact that the present is the child of the past and the parent
+of the future. It comes to this, that the living plants and animals we
+know are descended from ancestors on the whole simpler, and these from
+others likewise simpler, and so on, back and back--till we reach the
+first living creatures, of which, unfortunately, we know nothing.
+Evolution is a process of racial change in a definite direction, whereby
+new forms arise, take root, and flourish, alongside of or in the place
+of their ancestors, which were in most cases rather simpler in structure
+and behaviour.
+
+The rock-record, which cannot be wrong, though we may read it wrongly,
+shows clearly that there was once a time in the history of the Earth
+when the only backboned animals were Fishes. Ages passed, and there
+evolved Amphibians, with fingers and toes, scrambling on to dry land.
+Ages passed, and there evolved Reptiles, in bewildering profusion. There
+were fish-lizards and sea-serpents, terrestrial dragons and flying
+dragons, a prolific and varied stock. From the terrestrial Dinosaurs it
+seems that Birds and Mammals arose. In succeeding ages there evolved all
+the variety of Birds and all the variety of Mammals. Until at last arose
+the Man. The question is whether similar processes of evolution are
+still going on.
+
+We are so keenly aware of rapid changes in mankind, though these
+concern the social heritage much more than the flesh-and-blood natural
+inheritance, that we find no difficulty in the idea that evolution is
+going on in mankind. We know the contrast between modern man and
+primitive man, and we are convinced that in the past, at least, progress
+has been a reality. That degeneration may set in is an awful
+possibility--involution rather than evolution--but even if going back
+became for a time the rule, we cannot give up the hope that the race
+would recover itself and begin afresh to go forward. For although there
+have been retrogressions in the history of life, continued through
+unthinkably long ages, and although great races, the Flying Dragons for
+instance, have become utterly extinct, leaving no successors whatsoever,
+we feel sure that there has been on the whole a progress towards nobler,
+more masterful, more emancipated, more intelligent, and _better_ forms
+of life--a progress towards what mankind at its best has always regarded
+as best, i.e. affording most enduring satisfaction. So we think of
+evolution going on in mankind, evolution chequered by involution, but on
+the whole _progressive evolution_.
+
+
+Evolutionary Prospect for Man
+
+It is not likely that man's body will admit of _great_ change, but there
+is room for some improvement, e.g. in the superfluous length of the
+food-canal and the overcrowding of the teeth. It is likely, however,
+that there will be constitutional changes, e.g. of prolonged
+youthfulness, a higher standard of healthfulness, and a greater
+resistance to disease. It is justifiable to look forward to great
+improvements in intelligence and in control. The potentialities of the
+human brain, as it is, are far from being utilised to the full, and new
+departures of promise are of continual occurrence. What is of great
+importance is that the new departures or variations which emerge in fine
+children should be fostered, not nipped in the bud, by the social
+environment, education included. The evolutionary prospect for man is
+promising.
+
+[Illustration: PHOTOGRAPH OF A MEDIAN SECTION THROUGH THE SHELL OF THE
+PEARLY NAUTILUS
+
+It is only the large terminal chamber that is occupied by the animal.]
+
+[Illustration: PHOTOGRAPH OF THE ENTIRE SHELL OF THE PEARLY NAUTILUS
+
+The headquarters of the Nautilus are in the Indian and Pacific Oceans.
+They sometimes swim at the surface of the sea, but they usually creep
+slowly about on the floor of comparatively shallow water.]
+
+[Illustration: NAUTILUS
+
+A section through the Pearly Nautilus, _Nautilus pompilius_, common from
+Malay to Fiji. The shell is often about 9 inches long. The animal lives
+in the last chamber only, but a tube (S) runs through the empty
+chambers, perforating the partitions (SE). The bulk of the animal is
+marked VM; the eye is shown at E; a hood is marked H; round the mouth
+there are numerous lobes (L) bearing protrusible tentacles, some of
+which are shown. When the animal is swimming near the surface the
+tentacles radiate out in all directions, and it has been described as "a
+shell with something like a cauliflower sticking out of it." The Pearly
+Nautilus is a good example of a conservative type, for it began in the
+Triassic Era. But the family of Nautiloids to which it belongs
+illustrates very vividly what is meant by a dwindling race. The
+Nautiloids began in the Cambrian, reached their golden age in the
+Silurian, and began to decline markedly in the Carboniferous. There are
+2,500 extinct or fossil species of Nautiloids, and only 4 living
+to-day.]
+
+[Illustration: _Photo: W. S. Berridge._
+
+SHOEBILL
+
+A bird of a savage nature, never mixing with other marsh birds.
+According to Dr. Chalmers Mitchell, it shows affinities to herons,
+storks, pelicans, and gannets, and is a representative of a type equal
+to both herons and storks and falling between the two.]
+
+But it is very important to realise that among plant and animals
+likewise, _Evolution is going on_.
+
+
+The Fountain of Change: Variability
+
+On an ordinary big clock we do not readily see that even the minute hand
+is moving, and if the clock struck only once in a hundred years we can
+conceive of people arguing whether the hands did really move at all. So
+it often is with the changes that go on from generation to generation in
+living creatures. The flux is so slow, like the flowing of a glacier,
+that some people fail to be convinced of its reality. And it must, of
+course, be admitted that some kinds of living creatures, like the
+Lamp-shell _Ligula_ or the Pearly Nautilus, hardly change from age to
+age, whereas others, like some of the birds and butterflies, are always
+giving rise to something new. The Evening Primrose among plants, and the
+Fruit-fly, Drosophila, among animals, are well-known examples of
+organisms which are at present in a sporting or mutating mood.
+
+Certain dark varieties of moth, e.g. of the Peppered Moth, are taking
+the place of the paler type in some parts of England, and the same is
+true of some dark forms of Sugar-bird in the West Indian islands. Very
+important is the piece of statistics worked out by Professor R. C.
+Punnett, that "if a population contains .001 per cent of a new variety,
+and if that variety has even a 5 per cent selection advantage over the
+original form, the latter will almost completely disappear in less than
+a hundred generations." This sort of thing has been going on all over
+the world for untold ages, and the face of animate nature has
+consequently changed.
+
+We are impressed by striking novelties that crop up--a clever dwarf, a
+musical genius, a calculating boy, a cock with a 10 ft. tail, a
+"wonder-horse" with a mane reaching to the ground, a tailless cat, a
+white blackbird, a copper beech, a Greater Celandine with much cut up
+leaves; but this sort of mutation is common, and smaller, less brusque
+variations are commoner still. _They form the raw materials of possible
+evolution._ We are actually standing before an apparently inexhaustible
+fountain of change. This is evolution going on.
+
+
+The Sporting Jellyfish
+
+It is of interest to consider a common animal like the jellyfish
+Aurelia. It is admirably suited for a leisurely life in the open sea,
+where it swims about by contracting its saucer-shaped body, thus driving
+water out from its concavity. By means of millions of stinging cells on
+its four frilled lips and on its marginal tentacles it is able to
+paralyse and lasso minute crustaceans and the like, which it then wafts
+into its mouth. It has a very eventful life-history, for it has in its
+early youth to pass through a fixed stage, fastened to rock or seaweed,
+but it is a successful animal, well suited for its habitat, and
+practically cosmopolitan in its distribution. It is certainly an
+old-established creature. Yet it is very variable in colour and in size,
+and even in internal structure. Very often it is the size of a saucer or
+a soup-plate, but giants over two feet in diameter are well known. Much
+more important, however, than variation in colour and size are the
+inborn changes in structure. Normally a jellyfish has its parts in four
+or multiples of four. Thus it has four frilled lips, four tufts of
+digestive filaments in its stomach, and four brightly coloured
+reproductive organs. It has eight sense-organs round the margin of its
+disc, eight branched and eight unbranched radial canals running from the
+central stomach to a canal round the circumference. The point of giving
+these details is just this, that every now and then we find a jellyfish
+with its parts in sixes, fives, or threes, and with a multitude of minor
+idiosyncrasies. _Even in the well-established jellyfish there is a
+fountain of change._
+
+
+§ 1
+
+Evolution of Plants
+
+It is instructive to look at the various kinds of cabbages, such as
+cauliflower and Brussels sprouts, kale and curly greens, and remember
+that they are all scions of the not very promising wild cabbage found on
+our shores. And are not all the aristocrat apple-trees of our orchards
+descended from the plebeian crab-apple of the roadside? We know far too
+little about the precise origin of our cultivated plants, but there is
+no doubt that after man got a hold of them he took advantage of their
+variability to establish race after race, say, of rose and
+chrysanthemum, of potato and cereal. The evolution of cultivated plants
+is continuing before our eyes, and the creations of Mr. Luther Burbank,
+such as the stoneless plum and the primus berry, the spineless cactus
+and the Shasta daisy, are merely striking instances of what is always
+going on.
+
+There is reason to believe that the domestic dog has risen three times,
+from three distinct ancestors--a wolf, a jackal, and a coyote. So a
+multiple pedigree must be allowed for in the case of the dog, and the
+same is true in regard to some other domesticated animals. But the big
+fact is the great variety of breeds that man has been able to fix, after
+he once got started with a domesticated type. There are over 200
+well-marked breeds of domestic pigeons, and there is very strong
+evidence that all are descended from the wild rock-dove, just as the
+numerous kinds of poultry are descended from the jungle-fowl of some
+parts of India and the Malay Islands. Even more familiar is the way in
+which man has, so to speak, unpacked the complex fur of the wild rabbit,
+and established all the numerous colour-varieties which we see among
+domestic rabbits. And apart from colour-varieties there are long-haired
+Angoras and quaint lop-eared forms, and many more besides. All this
+points to evolution going on.
+
+
+The Romance of the Wheat
+
+It is well-known that Neolithic man grew wheat, and some authorities
+have put the date of the first wheat harvest at between fifteen thousand
+and ten thousand years ago. The ancient civilisations of Babylonia,
+Egypt, Crete, Greece, and Rome were largely based on wheat, and it is
+highly probable that the first great wheatfields were in the fertile
+land between the Tigris and the Euphrates. The oldest Egyptian tombs
+that contain wheat, which, by the way, never germinates after its
+millennia of rest, belong to the First Dynasty, and are about six
+thousand years old. But there must have been a long history of wheat
+before that.
+
+Now it is a very interesting fact that the almost certain ancestor of
+the cultivated wheat is at present living on the arid and rocky slopes
+of Mount Hermon. It is called _Triticum hermonis_, and it is varying
+notably to-day, as it did long ago when it gave rise to the emmer, which
+was cultivated in the Neolithic Age and is the ancestor of all our
+ordinary wheats. We must think of Neolithic man noticing the big seeds
+of this Hermon grass, gathering some of the heads, breaking the brittle
+spikelet-bearing axis in his fingers, knocking off the rough awns or
+bruising the spikelets in his hand till the glumes or chaff separated
+off and could be blown away, chewing a mouthful of the seeds--and
+resolving to sow and sow again.
+
+That was the beginning of a long story, in the course of which man took
+advantage of the numerous variations that cropped up in this sporting
+stock and established one successful race after another on his fields.
+Virgil refers in the "Georgics" to the gathering of the largest and
+fullest ears of wheat in order to get good seed for another sowing, but
+it was not till the first quarter of the nineteenth century that the
+great step was taken, by men like Patrick Sheriff of Haddington, of
+deliberately selecting individual ears of great excellence and
+segregating their progeny from mingling with mediocre stock. This is the
+method which has been followed with remarkable success in modern times.
+
+One of the factors that assisted the Allies in overcoming the food
+crisis in the darkest period of the war was the virtue of Marquis Wheat,
+a very prolific, early ripening, hard red spring wheat with excellent
+milling and baking qualities. It is now the dominant spring wheat in
+Canada and the United States, and it has enormously increased the real
+wealth of the world in the last ten years (1921). Now our point is
+simply that this Marquis Wheat is a fine example of evolution going on.
+In 1917 upwards of 250,000,000 bushels of this wheat were raised in
+North America, and in 1918 upwards of 300,000,000 bushels; yet the whole
+originated from a single grain planted in an experimental plot at Ottawa
+by Dr. Charles E. Saunders so recently as the spring of 1903.
+
+[Illustration: THE WALKING-FISH OR MUD-SKIPPER (PERIOPHTHALMUS), COMMON
+AT THE MOUTHS OF RIVERS IN TROPICAL AFRICA, ASIA, AND NORTH-WEST
+AUSTRALIA
+
+It skips about by means of its strong pectoral fins on the mud-flats; it
+jumps from stone to stone hunting small shore-animals; it climbs up the
+roots of the mangrove-trees. The close-set eyes protrude greatly and are
+very mobile. The tail seems to help in respiration.]
+
+[Illustration: _Photo: "The Times."_
+
+THE AUSTRALIAN MORE-PORK OR PODARGUS
+
+A bird with a frog-like mouth, allied to the British Nightjar. Now in
+the London Zoological Gardens.
+
+The capacious mouth is well suited for engulfing large insects such as
+locusts and mantises, which are mostly caught on the trees. During the
+day the More-pork or Frog-mouth sleeps upright on a branch, and its
+mottled brown plumage makes it almost invisible.]
+
+[Illustration: PELICAN'S BILL, ADAPTED FOR CATCHING AND STORING FISHES
+
+There is an enormous dilatable sac beneath the lower jaw.]
+
+[Illustration: HORNBILL'S BILL, ADAPTED FOR EXCAVATING A NEST IN A TREE,
+AND ALSO FOR SEIZING AND BREAKING DIVERSE FORMS OF FOOD, FROM MAMMALS TO
+TORTOISES, FROM ROOTS TO FRUITS
+
+The use of the helmet or casque is obscure.]
+
+[Illustration: SPOONBILL'S BILL, ADAPTED FOR SIFTING THE MUD AND
+CATCHING THE SMALL ANIMALS, E.G. FISHES, CRUSTACEANS, INSECT LARVÆ,
+WHICH LIVE THERE]
+
+[Illustration: FALCON'S BILL, ADAPTED FOR SEIZING, KILLING, AND TEARING
+SMALL MAMMALS AND BIRDS]
+
+[Illustration: AVOCET'S BILL, ADAPTED FOR A CURIOUS SIDEWAYS SCOOPING IN
+THE SHORE-POOLS AND CATCHING SMALL ANIMALS]
+
+[Illustration: PUFFIN'S BILL, ADAPTED FOR CATCHING SMALL FISHES NEAR THE
+SURFACE OF THE SEA, AND FOR HOLDING THEM WHEN CAUGHT AND CARRYING THEM
+TO THE NEST
+
+The scaly covering is moulted in the autumn.]
+
+We must not dwell too long on this particular instance of evolution,
+though it has meant much to our race. We wish, however, following
+Professor Buller's _Essays on Wheat_ (1919), to explain the method by
+which this good seed was discovered. From one we may learn all. The
+parent of Marquis Wheat on the male side was the mid-Europe Red Fife--a
+first-class cereal. The parent on the female side was less promising, a
+rather nondescript, not pure-bred wheat, called Red Calcutta, which was
+imported from India into Canada about thirty years ago. The father was
+part of a cargo that came from the Baltic to Glasgow, and was happily
+included in a sample sent on to David Fife in Ontario about 1842. From
+one kernel of this sample David Fife started his stock of Red Fife,
+which was crossed by Dr. Saunders with Hard Red Calcutta. The result of
+the cross was a medley of types, nearly a hundred varieties altogether,
+and it was in scrutinising these that Dr. Saunders hit upon Marquis. He
+worked steadily through the material, studying head after head of what
+resulted from sowing, and selecting out those that gave most promise.
+Each of the heads selected was propagated; most of the results were
+rejected; the elect were sifted again and yet again, and finally Marquis
+Wheat emerged, rich in constructive possibilities, probably the most
+valuable food-plant in the world. It is like a romance to read that "the
+first crop of the wheat that was destined within a dozen years to
+overtax the mightiest elevators in the land was stored away in the
+winter of 1904-5 in a paper packet no larger than an envelope."
+
+Thus from the Wild Wheat of Mount Hermon there evolved one of the most
+important food-plants of the world. This surely is _Evolution going on_.
+
+
+§ 2
+
+Changes in the Animal Life of a Country
+
+Nothing gives us a more convincing impression of evolution in being than
+a succession of pictures of the animal life of a country in different
+ages. Dr. James Ritchie, a naturalist of distinction, has written a
+masterly book, _The Influence of Man on Animal Life in Scotland_ (1920),
+in which we get this succession of pictures. "Within itself," he says,
+"a fauna is in a constant state of uneasy restlessness, an assemblage of
+creatures which in its parts ebbs and flows as one local influence or
+another plays upon it." There are temporary and local changes, endless
+disturbances and readjustments of the "balance of nature." One year
+there is a plague of field-voles, perhaps next year "grouse disease" is
+rife; in one place there is huge increase of starlings, in another place
+of rabbits; here cockchafers are in the ascendant, and there the moles
+are spoiling the pasture. "But while the parts fluctuate, the fauna as a
+whole follows a path of its own. As well as internal tides which swing
+to and fro about an average level, there is a drift which carries the
+fauna bodily along an 'irretraceable course.'" This is partly due to
+considerable changes of climate, for climate calls the tune to which
+living creatures dance, but it is also due to new departures among the
+animals themselves. We need not go back to the extinct animals and lost
+faunas of past ages--for Britain has plenty of relics of these--which
+"illustrate the reality of the faunal drift," but it may be very useful,
+in illustration of evolution in being, to notice what has happened in
+Scotland since the end of the Great Ice Age.
+
+Some nine thousand years ago or more, certain long-headed,
+square-jawed, short-limbed, but agile hunters and fishermen, whom we
+call Neolithic Man, established themselves in Scotland. What was the
+state of the country then?
+
+ It was a country of swamps, low forests of birch, alder, and willow,
+ fertile meadows, and snow-capped mountains. Its estuaries penetrated
+ further inland than they now do, and the sea stood at the level of
+ the Fifty-Foot Beach. On its plains and in its forests roamed many
+ creatures which are strange to the fauna of to-day--the Elk and the
+ Reindeer, Wild Cattle, the Wild Boar and perhaps Wild Horses, a
+ fauna of large animals which paid toll to the European Lynx, the
+ Brown Bear and the Wolf. In all likelihood, the marshes resounded to
+ the boom of the Bittern and the plains to the breeding calls of the
+ Crane and the Great Bustard.
+
+Such is Dr. Ritchie's initial picture.
+
+[Illustration: LIFE-HISTORY OF A FROG
+
+1, Before hatching; 2, newly hatched larvæ hanging on to water-weed; 3,
+with external gills; 4, external gills are covered over and are
+absorbed; 5, limbless larva about a month old with internal gills; 6,
+tadpole with hind-legs, about two months old; 7, with the fore-limbs
+emerging; 8, with all four legs free; 9, a young frog, about three
+months old, showing the almost complete absorption of the tail and the
+change of the tadpole mouth into a frog mouth.]
+
+[Illustration: _Photo: J. J. Ward. F.E.S._
+
+HIND-LEG OF WHIRLIGIG BEETLE WHICH HAS BECOME BEAUTIFULLY MODIFIED FOR
+AQUATIC LOCOMOTION
+
+The flattened tips form an expanding "fan" or paddle, which opens and
+closes with astonishing rapidity. The closing of the "fan," like the
+"feathering" of an oar, reduces friction when the leg is being moved
+forwards for the next stroke.]
+
+[Illustration: THE BIG ROBBER-CRAB (_Birgus Latro_), THAT CLIMBS THE
+COCO-NUT PALM AND BREAKS OFF THE NUTS
+
+It occurs on islands in the Indian Ocean and Pacific, and is often found
+far above sea-level. It is able to breathe dry air. One is seen emerging
+from its burrow, which is often lined with coco-nut fibre. The empty
+coco-nut shell is sometimes used by the Robber-Crab for the protection
+of its tail.]
+
+Now what happened in this kingdom of Caledonia which Neolithic Man had
+found? He began to introduce domesticated animals, and that meant a
+thinning of the ranks of predacious creatures. "Safety first" was the
+dangerous motto in obedience to which man exterminated the lynx, the
+brown bear, and the wolf. Other creatures, such as the great auk, were
+destroyed for food, and others like the marten for their furs. Small
+pests were destroyed to protect the beginnings of agriculture; larger
+animals like the boar were hunted out of existence; others, like the
+pearl-bearing river-mussels, yielded to subtler demands. No doubt there
+was protection also--protection for sport, for utility, for æsthetic
+reasons, and because of humane sentiments; even wholesome superstitions
+have safeguarded the robin redbreast and the wren. There were
+introductions too--the rabbit for utility, the pheasant for sport, and
+the peacock for amenity. And every introduction, every protection, every
+killing out had its far-reaching influences.
+
+But if we are to picture the evolution going on, we must think also of
+man's indirect interference with animal life. He destroyed the forests,
+he cultivated the wild, he made bridges, he allowed aliens, like rats
+and cockroaches, to get in unawares. Of course, he often did good, as
+when he drained swamps and got rid of the mosquitoes which once made
+malaria rife in Scotland.
+
+What has been the net result? Not, as one might think for a moment, a
+reduction in the _number_ of different kinds of animals. Fourteen or so
+species of birds and beasts have been banished from Scotland since man
+interfered, but as far as numbers go they have been more than replaced
+by deliberate introductions like fallow deer, rabbit, squirrel, and
+pheasant, and by accidental introductions like rats and cockroaches. But
+the change is rather in _quality_ than in quantity; the smaller have
+taken the place of the larger, rather paltry pigmies of noble giants.
+Thus we get a vivid idea that evolution, especially when man interferes,
+is not necessarily progressive. That depends on the nature of the sieves
+with which the living materials are sifted. As Dr. Ritchie well says,
+the standard of the wild fauna as regards size has fallen and is
+falling, and it is not in size only that there is loss, there is a
+deterioration of quality. "For how can the increase of Rabbits and
+Sparrows and Earthworms and Caterpillars, and the addition of millions
+of Rats and Cochroaches and Crickets and Bugs, ever take the place of
+those fine creatures round the memories of which the glamour of
+Scotland's past still plays--the Reindeer and the Elk, the Wolf, the
+Brown Bear, the Lynx, and the Beaver, the Bustard, the Crane, the
+Bumbling Bittern, and many another, lost or disappearing." Thus we see
+again that evolution is going on.
+
+
+§ 3
+
+The Adventurers
+
+All through the millions of years during which animals have tenanted the
+earth and the waters under the earth, there has been a search for new
+kingdoms to conquer, for new corners in which to make a home. And this
+still goes on. _It has been and is one of the methods of evolution to
+fill every niche of opportunity._ There is a spider that lives inside a
+pitcher-plant, catching some of the inquisitive insects which slip down
+the treacherous internal surface of the trap. There is another that
+makes its home in crevices among the rocks on the shore of the
+Mediterranean, or even in empty tubular shells, keeping the water out,
+more or less successfully, by spinning threads of silk across the
+entrance to its retreat. The beautiful brine-shrimp, _Artemia salina_,
+that used to occur in British salterns has found a home in the dense
+waters of the Great Salt Lake of Utah. Several kinds of earthworms have
+been found up trees, and there is a fish, Arges, that climbs on the
+stones of steep mountain torrents of the Andes. The intrepid explorers
+of the _Scotia_ voyage found quite a number of Arctic terns spending our
+winter within the summer of the Antarctic Circle--which means girdling
+the globe from pole to pole; and every now and then there are incursions
+of rare birds, like Pallas's Sand-grouse, into Britain, just as if they
+were prospecting in search of a promised land. Twice or thrice the
+distinctively North American Killdeer Plover has been found in Britain,
+having somehow or other got across the Atlantic. We miss part of the
+meaning of evolution if we do not catch this note of insurgence and
+adventure, which some animal or other never ceases to sound, though many
+establish themselves in a security not easily disturbed, and though a
+small minority give up the struggle against the stream and are content
+to acquiesce, as parasites or rottenness eaters, in a drifting life of
+ease.
+
+More important than very peculiar cases is the broad fact that over and
+over again in different groups of animals there have been attempts to
+master different kinds of haunts--such as the underground world, the
+trees, the freshwaters, and the air. There are burrowing amphibians,
+burrowing reptiles, burrowing birds, and burrowing mammals; there are
+tree-toads, tree-snakes, tree-lizards, tree-kangaroos, tree-sloths,
+tree-shrews, tree-mice, tree-porcupines, and so on; enough of a list to
+show, without mentioning birds, how many different kinds of animals
+have entered upon an arboreal apprenticeship--an apprenticeship often
+with far-reaching consequences. What the freeing of the hand from being
+an organ of terrestrial support has meant in the evolution of monkeys is
+a question that gives a spur to our imagination.
+
+
+The Case of the Robber Crab
+
+On some of the coral islands of the Indian and Pacific Oceans there
+lives a land-crab, Birgus, which has learned to breathe on land. It
+breathes dry air by means of curious blood-containing tufts in the upper
+part of its gill-cavity, and it has also rudimentary gills. It is often
+about a foot long, and it has very heavy great claws, especially on the
+left-hand side. With this great claw it hammers on the "eye-hole" of a
+coconut, from which it has torn off the fibrous husk. It hammers until a
+hole is made by which it can get at the pulp. Part of the shell is
+sometimes used as a protection for the soft abdomen--for the
+robber-crab, as it is called, is an offshoot from the hermit-crab stock.
+Every year this quaint explorer, which may go far up the hills and climb
+the coco-palms, has to go back to the sea to spawn. The young ones are
+hatched in the same state as in our common shore-crab. That is to say,
+they are free-swimming larvæ which pass through an open-water period
+before they settle down on the shore, and eventually creep up on to dry
+land. Just as open-water turtles lay their eggs on sandy shores, going
+back to their old terrestrial haunt, so the robber-crab, which has
+almost conquered the dry land, has to return to the seashore to breed.
+There is a peculiar interest in the association of the robber-crab with
+the coco-palm, for that tree is not a native of these coral islands, but
+has been introduced, perhaps from Mexico, by the Polynesian mariners
+before the discovery of America by Columbus. So the learning to deal
+with coconuts is a recent achievement, and we are face to face with a
+very good example of evolution going on.
+
+[Illustration: EARLY LIFE-HISTORY OF THE SALMON
+
+1. The fertilised egg, shed in the gravelly bed of the river.
+
+2. The embryo within the egg, just before hatching. The embryo has been
+constricted off from the yolk-laden portion of the egg.
+
+3. The newly hatched salmon, or alevin, encumbered with its legacy of
+yolk (Y.S.).
+
+4 and 5. The larval salmon, still being nourished from the yolk-sac
+(Y.S.), which is diminishing in size as the fish grows larger.
+
+6. The salmon fry about six weeks old, with the yolk fully absorbed, so
+that the young fish has now to feed for itself. The fry become parr,
+which go to the sea as smolts, and return as grilse.
+
+In all cases the small figures to the right indicate the natural size.]
+
+[Illustration: THE SALMON LEAPING AT THE FALL IS A MOST FASCINATING
+SPECTACLE
+
+Again and again we see them jumping out of the seething foam beneath the
+fall, casting themselves into the curtain of the down-rushing water,
+only to be carried back by it into the depths whence they have risen.
+One here and another there makes its effort good, touches the upper lip
+of the cataract, gives a swift stroke of its tail, and rushes on towards
+those upper reaches which are the immemorial spawning beds of its
+race.]
+
+
+The Story of the Salmon
+
+In late autumn or in winter the salmon spawn in the rivers. The female
+makes a shallow trough in the gravel by moving her tail from side to
+side, and therein lays many eggs. The male, who is in attendance,
+fertilises these with the milt, and then the female covers them deeply
+with gravel. The process is repeated over and over again for a week or
+more till all the eggs are shed. For three to four months the eggs
+develop, and eventually there emerge the larvæ or _alevins_, which lurk
+among the pebbles. They cannot swim much, for they are encumbered by a
+big legacy of yolk. In a few weeks, perhaps eight, the protruding bag of
+yolk has disappeared and the _fry_, about an inch long, begin to move
+about more actively and to fend for themselves. By the end of the year
+they have grown to be rather trout-like _parr_, about four inches long.
+In two years these are double that length. Usually in the second year,
+but it may be earlier or later, the parr become silvery _smolts_, which
+go out to sea, usually about the month of May. They feed on young
+herring and the like and grow large and strong. When they are about
+three and a half years old they come up the rivers as _grilse_ and may
+spawn. Or they may pass through the whole grilse stage in the sea and
+come up the rivers with all the characters of the full-grown fish. In
+many cases the salmon spawn only once, and some (they are called _kelts_
+after spawning) are so much exhausted by starting a new generation that
+they die or fall a victim to otters and other enemies. In the case of
+the salmon of the North Pacific (in the genus _Oncorhynchus_, not
+_Salmo_) all the individuals die after spawning, none being able to
+return to the sea. It must be remembered that full-grown salmon do not
+as a rule feed in fresh water, though they may be unable to resist
+snapping at the angler's strange creations. A very interesting fact is
+that the salmon keeps as it were a diary of its movements, which vary a
+good deal in different rivers. This diary is written in the scales, and
+a careful reading of the concentric lines on the scales shows the age of
+the fish, and when it went out to sea, and whether it has spawned or
+not, and more besides.
+
+
+Interpretation of the Salmon's Story
+
+When an animal frequents two different haunts, in one of which it
+breeds, it is very often safe to say that the breeding-place represents
+the original home. The flounder is quite comfortable far up the rivers,
+but it has to go to the shore-waters to spawn, and there is no doubt
+that the flounder is a marine fish which has recently learned to
+colonise the fresh waters. Its relatives, like plaice and sole, are
+strictly marine. But it is impossible to make a dogma of the rule that
+the breeding-place corresponds to the original home. Thus some kinds of
+bass, which belong to the marine family of sea-perches, live in the sea
+or in estuaries, while two have become permanent residents in fresh
+water. Or, again, the members of the herring family are very
+distinctively marine, but the shad, which belong to this family, spawn
+in rivers and may spend their lives there.
+
+So there are two different ways of interpreting the life-history of the
+salmon. Some authorities regard the salmon as a marine fish which is
+establishing itself in fresh water. But others read the story the other
+way and regard the salmon as a member of a freshwater race, that has
+taken to the sea for feeding purposes. In regard to trout, we know that
+the ranks of those in rivers and lakes are continually being reinforced
+by migrants from the sea, and that some trout go down to the sea while
+others remain in the freshwater. We know also in regard to a related
+fish, the char, that while the great majority of kinds are now permanent
+residents in cold and deep, isolated northern lakes, there are Arctic
+forms which live in the sea but enter the rivers to spawn. These facts
+favour the view that the salmon was originally a marine fish. But there
+are arguments on both sides, and, for our present purpose, the important
+fact is that the salmon is conquering _two_ haunts. Its evolution is
+going on.
+
+
+The Romance of the Eel
+
+Early in summer, at dates varying with the distance of the rivers from
+the open Atlantic, crowds of young eels or elvers come up-stream.
+Sometimes the procession or eel-fare includes thousands of individuals,
+each about the length of our first finger, and as thick as a stout
+knitting needle. They obey an inborn impulse to swim against the stream,
+seeking automatically to have both sides of their body equally
+stimulated by the current. So they go straight ahead. The obligation
+works only during the day, for when the sun goes down behind the hills
+the elvers snuggle under stones or beneath the bank and rest till dawn.
+In the course of time they reach the quiet upper reaches of the river or
+go up rivulets and drainpipes to the isolated ponds. Their impulse to go
+on must be very imperious, for they may wriggle up the wet moss by the
+side of a waterfall or even make a short excursion in a damp meadow.
+
+In the quiet-flowing stretches of the river or in the ponds they feed
+and grow for years and years. They account for a good many young fishes.
+Eventually, after five or six years in the case of the males, six to
+eight years in the case of the females, the well-grown fishes, perhaps a
+foot and a half to two feet long, are seized by a novel restlessness.
+They are beginning to be mature. They put on a silvery jacket and become
+large of eye, and they return to the sea. In getting away from the pond
+it may be necessary to wriggle through the damp meadow-grass before
+reaching the river. They travel by night and rather excitedly. The
+Arctic Ocean is too cold for them and the North Sea too shallow. They
+must go far out to sea, to where the old margin of the once larger
+continent of Europe slopes down to the great abysses, from the Hebrides
+southwards. Eels seem to spawn in the deep dark water; but the just
+liberated eggs have not yet been found. The young fry rises to near the
+surface and becomes a knife-blade-like larva, transparent all but its
+eye. It lives for many months in this state, growing to be about three
+inches long, rising and sinking in the water, and swimming gently.
+These open-sea young eels are known as Leptocephali, a name given to
+them before their real nature was proved. They gradually become shorter,
+and the shape changes from knife-blade-like to cylindrical. During this
+change they fast, and the weight of their delicate body decreases. They
+turn into glass-eels, about 2-1/2 inches long, like a knitting-needle in
+girth. They begin to move towards the distant shores and rivers, and
+they may be a year and a half old before they reach their destination
+and go up-stream as elvers. Those that ascend the rivers of the Eastern
+Baltic must have journeyed three thousand miles. It is certain that no
+eel ever matures or spawns in fresh water. It is practically certain
+that all the young eels ascending the rivers of North Europe have come
+in from the Atlantic, some of them perhaps from the Azores or further
+out still. It is interesting to inquire how the young eels circumvent
+the Falls of the Rhine and get into Lake Constance, or how their kindred
+on the other side of the Atlantic overcome the obstacle of Niagara; but
+it is more important to lay emphasis on the variety of habitats which
+this fish is trying--the deep waters, the open sea, the shore, the
+river, the pond, and even, it may be, a little taste of solid earth. It
+seems highly probable that the common eel is a deep-water marine fish
+which has learned to colonise the freshwaters. It has been adventurous
+and it has succeeded. The only shadow on the story of achievement is
+that there seems to be no return from the spawning. There is little
+doubt that death is the nemesis of their reproduction. In any case, no
+adult eel ever comes back from the deep sea. We are minded of Goethe's
+hard saying: "Death is Nature's expert advice to get plenty of life."
+
+
+§ 4
+
+Forming New Habits
+
+There is a well-known mudfish of Australia, Neoceratodus by name, which
+has turned its swim-bladder into a lung and comes to the surface to
+spout. It expels vitiated air with considerable force and takes fresh
+gulps. At the same time, like an ordinary fish, it has gills which allow
+the usual interchange of gases between the blood and the water. Now this
+Australian mudfish or double-breather (Dipnoan), which may be a long way
+over a yard in length, is a direct and little-changed descendant of an
+ancient extinct fish, Ceratodus, which lived in Mesozoic times, as far
+back as the Jurassic, which probably means over five millions of years
+ago. The Queensland mudfish is an antiquity, and there has not been much
+change in its lineage for millions of years. We might take it as an
+illustration of the inertia of evolution. And yet, though its structure
+has changed but little, the fish probably illustrates evolution in
+process, for it is a fish that is learning to breathe dry air. It cannot
+leave the water; but it can live comfortably in pools which are foul
+with decomposing animal and vegetable matter. In partially dried-up and
+foul waterholes, full of dead fishes of various kinds, Neoceratodus has
+been found vigorous and lively. Unless we take the view, which is
+_possible_, that the swim-bladder of fishes was originally a lung, the
+mud-fishes are learning to breathe dry air. They illustrate evolution
+agoing.
+
+[Illustration: DIAGRAM OF THE LIFE HISTORY OF THE COMMON EEL (_Anguilla
+Vulgalis_)
+
+1. The transparent open-sea knife-blade-like larva called a
+Leptocephalus.
+
+2 and 3. The gradual change of shape from knife-blade-like to
+cylindrical. The body becomes shorter and loses weight.
+
+4. The young elver, at least a year old, which makes its way from the
+open sea to the estuaries and rivers. It is 2/3 inches long and almost
+cylindrical.
+
+5. The fully-formed eel.]
+
+[Illustration: _Photo: Gambier Bolton._
+
+CASSOWARY
+
+Its bare head is capped with a helmet. Unlike the plumage of most birds
+its feathers are loose and hair-like, whilst its wings are merely
+represented by a few black quills. It is flightless and entirely
+dependent on its short powerful legs to carry it out of danger.]
+
+[Illustration: _Photo: Gambier Bolton._
+
+THE KIWI, ANOTHER FLIGHTLESS BIRD, OF REMARKABLE APPEARANCE, HABITS, AND
+STRUCTURE]
+
+The herring-gull is by nature a fish-eater; but of recent years, in some
+parts of Britain, it has been becoming in the summer months more and
+more of a vegetarian, scooping out the turnips, devouring potatoes,
+settling on the sheaves in the harvest field and gorging itself with
+grain. Similar experiments, usually less striking, are known in many
+birds; but the most signal illustration is that of the kea or Nestor
+parrot of New Zealand, which has taken to lighting on the loins of the
+sheep, tearing away the fleece, cutting at the skin, and gouging out
+fat. Now the parrot belongs to a vegetarian or frugivorous stock, and
+this change of diet in the relatively short time since sheep-ranches
+were established in New Zealand is very striking. Here, since we know
+the dates, we may speak of evolution going on under our eyes. It must be
+remembered that variations in habit may give an animal a new
+opportunity to test variations in structure which arise mysteriously
+from within, as expressions of germinal changefulness rather than as
+imprints from without. For of the transmissibility of the latter there
+is little secure evidence.
+
+
+Experiments in Locomotion
+
+It is very interesting to think of the numerous types of locomotion
+which animals have discovered--pulling and punting, sculling and rowing,
+and of the changes that are rung on these four main methods. How
+striking is the case of the frilled lizard (Chlamydosaurus) of
+Australia, which at the present time is, as it were, experimenting in
+bipedal progression--always a rather eventful thing to do. It gets up on
+its hind-legs and runs totteringly for a few feet, just like a baby
+learning to walk.
+
+How beautiful is the adventure which has led our dipper or
+water-ouzel--a bird allied to the wrens--to try walking and flying under
+water! How admirable is the volplaning of numerous parachutists--"flying
+fish," "flying frog," "flying dragon," "flying phalanger," "flying
+squirrel," and more besides, which take great leaps through the air. For
+are these not the splendid failures that might have succeeded in
+starting new modes of flight?
+
+Most daring of all, perhaps, are the aerial journeys undertaken by many
+small spiders. On a breezy morning, especially in the autumn, they mount
+on gate-posts and palings and herbage, and, standing with their head to
+the wind, pay out three or four long threads of silk. When the wind tugs
+at these threads, the spinners let go, and are borne, usually back
+downwards, on the wings of the wind from one parish to another. It is
+said that if the wind falls they can unfurl more sail, or furl if it
+rises. In any case, these wingless creatures make aerial journeys. When
+tens of thousands of the used threads sink to earth, there is a "shower
+of gossamer." On his _Beagle_ voyage Darwin observed that vast numbers
+of small gossamer spiders were borne on to the ship when it was sixty
+miles distant from the land.
+
+[Illustration: THE AUSTRALIAN FRILLED LIZARD, WHICH IS AT PRESENT TRYING
+TO BECOME A BIPED
+
+When it gets up on its hind-legs and runs for a short distance it folds
+its big collar round its neck.]
+
+[Illustration: A CARPET OF GOSSAMER
+
+The silken threads used by thousands of gossamer spiders in their
+migrations are here seen entangled in the grass, forming what is called
+a shower of gossamer. At the edge of the grass the gossamer forms a
+curtain, floating out and looking extraordinarily like waves breaking on
+a seashore.]
+
+[Illustration: THE WATER-SPIDER
+
+The spider is seen just leaving its diving-bell to ascend to the surface
+to capture air.
+
+The spider jerks its body and legs out at the surface and then dives--
+
+--carrying with it what looks like a silvery air-bubble--air entangled
+in the hair.
+
+The spider reaches its air-dome. Note how the touch of its legs indents
+the inflated balloon.
+
+Running down the side of the nest, the spider
+
+--brushes off the air at the entrance, and the bubble ascends into the
+silken balloon.
+
+_Photos: J. J. Ward, F.E.S._]
+
+
+New Devices
+
+It is impossible, we must admit, to fix dates, except in a few cases,
+relatively recent; but there is a smack of modernity in some striking
+devices which we can observe in operation to-day. Thus no one will
+dispute the statement that spiders are thoroughly terrestrial animals
+breathing dry air, but we have the fact of the water-spider conquering
+the under-water world. There are a few spiders about the seashore, and a
+few that can survive douching with freshwater, but the particular case
+of the true water-spider, _Argyroneta natans_, stands by itself because
+the creature, as regards the female at least, has _conquered_ the
+sub-aquatic environment. A flattish web is woven, somehow, underneath
+the water, and pegged down by threads of silk. Along a special vertical
+line the mother spider ascends to the surface and descends again, having
+entangled air in the hairs of her body. She brushes off this air
+underneath her web, which is thereby buoyed up into a sort of dome. She
+does this over and over again, never getting wet all the time, until the
+domed web has become like a diving-bell, full of dry air. In this
+eloquent anticipation of man's rational device, this creature--far from
+being endowed with reason--lays her eggs and looks after her young. The
+general significance of the facts is that when competition is keen, a
+new area of exploitation is a promised land. Thus spiders have spread
+over all the earth except the polar areas. But here is a spider with
+some spirit of adventure, which has endeavoured, instead of trekking, to
+find a new corner near at home. It has tackled a problem surely
+difficult for a terrestrial animal, the problem of living in great part
+under water, and it has solved it in a manner at once effective and
+beautiful.
+
+
+In Conclusion
+
+We have given but a few representative illustrations of a great theme.
+When we consider the changefulness of living creatures, the
+transformations of cultivated plants and domesticated animals, the
+gradual alterations in the fauna of a country, the search after new
+haunts, the forming of new habits, and the discovery of many inventions,
+are we not convinced that Evolution is going on? And why should it
+stop?
+
+
+
+
+VII
+
+THE DAWN OF MIND
+
+
+
+
+THE DAWN OF MIND
+
+
+In the story of evolution there is no chapter more interesting than the
+emergence of mind in the animal kingdom. But it is a difficult chapter
+to read, partly because "mind" cannot be seen or measured, only
+_inferred_ from the outward behaviour of the creature, and partly
+because it is almost impossible to avoid reading ourselves into the much
+simpler animals.
+
+
+§ 1
+
+Two Extremes to be Avoided
+
+The one extreme is that of uncritical generosity which credits every
+animal, like Brer Rabbit--who, by the way, was the hare--with human
+qualities. The other extreme is that of thinking of the animal as if it
+were an automatic machine, in the working of which there is no place or
+use for mind. Both these extremes are to be avoided.
+
+When Professor Whitman took the eggs of the Passenger Pigeon (which
+became extinct not long ago with startling rapidity) and placed them a
+few inches to one side of the nest, the bird looked a little uneasy and
+put her beak under her body as if to feel for something that was not
+there. But she did not try to retrieve her eggs, close at hand as they
+were. In a short time she flew away altogether. This shows that the mind
+of the pigeon is in some respects very different from the mind of man.
+On the other hand, when a certain clever dog, carrying a basket of eggs,
+with the handle in his mouth, came to a stile which had to be
+negotiated, he laid the basket on the ground, pushed it gently through a
+low gap to the other side, and then took a running leap over. We dare
+not talk of this dog as an automatic machine.
+
+
+A Caution in Regard to Instinct
+
+In studying the behaviour of animals, which is the only way of getting
+at their mind, for it is only of our own mind that we have direct
+knowledge, it is essential to give prominence to the fact that there has
+been throughout the evolution of living creatures a strong tendency to
+enregister or engrain capacities of doing things effectively. Thus
+certain abilities come to be inborn; they are parts of the inheritance,
+which will express themselves whenever the appropriate trigger is
+pulled. The newly born child does not require to learn its breathing
+movements, as it afterwards requires to learn its walking movements. The
+ability to go through the breathing movements is inborn, engrained,
+enregistered.
+
+In other words, there are hereditary pre-arrangements of nerve-cells and
+muscle-cells which come into activity almost as easily as the beating of
+the heart. In a minute or two the newborn pigling creeps close to its
+mother and sucks milk. It has not to learn how to do this any more than
+we have to learn to cough or sneeze. Thus animals have many useful
+ready-made, or almost ready-made, capacities of doing apparently clever
+things. In simple cases of these inborn pre-arrangements we speak of
+reflex actions; in more complicated cases, of instinctive behaviour. Now
+the caution is this, that while these inborn capacities usually work
+well in natural conditions, they sometimes work badly when the ordinary
+routine is disturbed. We see this when a pigeon continues sitting for
+many days on an empty nest, or when it fails to retrieve its eggs only
+two inches away. But it would be a mistake to call the pigeon, because
+of this, an unutterably stupid bird. We have only to think of the
+achievements of homing pigeons to know that this cannot be true. We must
+not judge animals in regard to those kinds of behaviour which have been
+handed over to instinct, and go badly agee when the normal routine is
+disturbed. In ninety-nine cases out of a hundred the enregistered
+instinctive capacities work well, and the advantage of their becoming
+stereotyped was to leave the animal more free for adventures at a higher
+level. Being "a slave of instinct" may give the animal a security that
+enables it to discover some new home or new food or new joy. Somewhat in
+the same way, a man of methodical habits, which he has himself
+established, may gain leisure to make some new departure of racial
+profit.
+
+[Illustration: _Photo: O. J. Wilkinson._
+
+JACKDAW BALANCING ON A GATEPOST
+
+The jackdaw is a big-brained, extremely alert, very educable, loquacious
+bird.]
+
+[Illustration: _From Ingersoll's "The Wit of the Wild."_
+
+TWO OPOSSUMS FEIGNING DEATH
+
+The Opossums are mainly arboreal marsupials, insectivorous and
+carnivorous, confined to the American Continent from the United States
+to Patagonia. Many have no pouch and carry their numerous young ones on
+their back, the tail of the young twined round that of the mother. The
+opossums are agile, clever creatures, and famous for "playing 'possum,"
+lying inert just as if they were dead.]
+
+[Illustration: MALE OF THREE-SPINED STICKLEBACK, MAKING A NEST OF
+WATER-WEED, GLUED TOGETHER BY VISCID THREADS SECRETED FROM THE KIDNEYS
+AT THE BREEDING SEASON]
+
+[Illustration: A FEMALE STICKLEBACK ENTERS THE NEST WHICH THE MALE HAS
+MADE, LAYS THE EGGS INSIDE, AND THEN DEPARTS
+
+In many cases two or three females use the same nest, the stickleback
+being polygamous. Above the nest the male, who mounts guard, is seen
+driving away an intruder.]
+
+When we draw back our finger from something very hot, or shut our eye to
+avoid a blow from a rebounding branch, we do not will the action; and
+this is more or less the case, probably, when a young mammal sucks its
+mother for the first time. Some Mound-birds of Celebes lay their eggs in
+warm volcanic ash by the shore of the sea, others in a great mass of
+fermenting vegetation; it is inborn in the newly hatched bird to
+struggle out as quickly as it can from such a strange nest, else it will
+suffocate. If it stops struggling too soon, it perishes, for it seems
+that the trigger of the instinct cannot be pulled twice. Similarly, when
+the eggs of the turtle, that have been laid in the sand of the shore,
+hatch out, the young ones make _instinctively_ for the sea. Some of the
+crocodiles bury their eggs two feet or so below the surface among sand
+and decaying vegetation--an awkward situation for a birthplace. When the
+young crocodile is ready to break out of the egg-shell, just as a chick
+does at the end of the three weeks of brooding, it utters
+_instinctively_ a piping cry. On hearing this, the watchful mother digs
+away the heavy blankets, otherwise the young crocodile would be buried
+alive at birth. Now there is no warrant for believing that the young
+Mound-birds, young crocodiles, and young turtles have an intelligent
+appreciation of what they do when they are hatched. They act
+instinctively, "as to the manner born." But this is not to say that
+their activity is not backed by endeavour or even suffused with a
+certain amount of awareness. Of course, it is necessarily difficult for
+man, who is so much a creature of intelligence, to get even an inkling
+of the mental side of instinctive behaviour.
+
+In many of the higher reaches of animal instinct, as in courtship or
+nest-building, in hunting or preparing the food, it looks as if the
+starting of the routine activity also "rang up" the higher centres of
+the brain and put the intelligence on the _qui vive_, ready to interpose
+when needed. So the twofold caution is this: (1) We must not depreciate
+the creature too much if, in unusual circumstances, it acts in an
+ineffective way along lines of behaviour which are normally handed over
+to instinct; and (2) we must leave open the possibility that even
+routine instinctive behaviour may be suffused with awareness and backed
+by endeavour.
+
+
+§ 2
+
+A Useful Law
+
+But how are we to know when to credit the animal with intelligence and
+when with something less spontaneous? Above all, how are we to know when
+the effective action, like opening the mouth the very instant it is
+touched by food in the mother's beak, is just a physiological action
+like coughing or sneezing, and when there is behind it--a mind at work?
+The answer to this question is no doubt that given by Prof. Lloyd
+Morgan, who may be called the founder of comparative psychology, that we
+must describe the piece of behaviour very carefully, just as it
+occurred, without reading anything into it, and that we must not ascribe
+it to a higher faculty if it can be satisfactorily accounted for in
+terms of a lower one. In following this principle we may be sometimes
+niggardly, for the behaviour may have a mental subtlety that we have
+missed; but in nine cases out of ten our conclusions are likely to be
+sound. It is the critical, scientific way.
+
+Bearing this law in mind, let us take a survey of the emergence of mind
+among backboned animals.
+
+
+Senses of Fishes
+
+Fishes cannot shut their eyes, having no true lids; but the eyes
+themselves are very well developed and the vision is acute, especially
+for moving objects. Except in gristly fishes, the external opening to
+the ear has been lost, so that sound-waves and coarser vibrations must
+influence the inner ear, which is well developed, through the
+surrounding flesh and bones. It seems that the main use of the ear in
+fishes is in connection with balancing, not with hearing. In many cases,
+however, the sense of hearing has been demonstrated; thus fishes will
+come to the side of a pond to be fed when a bell is rung or when a
+whistle is blown by someone not visible from the water. The fact that
+many fishes pay no attention at all to loud noises does not prove that
+they are deaf, for an animal may hear a sound and yet remain quite
+indifferent or irresponsive. This merely means that the sound has no
+vital interest for the animal. Some fishes, such as bullhead and
+dogfish, have a true sense of smell, detecting by their nostrils very
+dilute substances permeating the water from a distance. Others, such as
+members of the cod family, perceive their food in part at least by the
+sense of taste, which is susceptible to substances near at hand and
+present in considerable quantity. This sense of taste may be located on
+the fins as well as about the mouth. At this low level the senses of
+smell and taste do not seem to be very readily separated. The chief use
+of the sensitive line or lateral line seen on each side of a bony fish
+is to make the animal aware of slow vibrations and changes of pressure
+in the water. The skin responds to pressures, the ear to vibrations of
+high frequency; the lateral line is between the two in its function.
+
+
+Interesting Ways of Fishes
+
+The brain of the ordinary bony fish is at a very low level. Thus the
+cerebral hemispheres, destined to become more and more the seat of
+intelligence, are poorly developed. In gristly fishes, like skates and
+sharks, the brain is much more promising. But although the state of the
+brain does not lead one to expect very much from a bony fish like trout
+or eel, haddock or herring, illustrations are not wanting of what might
+be called pretty pieces of behaviour. Let us select a few cases.
+
+
+The Stickleback's Nest
+
+The three-spined and two-spined sticklebacks live equally well in fresh
+or salt water; the larger fifteen-spined stickleback is entirely marine.
+In all three species the male fish makes a nest, in fresh or brackish
+water in the first two cases, in shore-pools in the third case. The
+little species use the leaves and stems of water-plants; the larger
+species use seaweed and zoophyte. The leaves or fronds are entangled
+together and fastened by glue-like threads, secreted, strange to say, by
+the kidneys. It is just as if a temporary diseased condition had been
+regularised and turned to good purpose. Going through the nest several
+times, the male makes a little room in the middle. Partly by coercion
+and partly by coaxing he induces a female--first one and then
+another--to pass through the nest with two doors, depositing eggs during
+her short sojourn. The females go their way, and the male mounts guard
+over the nest. He drives off intruding fishes much bigger than himself.
+When the young are hatched, the male has for a time much to do, keeping
+his charges within bounds until they are able to move about with
+agility. It seems that sticklebacks are short-lived fishes, probably
+breeding only once; and it is reasonable to suppose that their success
+as a race depends to some extent on the paternal care. Now if we could
+believe that the nesting behaviour had appeared suddenly in its present
+form, we should be inclined to credit the fish with considerable mental
+ability. But we are less likely to be so generous if we reflect that the
+routine has been in all likelihood the outcome of a long racial process
+of slight improvements and critical testings. The secretion of the glue
+probably came about as a pathological variation; its utilisation was
+perhaps discovered by accident; the types that had wit enough to take
+advantage of this were most successful; the routine became enregistered
+hereditarily. The stickleback is not so clever as it looks.
+
+[Illustration: _Photo: Imperial War Museum._
+
+HOMING PIGEON
+
+A blue chequer hen, which during the War (in September of 1918) flew 22
+miles in as many minutes, saving the crew of an aeroplane in
+difficulties.]
+
+[Illustration: _Photo: Imperial War Museum._
+
+CARRIER PIGEON
+
+Carrier pigeons were much used in the War to carry messages. The
+photograph shows how the message is fixed to the carrier pigeon's leg,
+in the form of light rings.]
+
+[Illustration: _Photo: James's Press Agency._
+
+YELLOW-CROWNED PENGUIN
+
+Notice the flightless wings turned into flippers, which are often
+flapped very vigorously. The very strong feet are also noteworthy.
+Penguins are mostly confined to the Far South.]
+
+[Illustration: _Photo: Cagcombe & Co._
+
+PENGUINS ARE "A PECULIAR PEOPLE"
+
+Their wings have been turned into flippers for swimming in the sea and
+tobogganing on snow. The penguins come back over hundreds of miles of
+trackless waste to their birthplace, where they breed. When they reach
+the Antarctic shore they walk with determination to a suitable site,
+often at the top of a steep cliff. Some species waddle 130 steps per
+minute, 6 inches per step, two-thirds of a mile per hour.]
+
+
+The Mind of a Minnow
+
+To find solid ground on which to base an appreciation of the behaviour
+of fishes, it is necessary to experiment, and we may refer to Miss
+Gertrude White's interesting work on American minnows and sticklebacks.
+After the fishes had become quite at home in their artificial
+surroundings, their lessons began. Cloth packets, one of which contained
+meat and the other cotton, were suspended at opposite ends of the
+aquarium. The mud-minnows did not show that they perceived either
+packet, though they swam close by them; the sticklebacks were intrigued
+at once. Those that went towards the packet containing meat darted
+furiously upon it and pulled at it with great excitement. Those that
+went towards the cotton packet turned sharply away when they were within
+about two inches off. They then perceived what those at the other end
+were after and joined them--a common habit amongst fishes. Although the
+minnows were not interested in the tiny "bags of mystery," they were
+even more alert than the sticklebacks in perceiving moving objects in or
+on the water, and there is no doubt that both these shallow-water
+species discover their food largely by sense of sight.
+
+The next set of lessons had to do with colour-associations. The fishes
+were fed on minced snail, chopped earthworm, fragments of liver, and the
+like, and the food was given to them from the end of forceps held above
+the surface of the water, so that the fishes could not be influenced by
+smell. They had to leap out of the water to take the food from the
+forceps. Discs of coloured cardboard were slipped over the end of the
+forceps, so that what the fishes saw was a morsel of food in the centre
+of a coloured disc. After a week or so of preliminary training, they
+were so well accustomed to the coloured discs that the presentation of
+one served as a signal for the fishes to dart to the surface and spring
+out of the water. When baits of paper were substituted for the food, the
+fishes continued to jump at the discs. When, however, a blue disc was
+persistently used for the paper bait and a red disc for the real food,
+or _vice versa_, some of the minnows learned to discriminate infallibly
+between shadow and substance, both when these were presented alternately
+and when they were presented simultaneously. This is not far from the
+dawn of mind.
+
+In the course of a few lessons, both minnows and sticklebacks learned to
+associate particular colours with food, and other associations were also
+formed. A kind of larva that a minnow could make nothing of after
+repeated trials was subsequently ignored. The approach of the
+experimenter or anyone else soon began to serve as a food-signal. There
+can be no doubt that in the ordinary life of fishes there is a process
+of forming useful associations and suppressing useless responses. Given
+an inborn repertory of profitable movements that require no training,
+given the power of forming associations such as those we have
+illustrated, and given a considerable degree of sensory alertness along
+certain lines, fishes do not require much more. And in truth they have
+not got it. Moving with great freedom in three dimensions in a medium
+that supports them and is very uniform and constant, able in most cases
+to get plenty of food without fatiguing exertions and to dispense with
+it for considerable periods if it is scarce, multiplying usually in
+great abundance so that the huge infantile mortality hardly counts,
+rarely dying a natural death but usually coming with their strength
+unabated to a violent end, fishes hold their own in the struggle for
+existence without much in the way of mental endowment. Their brain has
+more to do with motion than with mentality, and they have remained at a
+low psychical level.
+
+Yet just as we should greatly misjudge our own race if we confined our
+attention to everyday routine, so in our total, as distinguished from
+our average, estimate of fishes, we must remember the salmon surmounting
+the falls, the wary trout eluding the angler's skill, the common
+mud-skipper (Periophthalmus) of many tropical shores which climbs on the
+rocks and the roots of the mangrove-trees, or actively hunts small
+shore-animals. We must remember the adventurous life-history of the eel
+and the quaint ways in which some fishes, males especially, look after
+their family. The male sea-horse puts the eggs in his breast-pocket; the
+male Kurtus carries them on the top of his head; the cock-paidle or
+lumpsucker guards them and aerates them in a corner of a shore-pool.
+
+
+§ 3
+
+The Mind of Amphibians
+
+Towards the end of the age of the Old Red Sandstone or Devonian, a great
+step in evolution was taken--the emergence of Amphibians. The earliest
+representatives had fish-like characters even more marked than those
+which may be discerned in the tadpoles of our frogs and toads, and there
+is no doubt that amphibians sprang from a fish stock. But they made
+great strides, associated in part with their attempts to get out of the
+water on to dry land. From fossil forms we cannot say much in regard to
+soft parts; but if we consider the living representatives of the class,
+we may credit amphibians with such important acquisitions as fingers and
+toes, a three-chambered heart, true ventral lungs, a drum to the ear, a
+mobile tongue, and vocal cords. When animals began to be able to grasp
+an object and when they began to be able to utter sufficient sounds, two
+new doors were opened. Apart from insects, whose instrumental music had
+probably begun before the end of the Devonian age, amphibians were the
+first animals to have a voice. The primary meaning of this voice was
+doubtless, as it is to-day in our frogs, a sex-call; but it was the
+beginning of what was destined to play a very important part in the
+evolution of the mind. In the course of ages the significance of the
+voice broadened out; it became a parental call; it became an infant's
+cry. Broadening still, it became a very useful means of recognition
+among kindred, especially in the dark and in the intricacies of the
+forest. Ages passed, and the voice rose on another turn of the
+evolutionary spiral to be expressive of particular emotions beyond the
+immediate circle of sex--emotions of joy and of fear, of jealousy and of
+contentment. Finally, we judge, the animal--perhaps the bird was
+first--began to give utterance to particular "words," indicative not
+merely of emotions, but of particular things with an emotional halo,
+such as "food," "enemy," "home." Long afterwards, words became _in man_
+the medium of reasoned discourse. Sentences were made and judgments
+expressed. But was not the beginning in the croaking of Amphibia?
+
+
+Senses of Amphibians
+
+Frogs have good eyes, and the toad's eyes are "jewels." There is
+evidence of precise vision in the neat way in which a frog catches a
+fly, flicking out its tongue, which is fixed in front and loose behind.
+There is also experimental proof that a frog discriminates between red
+and blue, or between red and white, and an interesting point is that
+while our skin is sensitive to heat rays but not to light, the skin of
+the frog answers back to light rays as well. Professor Yerkes
+experimented with a frog which had to go through a simple labyrinth if
+it wished to reach a tank of water. At the first alternative between two
+paths, a red card was placed on the wrong side and a white one on the
+other. When the frog had learned to take the correct path, marked by the
+white card, Prof. Yerkes changed the cards. The confusion of the frog
+showed how thoroughly it had learned its lesson.
+
+We know very little in regard to sense of smell or taste in amphibians;
+but the sense of hearing is well developed, more developed than might be
+inferred from the indifference that frogs show to almost all sounds
+except the croaking of their kindred and splashes in the water.
+
+The toad looks almost sagacious when it is climbing up a bank, and some
+of the tree-frogs are very alert; but there is very little that we dare
+say about the amphibian mind. We have mentioned that frogs may learn the
+secret of a simple maze, and toads sometimes make for a particular
+spawning-pond from a considerable distance. But an examination of their
+brains, occupying a relatively small part of the broad, flat skull,
+warns us not to expect much intelligence. On the other hand, when we
+take frogs along a line that is very vital to them, namely, the
+discrimination of palatable and unpalatable insects, we find, by
+experiment, that they are quick to learn and that they remember their
+lessons for many days. Frogs sometimes deposit their eggs in very
+unsuitable pools of water; but perhaps that is not quite so stupid as it
+looks. The egg-laying is a matter that has been, as it were, handed over
+to instinctive registration.
+
+[Illustration: _Photo: W. S. Berridge._
+
+HARPY-EAGLE
+
+"Clean and dainty and proud as a Spanish Don."
+
+It is an arboreal and cliff-loving bird, feeding chiefly on mammals,
+very fierce and strong. The under parts are mostly white, with a greyish
+zone on the chest. The upper parts are blackish-grey. The harpy occurs
+from Mexico to Paraguay and Bolivia.]
+
+[Illustration: _Photo: W. S. Berridge, F.Z.S._
+
+THE DINGO OR WILD DOG OF AUSTRALIA, PERHAPS AN INDIGENOUS WILD SPECIES,
+PERHAPS A DOMESTICATED DOG THAT HAS GONE WILD OR FERAL
+
+It does much harm in destroying sheep. It is famous for its persistent
+"death-feigning," for an individual has been known to allow part of its
+skin to be removed, in the belief that it was dead, before betraying its
+vitality.]
+
+[Illustration: WOODPECKER, HAMMERING AT A COTTON-REEL, ATTACHED TO A
+TREE
+
+Notice how the stiff tail-feathers braced against the stem help the bird
+to cling on with its toes. The original hole, in which this woodpecker
+inserted nuts for the purposes of cracking the shell and extracting the
+kernel, is seen towards the top of the tree. But the taker of the
+photograph tied on a hollowed-out cotton-reel as a receptacle for a nut,
+and it was promptly discovered and used by the bird.]
+
+
+Experiments in Parental Care
+
+It must be put to the credit of amphibians that they have made many
+experiments in methods of parental care, as if they were feeling their
+way to new devices. A common frog lays her clumps of eggs in the cradle
+of the water, sometimes far over a thousand together; the toad winds two
+long strings round and between water-weeds; and in both cases that is
+all. There is no parental care, and the prolific multiplication covers
+the enormous infantile mortality. This is the spawning solution of the
+problem of securing the continuance of the race. But there is another
+solution, that of parental care associated with an economical reduction
+of the number of eggs. Thus the male of the Nurse-Frog (Alytes), not
+uncommon on the Continent, fixes a string of twenty to fifty eggs to the
+upper part of his hind-legs, and retires to his hole, only coming out at
+night to get some food and to keep up the moisture about the eggs. In
+three weeks, when the tadpoles are ready to come out, he plunges into
+the pond and is freed from his living burden and his family cares. In
+the case of the thoroughly aquatic Surinam Toad (Pipa), the male helps
+to press the eggs, perhaps a hundred in number, on to the back of the
+female, where each sinks into a pocket of skin with a little lid. By and
+by fully formed young toads jump out of the pockets.
+
+In the South American tree-frogs called Nototrema there is a pouch on
+the back of the female in which the eggs develop, and it is interesting
+to find that in some species what come out are ordinary tadpoles, while
+in other species the young emerge as miniatures of their parents.
+Strangest of all, perhaps, is the case of Darwin's Frog (Rhinoderma of
+Chili), where the young, about ten to fifteen in number, develop in the
+male's croaking-sacs, which become in consequence enormously distended.
+Eventually the strange spectacle is seen of miniature frogs jumping out
+of their father's mouth. Needless to say we are not citing these methods
+of parental care as examples of intelligence; but perhaps they correct
+the impression of amphibians as a rather humdrum race. Whatever be the
+mental aspect of the facts, there has certainly been some kind of
+experimenting, and the increase of parental care, so marked in many
+amphibians, with associated reduction of the number of offspring is a
+finger-post on the path of progress.
+
+
+§ 4
+
+The Reptilian Mind
+
+We speak of the wisdom of the serpent; but it is not very easy to
+justify the phrase. Among all the multitude of reptiles--snakes,
+lizards, turtles, and crocodiles, a motley crowd--we cannot see much
+more than occasional traces of intelligence. The inner life remains a
+tiny rill.
+
+No doubt many reptiles are very effective; but it is an instinctive
+rather than an intelligent efficiency. The well-known "soft-shell"
+tortoise of the United States swims with powerful strokes and runs so
+quickly that it can hardly be overtaken. It hunts vigorously for
+crayfish and insect larvæ in the rivers. It buries itself in the mud
+when cold weather comes. It may lie on a floating log ready to slip into
+the water at a moment's notice; it may bask on a sunny bank or in the
+warm shallows. Great wariness is shown in choosing times and places for
+egg-laying. The mother tramps the earth down upon the buried eggs. All
+is effective. Similar statements might be made in regard to scores of
+other reptiles; but what we see is almost wholly of the nature of
+instinctive routine, and we get little glimpse of more than efficiency
+and endeavour.
+
+In a few cases there is proof of reptiles finding their way back to
+their homes from a considerable distance, and recognition of persons is
+indubitable. Gilbert White remarks of his tortoise: "Whenever the good
+old lady came in sight who had waited on it for more than thirty years,
+it always hobbled with awkward alacrity towards its benefactress, while
+to strangers it was altogether inattentive." Of definite learning there
+are a few records. Thus Professor Yerkes studied a sluggish turtle of
+retiring disposition, taking advantage of its strong desire to efface
+itself. On the path of the darkened nest of damp grass he interposed a
+simple maze in the form of a partitioned box. After wandering about
+constantly for thirty-five minutes the turtle found its way through the
+maze by chance. Two hours afterwards it reached the nest in fifteen
+minutes; and after another interval of two hours it only required five
+minutes. After the third trial, the routes became more direct, there was
+less aimless wandering. The time of the twentieth trial was forty-five
+seconds; that of the thirtieth, forty seconds. In the thirtieth case,
+the path followed was quite direct, and so it was on the fiftieth trip,
+which only required thirty-five seconds. Of course, the whole thing did
+not amount to very much; but there was a definite learning, _a learning
+from experience_, which has played an important part in the evolution of
+animal behaviour.
+
+Comparing reptiles with amphibians, we may recognise an increased
+masterliness of behaviour and a hint of greater plasticity. The records
+of observers who have made pets of reptiles suggest that the life of
+feeling or emotion is growing stronger, and so do stories, if they can
+be accepted, which suggest the beginning of conjugal affection.
+
+The error must be guarded against of interpreting in terms of
+intelligence what is merely the outcome of long-continued structure
+adaptation. When the limbless lizard called the Slow-worm is suddenly
+seized by the tail, it escapes by surrendering the appendage, which
+breaks across a preformed weak plane. But this is a reflex action, not a
+reflective one. It is comparable to our sudden withdrawal of our finger
+from a very hot cinder. The Egg-eating African snake Dasypeltis gets the
+egg of a bird into its gullet unbroken, and cuts the shell against
+downward-projecting sharp points of the vertebræ. None of the precious
+contents is lost and the broken "empties" are returned. It is admirable,
+indeed unsurpassable; but it is not intelligent.
+
+
+§ 5
+
+Mind in Birds
+
+Sight and hearing are highly developed in birds, and the senses, besides
+pulling the triggers of inborn efficiencies, supply the raw materials
+for intelligence. There is some truth, though not the whole truth, in
+the old philosophical dictum, that there is nothing in the intellect
+which was not previously in the senses. Many people have admired the
+certainty and alacrity with which gulls pick up a fragment of biscuit
+from the white wake of a steamer, and the incident is characteristic. In
+their power of rapidly altering the focus of the eye, birds are
+unsurpassed.
+
+To the sense of sight in birds, the sense of hearing comes a good
+second. A twig breaks under our feet, and out sounds the danger-call of
+the bird we were trying to watch. Many young birds, like partridges,
+respond when two or three hours old to the anxious warning note of the
+parents, and squat motionless on the ground, though other sounds, such
+as the excited clucking of a foster-mother hen, leave them indifferent.
+They do not know what they are doing when they squat; they are obeying
+the living hand of the past which is within them. Their behaviour is
+instinctive. But the present point is the discriminating quality of the
+sense of hearing; and that is corroborated by the singing of birds.
+It is emotional art, expressing feelings in the medium of sound. On the
+part of the females, who are supposed to listen, it betokens a
+cultivated ear.
+
+[Illustration: THE BEAVER
+
+The beaver will gnaw through trees a foot in diameter; to save itself
+more trouble than is necessary, it will stop when it has gnawed the
+trunk till there is only a narrow core left, having the wit to know that
+the autumn gales will do the rest.]
+
+[Illustration: _Photo: F. R. Hinkins & Son._
+
+THE THRUSH AT ITS ANVIL
+
+The song-thrush takes the snail's shell in its bill, and knocks it
+against a stone until it breaks, making the palatable flesh available.
+
+Many broken shells are often found around the anvil.]
+
+As to the other senses, touch is not highly developed except about the
+bill, where it reaches a climax in birds like the wood-cock, which probe
+for unseen earthworms in the soft soil. Taste seems to be poorly
+developed, for most birds bolt their food, but there is sometimes an
+emphatic rejection of unpalatable things, like toads and caterpillars.
+Of smell in birds little is known, but it has been proved to be present
+in certain cases, e.g. in some nocturnal birds of prey. It seems certain
+that it is by sight, not by smell, that the eagles gather to the
+carcass; but perhaps there is more smell in birds than they are usually
+credited with. One would like to experiment with the oil from the preen
+gland of birds to see whether the scent of this does not help in the
+recognition of kin by kin at night or amid the darkness of the forest.
+There may be other senses in birds, such as a sense of temperature and a
+sense of balance; but no success has attended the attempts made to
+demonstrate a magnetic sense, which has been impatiently postulated by
+students of bird migration in order to "explain" how the birds find
+their way. The big fact is that in birds there are two widely open
+gateways of knowledge, the sense of sight and the sense of hearing.
+
+
+Instinctive Aptitudes
+
+Many a young water-bird, such as a coot, swims right away when it is
+tumbled into water for the first time. So chicks peck without any
+learning or teaching, very young ducklings catch small moths that flit
+by, and young plovers lie low when the danger-signal sounds. But birds
+seem strangely limited as regards many of these instinctive
+capacities--limited when compared with the "little-brained" ants and
+bees, which have from the first such a rich repertory of ready-made
+cleverness. The limitation in birds is of great interest, for it means
+that intelligence is coming to its own and is going to take up the
+reins at many corners of the daily round. Professor Lloyd Morgan
+observed that his chickens incubated in the laboratory had no
+instinctive awareness of the significance of their mother's cluck when
+she was brought outside the door. Although thirsty and willing to drink
+from a moistened finger-tip, they did not instinctively recognize water,
+even when they walked through a saucerful. Only when they happened to
+peck their toes as they stood in the water did they appreciate water as
+the stuff they wanted, and raise their bills up to the sky. Once or
+twice they actually stuffed their crops with "worms" of red worsted!
+
+Instinctive aptitudes, then, the young birds have, but these are more
+limited than in ants, bees, and wasps; and the reason is to be found in
+the fact that the brain is now evolving on the tack of what Sir Ray
+Lankester has called "educability." Young birds _learn_ with prodigious
+rapidity; the emancipation of the mind from the tyranny of hereditary
+obligations has begun. Young birds make mistakes, like the red worsted
+mistake, but they do not make the same mistakes often. They are able to
+profit by experience in a very rapid way. We do not mean that creatures
+of the little-brain type, like ants, bees, and wasps, are unable to
+profit by experience or are without intelligence. There are no such
+hard-and-fast lines. We mean that in the ordinary life of insects the
+enregistered instinctive capacities are on the whole sufficient for the
+occasion, and that intelligent educability is very slightly developed.
+Nor do we mean that birds are quite emancipated from the tyranny of
+engrained instinctive obligations, and can always "ring up" intelligence
+in a way that is impossible for the stereotyped bee. The sight of a
+pigeon brooding on an empty nest, while her two eggs lie disregarded
+only a couple of inches away, is enough to show that along certain lines
+birds may find it impossible to get free from the trammels of instinct.
+The peculiar interest of birds is that they have many instincts and yet
+a notable power of learning intelligently.
+
+
+Intelligence co-operating with Instinct
+
+Professor Lloyd Morgan was foster-parent to two moorhens which grew up
+in isolation from their kindred. They swam instinctively, but they would
+not dive, neither in a large bath nor in a current. But it happened one
+day when one of these moorhens was swimming in a pool on a Yorkshire
+stream, that a puppy came barking down the bank and made an awkward
+feint towards the young bird. In a moment the moorhen dived, disappeared
+from view, and soon partially reappeared, his head just peeping above
+the water beneath the overhanging bank. This was the first time the bird
+had dived, and the performance was absolutely true to type.
+
+There can be little doubt as to the meaning of this observation. The
+moorhen has an hereditary or instinctive capacity for swimming and
+diving, but the latter is not so easily called into activity as the
+former. The particular moorhen in question had enjoyed about two months
+of swimming experience, which probably counted for something, but in the
+course of that experience nothing had pulled the trigger of the diving
+capacity. On an eventful day the young moorhen saw and heard the dog; it
+was emotionally excited; it probably did to some extent intelligently
+appreciate a novel and meaningful situation. Intelligence cooperated
+with instinct, and the bird dived appropriately.
+
+Birds have inborn predispositions to certain effective ways of pecking,
+scratching, swimming, diving, flying, crouching, lying low,
+nest-building, and so on; but they are marked off from the much more
+purely instinctive ants and bees by the extent to which individual
+"nurture" seems to mingle with the inherited "nature." The two together
+result in the fine product which we call the bird's behaviour. After
+Lloyd Morgan's chicks had tried a few conspicuous and unpalatable
+caterpillars, they had no use for any more. They learned in their early
+days with prodigious rapidity, illustrating the deep difference between
+the "big-brain" type, relatively poor in its endowment of instinctive
+capacities, but eminently "educable," and the "little-brain" type, say,
+of ants and bees, richly endowed with instinctive capacities, but very
+far from being quick or glad to learn. We owe it to Sir Ray Lankester to
+have made it clear that these two types of brain are, as it were, on
+different tacks of evolution, and should not be directly pitted against
+one another. The "little-brain" type makes for a climax in the ant,
+where instinctive behaviour reaches a high degree of perfection; the
+"big-brain" type reaches its climax in horse and dog, in elephant and
+monkey. The particular interest that attaches to the behaviour of birds
+is in the combination of a good deal of instinct with a great deal of
+intelligent learning. This is well illustrated when birds make a nest
+out of new materials or in some quite novel situation. It is clearly
+seen when birds turn to some new kind of food, like the Kea parrot,
+which attacks the sheep in New Zealand.
+
+Some young woodpeckers are quite clever in opening fir cones to get at
+the seeds, and this might be hastily referred to a well-defined
+hereditary capacity. But the facts are that the parents bring their
+young ones first the seeds themselves, then partly opened cones, and
+then intact ones. There is an educative process, and so it is in scores
+of cases.
+
+
+Using their Wits
+
+When the Greek eagle lifts the Greek tortoise in its talons, and lets it
+fall from a height so that the strong carapace is broken and the flesh
+exposed, it is making intelligent use of an expedient. Whether it
+discovered the expedient by experimenting, as is possible, or by chance,
+as is more likely, it uses it intelligently. In the same way
+herring-gulls lift sea-urchins and clams in their bills, and let them
+fall on the rocks so that the shells are broken. In the same way rooks
+deal with freshwater mussels.
+
+
+The Thrush's Anvil
+
+A very instructive case is the behaviour of the song-thrush when it
+takes a wood-snail in its beak and hammers it against a stone, its
+so-called anvil. To a young thrush, which she had brought up by hand,
+Miss Frances Pitt offered some wood-snails, but it took no interest in
+them until one put out its head and began to move about. The bird then
+pecked at the snail's horns, but was evidently puzzled when the creature
+retreated within the shelter of the shell. This happened over and over
+again, the thrush's inquisitive interest increasing day by day. It
+pecked at the shell and even picked it up by the lip, but no real
+progress was made till the sixth day, when the thrush seized the snail
+and beat it on the ground as it would a big worm. On the same day it
+picked up a shell and knocked it repeatedly against a stone, trying
+first one snail and then another. After fifteen minutes' hard work, the
+thrush managed to break one, and after that it was all easy. A certain
+predisposition to beat things on the ground was doubtless present, but
+the experiment showed that the use of an anvil could be arrived at by an
+untutored bird. After prolonged trying it found out how to deal with a
+difficult situation. It may be said that in more natural conditions this
+might be picked up by imitation, but while this is quite possible, it is
+useful to notice that experiments with animals lead us to doubt whether
+imitation counts for nearly so much as used to be believed.
+
+
+§ 6
+
+The Mind of the Mammal
+
+When we watch a collie at a sheep-driving competition, or an elephant
+helping the forester, or a horse shunting waggons at a railway siding,
+we are apt to be too generous to the mammal mind. For in the cases we
+have just mentioned, part of man's mind has, so to speak, got into the
+animal's. On the other hand, when we study rabbits and guinea-pigs, we
+are apt to be too stingy, for these rodents are under the average of
+mammals, and those that live in domestication illustrate the stupefying
+effect of a too sheltered life. The same applies to domesticated sheep
+contrasted with wild sheep, or even with their own lambs. If we are to
+form a sound judgment on the intelligence of mammals we must not attend
+too much to those that have profited by man's training, nor to those
+whose mental life has been dulled by domestication.
+
+
+Instinctive Aptitudes
+
+What is to be said of the behaviour of beavers who gnaw the base of a
+tree with their chisel-edged teeth till only a narrow core is left--to
+snap in the first gale, bringing the useful branches down to the ground?
+What is to be said of the harvest-mouse constructing its nest, or of the
+squirrel making cache after cache of nuts? These and many similar pieces
+of behaviour are fundamentally instinctive, due to inborn
+predispositions of nerve-cells and muscle-cells. But in mammals they
+seem to be often attended by a certain amount of intelligent attention,
+saving the creature from the tyranny of routine so marked in the ways of
+ants and bees.
+
+
+Sheer Dexterity
+
+Besides instinctive aptitudes, which are exhibited in almost equal
+perfection by all the members of the same species, there are acquired
+dexterities which depend on individual opportunities. They are also
+marked by being outside and beyond ordinary routine--not that any
+rigorous boundary line can be drawn. We read that at Mathura on the
+Jumna doles of food are provided by the piety of pilgrims for the sacred
+river-tortoises, which are so crowded when there is food going that
+their smooth carapaces form a more or less continuous raft across the
+river. On that unsteady slippery bridge the Langur monkeys
+(_Semnopithecus entellus_) venture out and in spite of vicious snaps
+secure a share of the booty. This picture of the monkeys securing a
+footing on the moving mass of turtle-backs is almost a diagram of sheer
+dexterity. It illustrates the spirit of adventure, the will to
+experiment, which is, we believe, the main motive-force in new
+departures in behaviour.
+
+[Illustration: _Photo: Lafayette_
+
+ALSATIAN WOLF-DOG
+
+An animal of acute senses and great intelligence. It was of great
+service in the war.
+
+(The dog shown, Arno von Indetal, is a trained police dog and did
+service abroad during the war.)]
+
+[Illustration: _Photo: W. S. Berridge._
+
+THE POLAR BEAR OF THE FAR NORTH
+
+An animal of extraordinary strength, able with a stroke of its paw to
+lift a big seal right out of the water and send it crashing along the
+ice. The food consists chiefly of seals. The sexes wander separately. A
+hole is often dug as a winter retreat, but there is no hibernation. A
+polar bear in captivity has been seen making a current with its paw in
+the water of its pool in order to secure floating buns without
+trouble--an instance of sheer intelligence.]
+
+[Illustration: _From the Smithsonian Report_, 1914
+
+AN ALLIGATOR "YAWNING" IN EXPECTATION OF FOOD
+
+Note the large number of sharp conical teeth fixed in sockets along the
+jaws.]
+
+
+Power of Association
+
+A bull-terrier called Jasper, studied by Prof. J. B. Watson, showed
+great power of associating certain words with certain actions. From a
+position invisible to the dog the owner would give certain commands,
+such as "Go into the next room and bring me a paper lying on the floor."
+Jasper did this at once, and a score of similar things.
+
+Lord Avebury's dog Van was accustomed to go to a box containing a small
+number of printed cards and select the card TEA or OUT, as the occasion
+suggested. It had established an association between certain black marks
+on a white background and the gratification of certain desires. It is
+probable that some of the extraordinary things horses and dogs have been
+known to do in the way of stamping a certain number of times in supposed
+indication of an answer to an arithmetical question (in the case of
+horses), or of the name of an object drawn (in the case of dogs), are
+dependent on clever associations established by the teacher between
+minute signs and a number of stampings. What is certain is that mammals
+have in varying degrees a strong power of establishing associations.
+There is often some delicacy in the association established. Everyone
+knows of cases where a dog, a cat, or a horse will remain quite
+uninterested, to all appearance, in its owner's movements until some
+little detail, such as taking a key from its peg, pulls the trigger. Now
+the importance of this in the wild life of the fox or the hare, the
+otter or the squirrel, is obviously that the young animals learn to
+associate certain sounds in their environment with definite
+possibilities. They have to learn an alphabet of woodcraft, the letters
+of which are chiefly sounds and scents.
+
+
+The Dancing Mouse as a Pupil
+
+The dancing or waltzing mouse is a Japanese variety with many
+peculiarities, such as having only one of the three semicircular canals
+of the ear well developed. It has a strong tendency to waltz round and
+round in circles without sufficient cause and to trip sideways towards
+its dormitory instead of proceeding in the orthodox head-on fashion. But
+this freak is a very educable creature, as Professor Yerkes has shown.
+In a careful way he confronted his mouse-pupil with alternative pathways
+marked by different degrees of illumination, or by different colours. If
+the mouse chose compartment A, it found a clear passage direct to its
+nest; if it chose compartment B, it was punished by a mild electric
+shock and it had to take a roundabout road home. Needless to say, the A
+compartment was sometimes to the right hand, sometimes to the left, else
+mere position would have been a guide. The experiments showed that the
+dancing mice learn to discriminate the right path from the wrong, and
+similar results have been got from other mammals, such as rats and
+squirrels. There is no proof of learning by ideas, but there is proof of
+learning by experience. And the same must be true in wild life.
+
+Many mammals, such as cats and rats, learn how to manipulate
+puzzle-boxes and how to get at the treasure at the heart of a Hampton
+Court maze. Some of the puzzle-boxes, with a reward of food inside, are
+quite difficult, for the various bolts and bars have to be dealt with in
+a particular order, and yet many mammals master the problem. What is
+plain is that they gradually eliminate useless movements, that they make
+fewer and fewer mistakes, that they eventually succeed, and that they
+register the solution within themselves so that it remains with them for
+a time. It looks a little like the behaviour of a man who learns a game
+of skill without thinking. It is a learning by experience, not by ideas
+or reflection. Thus it is very difficult to suppose that a rat or a cat
+could form any idea or even picture of the Hampton Court maze--which
+they nevertheless master.
+
+
+Learning Tricks
+
+Given sufficient inducement many of the cleverer mammals will learn to
+do very sensible things, and no one is wise enough to say that they
+never understand what they are doing. Yet it is certain that trained
+animals often exhibit pieces of behaviour which are not nearly so clever
+as they look. The elephant at the Belle Vue Gardens in Manchester used
+to collect pennies from benevolent visitors. When it got a penny in its
+trunk it put it in the slot of an automatic machine which delivered up a
+biscuit. When a visitor gave the elephant a halfpenny it used to throw
+it back with disgust. At first sight this seemed almost wise, and there
+was no doubt some intelligent appreciation of the situation. But it was
+largely a matter of habituation, the outcome of careful and prolonged
+training. The elephant was laboriously taught to put the penny in the
+slot and to discriminate between the useful pennies and the useless
+halfpennies. It was not nearly so clever as it looked.
+
+
+Using their Wits
+
+In the beautiful Zoological Park in Edinburgh the Polar Bear was wont to
+sit on a rocky peninsula of a water-filled quarry. The visitors threw in
+buns, some of which floated on the surface. It was often easy for the
+Polar Bear to collect half a dozen by plunging into the pool. But it had
+discovered a more interesting way. At the edge of the peninsula it
+scooped the water gently with its huge paw and made a current which
+brought the buns ashore. This was a simple piece of behaviour, but it
+has the smack of intelligence--of putting two and two together in a
+novel way. It suggests the power of making what is called a "perceptual
+inference."
+
+On the occasion of a great flood in a meadow it was observed that a
+number of mares brought their foals to the top of a knoll, and stood
+round about them protecting them against the rising water. A dog has
+been known to show what was at any rate a plastic appreciation of a
+varying situation in swimming across a tidal river. It changed its
+starting-point, they say, according to the flow or ebb of the tide.
+Arctic foxes and some other wild mammals show great cleverness in
+dealing with traps, and the manipulative intelligence of elephants is
+worthy of all our admiration.
+
+
+§ 7
+
+Why is there not more Intelligence?
+
+When we allow for dexterity and power of association, when we recognise
+a certain amount of instinctive capacity and a capacity for profiting by
+experience in an intelligent way, we must admit a certain degree of
+disappointment when we take a survey of the behaviour of mammals,
+especially of those with very fine brains, from which we should
+naturally expect great things. Why is there not more frequent exhibition
+of intelligence in the stricter sense?
+
+The answer is that most mammals have become in the course of time very
+well adapted to the ordinary conditions of their life, and tend to leave
+well alone. They have got their repertory of efficient answers to the
+ordinary questions of everyday life, and why should they experiment? In
+the course of the struggle for existence what has been established is
+efficiency in normal circumstances, and therefore even the higher
+animals tend to be no cleverer than is necessary. So while many mammals
+are extraordinarily efficient, they tend to be a little dull. Their
+mental equipment is adequate for the everyday conditions of their life,
+but it is not on sufficiently generous lines to admit of, let us say, an
+interest in Nature or adventurous experiment. Mammals always tend to
+"play for safety."
+
+We hasten, however, to insert here some very interesting saving clauses.
+
+
+Experimentation in Play
+
+A glimpse of what mammals are capable of, were it necessary, may be
+obtained by watching those that are playful, such as lambs and kids,
+foals and calves, young foxes and others. For these young creatures let
+themselves go irresponsibly, they are still unstereotyped, they test
+what they and their fellows can do. The experimental character of much
+of animal play is very marked.
+
+It is now recognised by biologists that play among animals is the young
+form of work, and that the playing period, often so conspicuous, is
+vitally important as an apprenticeship to the serious business of life
+and as an opportunity for learning the alphabet of Nature. But the
+playing period is much more; it is one of the few opportunities animals
+have of making experiments without too serious responsibilities. Play is
+Nature's device for allowing elbow-room for new departures
+(behaviour-variations) which may form part of the raw materials of
+progress. Play, we repeat, gives us a glimpse of the possibilities of
+the mammal mind.
+
+
+Other Glimpses of Intelligence
+
+A squirrel is just as clever as it needs to be and no more; and of some
+vanishing mammals, like the beaver, not even this can be said. Humdrum
+non-plastic efficiency is apt to mean stagnation. Now we have just seen
+that in the play of young mammals there is an indication of unexhausted
+possibilities, and we get the same impression when we think of three
+other facts. (_a_) In those mammals, like dog and horse, which have
+entered into active cooperative relations with man, we see that the mind
+of the mammal is capable of much more than the average would lead us to
+think. When man's sheltering is too complete and the domesticated
+creature is passive in his grip, the intelligence deteriorates. (_b_)
+When we study mammals, like the otter, which live a versatile life in a
+very complex and difficult environment, we get an inspiriting picture of
+the play of wits. (_c_) Thirdly, when we pass to monkeys, where the
+fore-limb has become a free hand, where the brain shows a relatively
+great improvement, where "words" are much used, we cannot fail to
+recognise the emergence of something new--a restless inquisitiveness, a
+desire to investigate the world, an unsatisfied tendency to experiment.
+We are approaching the Dawn of Reason.
+
+
+THE MIND OF MONKEYS
+
+§ 8
+
+There is a long gamut between the bushy-tailed, almost squirrel-like
+marmosets and the big-brained chimpanzee. There is great variety of
+attainment at different levels in the Simian tribe.
+
+
+Keen Senses
+
+To begin at the beginning, it is certain that monkeys have a first-class
+sensory equipment, especially as regards sight, hearing, and touch. The
+axes of the two eyes are directed forwards as in man, and a large
+section of the field of vision is common to both eyes. In other words,
+monkeys have a more complete stereoscopic vision than the rest of the
+mammals enjoy. They look more and smell less. They can distinguish
+different colours, apart from different degrees of brightness in the
+coloured objects. They are quick to discriminate differences in the
+shapes of things, e.g. boxes similar in size but different in shape, for
+if the prize is always put in a box of the same shape they soon learn
+(by association) to select the profitable one. They learn to
+discriminate cards with short words or with signs printed on them,
+coming down when the "Yes" card is shown, remaining on their perch when
+the card says "No." Bred to a forest life where alertness is a
+life-or-death quality, they are quick to respond to a sudden movement or
+to pick out some new feature in their surroundings. And what is true of
+vision holds also for hearing.
+
+
+Power of Manipulation
+
+Another quality which separates monkeys very markedly from ordinary
+mammals is their manipulative expertness, the co-ordination of hand
+and eye. This great gift follows from the fact that among monkeys the
+fore-leg has been emancipated. It has ceased to be indispensable as an
+organ of support; it has become a climbing, grasping, lifting, handling
+organ. The fore-limb has become a free hand, and everyone who knows
+monkeys at all is aware of the zest with which they use their tool. They
+enjoy pulling things to pieces--a kind of dissection--or screwing the
+handle off a brush and screwing it on again.
+
+[Illustration: _Photo: W. P. Dando_
+
+BABY ORANG
+
+Notice the small ears and the suggestion of good temper. The mother
+orang will throw prickly fruits and pieces of branches at those who
+intrude on her maternal care.]
+
+[Illustration: _Photo: Gambier Bolton._
+
+ORANG-UTAN
+
+A large and heavy ape, frequenting forests in Sumatra and Borneo, living
+mainly in trees, where a temporary nest is made. The expression is
+melancholy, the belly very protuberant, the colour yellow-brown, the
+movements are cautious and slow.]
+
+[Illustration: 1. CHIMPANZEE
+
+2. BABY ORANG-UTAN
+
+3. ORANG-UTAN
+
+4. BABY CHIMPANZEES
+
+_Photos: James's Press Agency._
+
+In his famous book on _The Expression of the Emotions in Man and
+Animals_ (1872) Charles Darwin showed that many forms of facial
+expression familiar in man have their counterparts in apes and other
+mammals. He also showed how important the movements of expression are as
+means of communication between mother and offspring, mate and mate, kith
+and kin.
+
+The anthropoid apes show notable differences of temperament as the
+photographs show. The chimpanzee is lively, cheerful, and educable. The
+orang is also mild of temper, but often and naturally appears melancholy
+in captivity. This is not suggested, however, by our photograph of the
+adult. Both chimpanzee and orang are markedly contrasted with the fierce
+and gloomy gorilla.]
+
+
+Activity for Activity's Sake
+
+Professor Thorndike hits the nail on the head when he lays stress on the
+intensity of activity in monkeys--activity both of body and mind. They
+are pent-up reservoirs of energy, which almost any influence will tap.
+Watch a cat or a dog, Professor Thorndike says; it does comparatively
+few things and is content for long periods to do nothing. It will be
+splendidly active in response to some stimulus such as food or a friend
+or a fight, but if nothing appeals to its special make-up, which is very
+utilitarian in its interests, it will do nothing. "Watch a monkey and
+you cannot enumerate the things he does, cannot discover the stimuli to
+which he reacts, cannot conceive the _raison d'etre_ of his pursuits.
+Everything appeals to him. He likes to be active for the sake of
+activity."
+
+This applies to mental activity as well, and the quality is one of
+extraordinary interest, for it shows the experimenting mood at a higher
+turn of the spiral than in any other creature, save man. It points
+forward to the scientific spirit. We cannot, indeed, believe in the
+sudden beginning of any quality, and we recall the experimenting of
+playing mammals, such as kids and kittens, or of inquisitive adults like
+Kipling's mongoose, Riki-Tiki-Tavi, which made it his business in life
+to find out about things. But in monkeys the habit of restless
+experimenting rises to a higher pitch. They appear to be curious about
+the world. The psychologist whom we have quoted tells of a monkey which
+happened to hit a projecting wire so as to make it vibrate. He went on
+repeating the performance hundreds of times during the next few days. Of
+course, he got nothing out of it, save fun, but it was grist to his
+mental mill. "The fact of mental life is to monkeys it own reward." The
+monkey's brain is "tender all over, functioning throughout, set off in
+action by anything and everything."
+
+
+Sheer Quickness
+
+Correlated with the quality of restless inquisitiveness and delight in
+activity for its own sake there is the quality of quickness. We mean not
+merely the locomotor agility that marks most monkeys, but quickness of
+perception and plan. It is the sort of quality that life among the
+branches will engender, where it is so often a case of neck or nothing.
+It is the quality which we describe as being on the spot, though the
+phrase has slipped from its original moorings. Speaking of his Bonnet
+Monkey, an Indian macaque, second cousin to the kind that lives on the
+Rock of Gibraltar, Professor S. J. Holmes writes: "For keenness of
+perception, rapidity of action, facility in forming good practical
+judgments about ways and means of escaping pursuit and of attaining
+various other ends, Lizzie had few rivals in the animal world.... Her
+perceptions and decisions were so much more rapid than my own that she
+would frequently transfer her attention, decide upon a line of action,
+and carry it into effect before I was aware of what she was about. Until
+I came to guard against her nimble and unexpected manoeuvres, she
+succeeded in getting possession of many apples and peanuts which I had
+not intended to give her except upon the successful performance of some
+task."
+
+
+Quick to Learn
+
+Quite fundamental to any understanding of animal behaviour is the
+distinction so clearly drawn by Sir Ray Lankester between the
+"little-brain" type, rich in inborn or instinctive capacities, but
+relatively slow to learn, and the "big-brain" type, with a relatively
+poor endowment of specialised instincts, but with great educability. The
+"little-brain" type finds its climax in ants and bees; the "big-brain"
+type in horses and dogs, elephants and monkeys. And of all animals
+monkeys are the quickest to learn, if we use the word "learn" to mean
+the formation of useful associations between this and that, between a
+given sense-presentation and a particular piece of behaviour.
+
+
+The Case of Sally
+
+Some of us remember Sally, the chimpanzee at the "Zoo" with which Dr.
+Romanes used to experiment. She was taught to give her teacher the
+number of straws he asked for, and she soon learned to do so up to five.
+If she handed a number not asked for, her offer was refused; if she gave
+the proper number, she got a piece of fruit. If she was asked for five
+straws, she picked them up individually and placed them in her mouth,
+and when she had gathered five she presented them together in her hand.
+Attempts to teach her to give six to ten straws were not very
+successful. For Sally "above six" meant "many," and besides, her limits
+of patience were probably less than her range of computation. This was
+hinted at by the highly interesting circumstance that when dealing with
+numbers above five she very frequently doubled over a straw so as to
+make it present two ends and thus appear as two straws. The doubling of
+the straw looked like an intelligent device to save time, and it was
+persistently resorted to in spite of the fact that her teacher always
+refused to accept a doubled straw as equivalent to two straws. Here we
+get a glimpse of something beyond the mere association of a
+sound--"Five"--and that number of straws.
+
+
+The Case of Lizzie
+
+The front of the cage in which Professor Holmes kept Lizzie was made of
+vertical bars which allowed her to reach out with her arm. On a board
+with an upright nail as handle, there was placed an apple--out of
+Lizzie's reach. She reached immediately for the nail, pulled the board
+in and got the apple. "There was no employment of the method of trial
+and error; there was direct appropriate action following the perception
+of her relation to board, nail, and apple." Of course her ancestors may
+have been adepts at drawing a fruit-laden branch within their reach, but
+the simple experiment was very instructive. All the more instructive
+because in many other cases the experiments indicate a gradual sifting
+out of useless movements and an eventful retention of the one that pays.
+When Lizzie was given a vaseline bottle containing a peanut and closed
+with a cork, she at once pulled the cork out with her teeth, obeying the
+instinct to bite at new objects, but she never learned to turn the
+bottle upside down and let the nut drop out. She often got the nut, and
+after some education she got it more quickly than she did at first, but
+there was no indication that she ever perceived the fit and proper way
+of getting what she wanted. "In the course of her intent efforts her
+mind seemed so absorbed with the object of desire that it was never
+focussed on the means of attaining that object. There was no
+deliberation, and no discrimination between the important and the
+unimportant elements in her behaviour. The gradually increasing facility
+of her performances depended on the apparently unconscious elimination
+of useless movements." This may be called learning, but it is learning
+at a very low level; it is far from learning by ideas; it is hardly even
+learning by experiment; it is not more than learning by experience, it
+is not more than fumbling at learning!
+
+
+Trial and Error
+
+A higher note is struck in the behaviour of some more highly endowed
+monkeys. In many experiments, chiefly in the way of getting into boxes
+difficult to open, there is evidence (1) of attentive persistent
+experiment (2) of the rapid elimination of ineffective movements, and
+(3) of remembering the solution when it was discovered. Kinnaman taught
+two macaques the Hampton Court Maze, a feat which probably means a
+memory of movements, and we get an interesting glimpse in his
+observation that they began to smack their lips audibly when they
+reached the latter part of their course, and began to feel, dare one
+say, "We are right this time."
+
+In getting into "puzzle-boxes" and into "combination-boxes" (where the
+barriers must be overcome in a definite order), monkeys learn by the
+trial and error method much more quickly than cats and dogs do, and a
+very suggestive fact emphasized by Professor Thorndike is "a process of
+sudden acquisition by a rapid, often apparently instantaneous
+abandonment of the unsuccessful movements and selection of the
+appropriate one, which rivals in suddenness the selections made by human
+beings in similar performances." A higher note still was sounded by one
+of Thorndike's monkeys which opened a puzzle-box at once, eight months
+after his previous experience with it. For here was some sort of
+registration of a solution.
+
+
+Imitation
+
+Two chimpanzees in the Dublin Zoo were often to be seen washing the two
+shelves of their cupboard and "wringing" the wet cloth in the approved
+fashion. It was like a caricature of a washerwoman, and someone said,
+"What mimics they are!" Now we do not know whether that was or was not
+the case with the chimpanzees, but the majority of the experiments that
+have been made do not lead us to attach to imitation so much importance
+as is usually given to it by the popular interpreter. There are
+instances where a monkey that had given up a puzzle in despair returned
+to it when it had seen its neighbour succeed, but most of the
+experiments suggested that the creature has to find out for itself. Even
+with such a simple problem as drawing food near with a stick, it often
+seems of little use to show the monkey how it is done. Placing a bit of
+food outside his monkey's cage, Professor Holmes "poked it about with
+the stick so as to give her a suggestion of how the stick might be
+employed to move the food within reach, but although the act was
+repeated many times Lizzie never showed the least inclination to use the
+stick to her advantage." Perhaps the idea of a "tool" is beyond the
+Bonnet Monkey, yet here again we must be cautious, for Professor L. T.
+Hobhouse had a monkey of the same macaque genus which learned in the
+course of time to use a crooked stick with great effect.
+
+
+The Case of Peter
+
+Perhaps the cleverest monkey as yet studied was a performing chimpanzee
+called Peter, which has been generally described by Dr. Lightner Witmer.
+Peter could skate and cycle, thread needles and untie knots, smoke a
+cigarette and string beads, screw in nails and unlock locks. But what
+Peter was thinking about all the time it was hard to guess, and there is
+very little evidence to suggest that his rapid power of putting two and
+two together ever rose above a sort of concrete mental experimenting,
+which Dr. Romanes used to call perceptual inference. Without supposing
+that there are hard-and-fast boundary lines, we cannot avoid the general
+conclusion that, while monkeys are often intelligent, they seldom, if
+ever, show even hints of reason, i.e. of working or playing with general
+ideas. That remains Man's prerogative.
+
+
+The Bustle of the Mind
+
+In mammals like otters, foxes, stoats, hares, and elephants, what a
+complex of tides and currents there must be in the brain-mind! We may
+think of a stream with currents at different levels. Lowest there are
+the _basal appetites_ of hunger and sex, often with eddies rising to the
+surface. Then there are the _primary emotions_, such as fear of
+hereditary enemies and maternal affection for offspring. Above these are
+_instinctive aptitudes_, inborn powers of doing clever things without
+having to learn how. But in mammals these are often expressed along
+with, or as it were through, the controlled life of _intelligent
+activity_, where there is more clear-cut perceptual influence.
+
+[Illustration: _Photo: W. P. Dando._
+
+CHIMPANZEE
+
+An African ape, at home in the equatorial forests, a lively and playful
+creature, eminently educable.]
+
+[Illustration: _Photo: W. S. Berridge._
+
+YOUNG CHEETAHS, OR HUNTING LEOPARDS
+
+Trained to hunt from time immemorial and quite easily tamed. Cheetahs
+occur in India, Persia, Turkestan, and Africa.]
+
+[Illustration: _Photo: C. Reid._
+
+COMMON OTTER
+
+One of the most resourceful of animals and the "most playsomest crittur
+on God's earth." It neither stores nor hibernates, but survives in
+virtue of its wits and because of the careful education of the young.
+The otter is a roving animal, often with more than one resting-place; it
+has been known to travel fifteen miles in a night.]
+
+Higher still are the records or memories of individual experience and
+the registration of individual habits, while on the surface is the
+instreaming multitude of messages from the outside world, like raindrops
+and hailstones on the stream, some of them penetrating deeply, being, as
+we say, full of meaning. The mind of the higher animal is in some
+respects like a child's mind, in having little in the way of clear-cut
+ideas, in showing no reason in the strict sense, and in its
+extraordinary educability, but it differs from the child's mind entirely
+in the sure effectiveness of a certain repertory of responses. It is
+efficient to a degree.
+
+
+"Until at last arose the Man."
+
+Man's brain is more complicated than that of the higher apes--gorilla,
+orang, and chimpanzee--and it is relatively larger. But the improvements
+in structure do not seem in themselves sufficient to account for man's
+great advance in intelligence. The rill of inner life has become a swift
+stream, sometimes a rushing torrent. Besides perceptual inference or
+_Intelligence_--a sort of picture-logic, which some animals likewise
+have--there is conceptual inference--or _Reason_--an internal
+experimenting with general ideas. Even the cleverest animals, it would
+seem, do not get much beyond playing with "particulars"; man plays an
+internal game of chess with "universals." Intelligent behaviour may go a
+long way with mental images; rational conduct demands general ideas. It
+may be, however, that "percepts" and "concepts" differ rather in degree
+than in kind, and that the passage from one to the other meant a higher
+power of forming associations. A clever dog has probably a generalised
+percept of man, as distinguished from a memory-image of the particular
+men it has known, but man alone has the concept Man, or Mankind, or
+Humanity. Experimenting with concepts or general ideas is what we call
+Reason.
+
+Here, of course, we get into deep waters, and perhaps it is wisest not
+to attempt too much. So we shall content ourselves here with pointing
+out that Man's advance in intelligence and from intelligence to reason
+is closely wrapped up with his power of speech. What animals began--a
+small vocabulary--he has carried to high perfection. But what is
+distinctive is not the vocabulary so much as the habit of making
+sentences, of expressing judgments in a way which admitted of
+communication between mind and mind. The multiplication of words meant
+much, the use of words as symbols of general ideas meant even more, for
+it meant the possibility of playing the internal game of thinking; but
+perhaps the most important advance of all was the means of comparing
+notes with neighbours, of corroborating individual experience by social
+intercourse. With words, also, it became easier to enregister outside
+himself the gains of the past. It is not without significance that the
+Greek Logos, which may be translated "the word," may also be translated
+Mind.
+
+
+§ 9
+
+Looking Backwards
+
+When we take a survey of animal behaviour we see a long inclined plane.
+The outer world provokes simple creatures to answer back; simple
+creatures act experimentally on their surroundings. From the beginning
+this twofold process has been going on, receiving stimuli from the
+environment and acting upon the environment, and according to the
+efficiency of the reactions and actions living creatures have been
+sifted for millions of years. One main line of advance has been opening
+new gateways of knowledge--the senses, which are far more than five in
+number. The other main line of advance has been in most general terms,
+experimenting or testing, probing and proving, trying one key after
+another till a door is unlocked. There is progress in multiplying the
+gateways of knowledge and making them more discriminating, and there is
+progress in making the modes of experimenting more wide-awake, more
+controlled, and more resolute. But behind both of these is the
+characteristically vital power of enregistering within the organism the
+lessons of the past. In the life of the individual these enregistrations
+are illustrated by memories and habituations and habits; in the life of
+the race they are illustrated by reflex actions and instinctive
+capacities.
+
+
+Body and Mind
+
+We must not shirk the very difficult question of the relation between
+the bodily and the mental side of behaviour.
+
+(_a_) Some great thinkers have taught that the mind is a reality by
+itself which plays upon the instrument of the brain and body. As the
+instrument gets worn and dusty the playing is not so good as it once
+was, but the player is still himself. This theory of the essential
+independence of the mind is a very beautiful one, but those who like it
+when applied to themselves are not always so fond of it when it is
+applied to other intelligent creatures like rooks and elephants. It may
+be, however, that there is a gradual emancipation of the mind which has
+gone furthest in Man and is still progressing.
+
+(_b_) Some other thinkers have taught that the inner life of thought and
+feeling is only, as it were, an echo of the really important
+activity--that of the body and brain. Ideas are just foam-bells on the
+hurrying streams and circling eddies of matter and energy that make up
+our physiological life. To most of us this theory is impossible, because
+we are quite sure that ideas and feelings and purposes, which cannot be
+translated into matter and motion, are the clearest realities in our
+experience, and that they count for good and ill all through our life.
+They are more than the tickings of the clock; they make the wheels go
+round.
+
+(_c_) There are others who think that the most scientific position is
+simply to recognise both the bodily and the mental activities as equally
+important, and so closely interwoven that they cannot be separated.
+Perhaps they are just the outer and the inner aspects of one
+reality--the life of the creature. Perhaps they are like the concave and
+convex curves of a dome, like the two sides of a shield. Perhaps the
+life of the organism is always a unity, at one time appearing more
+conspicuously as Mind-body, at another time as Body-mind. The most
+important fact is that neither aspect can be left out. By no jugglery
+with words can we get Mind out of Matter and Motion. And since we are in
+ourselves quite sure of our Mind, we are probably safe in saying that in
+the beginning was Mind. This is in accordance with Aristotle's saying
+that there is nothing in the end which was not also in kind present in
+the beginning--whatever we mean by beginning.
+
+
+In conclusion
+
+What has led to the truly wonderful result which we admire in a creature
+like a dog or an otter, a horse or a hare? In general, we may say, just
+two main processes--(1) testing all things, and (2) holding fast that
+which is good. New departures occur and these are tested for what they
+are worth. Idiosyncrasies crop up and they are sifted. New cards come
+mysteriously from within into the creature's hand, and they are
+played--for better or for worse. So by new variations and their sifting,
+by experimenting and enregistering the results, the mind has gradually
+evolved and will continue to evolve.
+
+
+
+
+VIII
+
+FOUNDATIONS OF THE UNIVERSE
+
+
+
+
+THE WORLD OF ATOMS
+
+
+Most people have heard of the oriental race which puzzled over the
+foundations of the universe, and decided that it must be supported on
+the back of a giant elephant. But the elephant? They put it on the back
+of a monstrous tortoise, and there they let the matter end. If every
+animal in nature had been called upon, they would have been no nearer a
+foundation. Most ancient peoples, indeed, made no effort to find a
+foundation. The universe was a very compact little structure, mainly
+composed of the earth and the great canopy over the earth which they
+called the sky. They left it, as a whole, floating in nothing. And in
+this the ancients were wiser than they knew. Things do not fall down
+unless they are pulled down by that mysterious force which we call
+gravitation. The earth, it is true, is pulled by the sun, and would fall
+into it; but the earth escapes this fiery fate by circulating at great
+speed round the sun. The stars pull each other; but it has already been
+explained that they meet this by travelling rapidly in gigantic orbits.
+Yet we do, in a new sense of the word, need foundations of the universe.
+Our mind craves for some explanation of the matter out of which the
+universe is made. For this explanation we turn to modern Physics and
+Chemistry. Both these sciences study, under different aspects, matter
+and energy; and between them they have put together a conception of the
+fundamental nature of things which marks an epoch in the history of
+human thought.
+
+
+§ 1
+
+The Bricks of the Cosmos
+
+More than two thousand years ago the first men of science, the Greeks of
+the cities of Asia Minor, speculated on the nature of matter. You can
+grind a piece of stone into dust. You can divide a spoonful of water
+into as many drops as you like. Apparently you can go on dividing as
+long as you have got apparatus fine enough for the work. But there must
+be a limit, these Greeks said, and so they supposed that all matter was
+ultimately composed of minute particles which were indivisible. That is
+the meaning of the Greek word "atom."
+
+Like so many other ideas of these brilliant early Greek thinkers, the
+atom was a sound conception. We know to-day that matter is composed of
+atoms. But science was then so young that the way in which the Greeks
+applied the idea was not very profound. A liquid or a gas, they said,
+consisted of round, smooth atoms, which would not cling together. Then
+there were atoms with rough surfaces, "hooky" surfaces, and these stuck
+together and formed solids. The atoms of iron or marble, for instance,
+were so very hooky that, once they got together, a strong man could not
+tear them apart. The Greeks thought that the explanation of the universe
+was that an infinite number of these atoms had been moving and mixing in
+an infinite space during an infinite time, and had at last hit by chance
+on the particular combination which is our universe.
+
+This was too simple and superficial. The idea of atoms was cast aside,
+only to be advanced again in various ways. It was the famous Manchester
+chemist, John Dalton, who restored it in the early years of the
+nineteenth century. He first definitely formulated the atomic theory as
+a scientific hypothesis. The whole physical and chemical science of that
+century was now based upon the atom, and it is quite a mistake to
+suppose that recent discoveries have discredited "atomism." An atom is
+the smallest particle of a chemical element. No one has ever seen an
+atom. Even the wonderful new microscope which has just been invented
+cannot possibly show us particles of matter which are a million times
+smaller than the breadth of a hair; for that is the size of atoms. We
+can weigh them and measure them, though they are invisible, and we know
+that all matter is composed of them. It is a new discovery that atoms
+are not indivisible. They consist themselves of still smaller particles,
+as we shall see. But the atoms exist all the same, and we may still say
+that they are the bricks of which the material universe is built.
+
+[Illustration: _Photo: Elliott & Fry._
+
+SIR ERNEST RUTHERFORD
+
+One of our most eminent physicists who has succeeded Sir J. J. Thomson
+as Cavendish Professor of Physics at the University of Cambridge. The
+modern theory of the structure of the atom is largely due to him.]
+
+[Illustration: _Photo: Rischgitz Collection._
+
+J. CLERK-MAXWELL
+
+One of the greatest scientific men who have ever lived. He
+revolutionised physics with his electro-magnetic theory of light, and
+practically all modern researches have had their origin, direct or
+indirect, in his work. Together with Faraday he constitutes one of the
+main scientific glories of the nineteenth century.]
+
+[Illustration: _Photo: Ernest H. Mills._
+
+SIR WILLIAM CROOKES
+
+Sir William Crookes experimented on the electric discharge in vacuum
+tubes and described the phenomena as a "fourth state of matter." He was
+actually observing the flight of electrons, but he did not fully
+appreciate the nature of his experiments.]
+
+[Illustration: _Photo: Photo Press_
+
+PROFESSOR SIR W. H. BRAGG
+
+One of the most distinguished physicists of the present day.]
+
+But if we had some magical glass by means of which we could see into the
+structure of material things, we should not see the atoms put evenly
+together as bricks are in a wall. As a rule, two or more atoms first
+come together to form a larger particle, which we call a "molecule."
+Single atoms do not, as a rule, exist apart from other atoms; if a
+molecule is broken up, the individual atoms seek to unite with other
+atoms of another kind or amongst themselves. For example, three atoms of
+oxygen form what we call ozone; two atoms of hydrogen uniting with one
+atom of oxygen form water. It is molecules that form the mass of matter;
+a molecule, as it has been expressed, is a little building of which
+atoms are the bricks.
+
+In this way we get a useful first view of the material things we handle.
+In a liquid the molecules of the liquid cling together loosely. They
+remain together as a body, but they roll over and away from each other.
+There is "cohesion" between them, but it is less powerful than in a
+solid. Put some water in a kettle over the lighted gas, and presently
+the tiny molecules of water will rush through the spout in a cloud of
+steam and scatter over the kitchen. The heat has broken their bond of
+association and turned the water into something like a gas; though we
+know that the particles will come together again, as they cool, and form
+once more drops of water.
+
+In a gas the molecules have full individual liberty. They are in a
+state of violent movement, and they form no union with each other. If we
+want to force them to enter into the loose sort of association which
+molecules have in a liquid, we have to slow down their individual
+movements by applying severe cold. That is how a modern man of science
+liquefies gases. No power that we have will liquefy air at its ordinary
+temperature. In _very_ severe cold, on the other hand, the air will
+spontaneously become liquid. Some day, when the fires of the sun have
+sunk very low, the temperature of the earth will be less than -200° C.:
+that is to say, more than two hundred degrees Centigrade below
+freezing-point. It will sink to the temperature of the moon. Our
+atmosphere will then be an ocean of liquid air, 35 feet deep, lying upon
+the solidly frozen masses of our water-oceans.
+
+In a solid the molecules cling firmly to each other. We need a force
+equal to twenty-five tons to tear asunder the molecules in a bar of iron
+an inch thick. Yet the structure is not "solid" in the popular sense of
+the word. If you put a piece of solid gold in a little pool of mercury,
+the gold will take in the mercury _between_ its molecules, as if it were
+porous like a sponge. The hardest solid is more like a lattice-work than
+what we usually mean by "solid"; though the molecules are not fixed,
+like the bars of a lattice-work, but are in violent motion; they vibrate
+about equilibrium positions. If we could see right into the heart of a
+bit of the hardest steel, we should see billions of separate molecules,
+at some distance from each other, all moving rapidly to and fro.
+
+This molecular movement can, in a measure, be made visible. It was
+noticed by a microscopist named Brown that, in a solution containing
+very fine suspended particles, the particles were in constant movement.
+Under a powerful microscope these particles are seen to be violently
+agitated; they are each independently darting hither and thither
+somewhat like a lot of billiard balls on a billiard table, colliding and
+bounding about in all directions. Thousands of times a second these
+encounters occur, and this lively commotion is always going on, this
+incessant colliding of one molecule with another is the normal
+condition of affairs; not one of them is at rest. The reason for this
+has been worked out, and it is now known that these particles move about
+because they are being incessantly bombarded by the molecules of the
+liquid. The molecules cannot, of course, be seen, but the fact of their
+incessant movement is revealed to the eye by the behaviour of the
+visible suspended particles. This incessant movement in the world of
+molecules is called the Brownian movement, and is a striking proof of
+the reality of molecular motions.
+
+
+§ 2
+
+The Wonder-World of Atoms
+
+The exploration of this wonder-world of atoms and molecules by the
+physicists and chemists of to-day is one of the most impressive triumphs
+of modern science. Quite apart from radium and electrons and other
+sensational discoveries of recent years, the study of ordinary matter is
+hardly inferior, either in interest or audacity, to the work of the
+astronomer. And there is the same foundation in both cases--marvellous
+apparatus, and trains of mathematical reasoning that would have
+astonished Euclid or Archimedes. Extraordinary, therefore, as are some
+of the facts and figures we are now going to give in connection with the
+minuteness of atoms and molecules, let us bear in mind that we owe them
+to the most solid and severe processes of human thought.
+
+Yet the principle can in most cases be made so clear that the reader
+will not be asked to take much on trust. It is, for instance, a matter
+of common knowledge that gold is soft enough to be beaten into gold
+leaf. It is a matter of common sense, one hopes, that if you beat a
+measured cube of gold into a leaf six inches square, the mathematician
+can tell the thickness of that leaf without measuring it. As a matter of
+fact, a single grain of gold has been beaten into a leaf seventy-five
+inches square. Now the mathematician can easily find that when a single
+grain of gold is beaten out to that size, the leaf must be 1/367,000 of
+an inch thick, or about a thousand times thinner than the paper on
+which these words are printed; yet the leaf must be several molecules
+thick.
+
+The finest gold leaf is, in fact, too thick for our purpose, and we turn
+with a new interest to that toy of our boyhood the soap-bubble. If you
+carefully examine one of these delicate films of soapy water, you notice
+certain dark spots or patches on them. These are their thinnest parts,
+and by two quite independent methods--one using electricity and the
+other light--we have found that at these spots the bubble is less than
+the three-millionth of an inch thick! But the molecules in the film
+cling together so firmly that they must be at least twenty or thirty
+deep in the thinnest part. A molecule, therefore, must be far less than
+the three-millionth of an inch thick.
+
+We found next that a film of oil on the surface of water may be even
+thinner than a soap-bubble. Professor Perrin, the great French authority
+on atoms, got films of oil down to the fifty-millionth of an inch in
+thickness! He poured a measured drop of oil upon water. Then he found
+the exact limits of the area of the oil-sheet by blowing upon the water
+a fine powder which spread to the edge of the film and clearly outlined
+it. The rest is safe and simple calculation, as in the case of the
+beaten grain of gold. Now this film of oil must have been at least two
+molecules deep, so a single molecule of oil is considerably less than a
+hundred-millionth of an inch in diameter.
+
+Innumerable methods have been tried, and the result is always the same.
+A single grain of indigo, for instance, will colour a ton of water. This
+obviously means that the grain contains billions of molecules which
+spread through the water. A grain of musk will scent a room--pour
+molecules into every part of it--for several years, yet not lose
+one-millionth of its mass in a year. There are a hundred ways of showing
+the minuteness of the ultimate particles of matter, and some of these
+enable us to give definite figures. On a careful comparison of the best
+methods we can say that the average molecule of matter is less than
+the 1/125,000,000 of an inch in diameter. In a single cubic centimetre
+of air--a globule about the size of a small marble--there are thirty
+million trillion molecules. And since the molecule is, as we saw, a
+group or cluster of atoms, the atom itself is smaller. Atoms, for
+reasons which we shall see later, differ very greatly from each other in
+size and weight. It is enough to say that some of them are so small that
+it would take 400,000,000 of them, in a line, to cover an inch of space;
+and that it takes at least a quintillion atoms of gold to weigh a single
+gramme. Five million atoms of helium could be placed in a line across
+the diameter of a full stop.
+
+[Illustration: An atom is the smallest particle of a chemical element.
+Two or more atoms come together to form a molecule: thus molecules form
+the mass of matter. A molecule of water is made up of two atoms of
+hydrogen and one atom of oxygen. Molecules of different substances,
+therefore, are of different sizes according to the number and kind of
+the particular atoms of which they are composed. A starch molecule
+contains no less than 25,000 atoms.
+
+Molecules, of course, are invisible. The above diagram illustrates the
+_comparative_ sizes of molecules.]
+
+[Illustration: INCONCEIVABLE NUMBERS AND INCONCEIVABLY SMALL PARTICLES
+
+The molecules, which are inconceivably small, are, on the other hand, so
+numerous that if one was able to place, end to end, all those contained
+in, for example, a cubic centimetre of gas (less than a fifteenth of a
+cubic inch), one would obtain a line capable of passing two hundred
+times round the earth.]
+
+[Illustration: WHAT IS A MILLION?
+
+In dealing with the infinitely small, it is difficult to apprehend the
+vast figures with which scientists confront us. A million is one
+thousand thousand. We may realise what this implies if we consider that
+a clock, beating seconds, takes approximately 278 hours (i.e. one week
+four days fourteen hours) to tick one million times. A billion is one
+million million. To tick a billion the clock would tick for over 31,735
+years.
+
+(In France and America a thousand millions is called a billion.)]
+
+[Illustration: THE BROWNIAN MOVEMENT
+
+A diagram, constructed from actual observations, showing the erratic
+paths pursued by very fine particles suspended in a liquid, when
+bombarded by the molecules of the liquid. This movement is called the
+Brownian movement, and it furnishes a striking illustration of the truth
+of the theory that the molecules of a body are in a state of continual
+motion.]
+
+
+The Energy of Atoms
+
+And this is only the beginning of the wonders that were done with
+"ordinary matter," quite apart from radium and its revelations, to which
+we will come presently. Most people have heard of "atomic energy," and
+the extraordinary things that might be accomplished if we could harness
+this energy and turn it to human use. A deeper and more wonderful source
+of this energy has been discovered in the last twenty years, but it is
+well to realise that the atoms themselves have stupendous energy. The
+atoms of matter are vibrating or gyrating with extraordinary vigour. The
+piece of cold iron you hold in your hand, the bit of brick you pick up,
+or the penny you take from your pocket is a colossal reservoir of
+energy, since it consists of trillions of moving atoms. To realise the
+total energy, of course, we should have to witness a transformation such
+as we do in atoms of radio-active elements, about which we shall have
+something to say presently.
+
+If we put a grain of indigo in a glass of water, or a grain of musk in a
+perfectly still room, we soon realise that molecules travel. Similarly,
+the fact that gases spread until they fill every "empty" available space
+shows definitely that they consist of small particles travelling at
+great speed. The physicist brings his refined methods to bear on these
+things, and he measures the energy and velocity of these infinitely
+minute molecules. He tells us that molecules of oxygen, at the
+temperature of melting ice, travel at the rate of about 500 yards a
+second--more than a quarter of a mile a second. Molecules of hydrogen
+travel at four times that speed, or three times the speed with which a
+bullet leaves a rifle. Each molecule of the air, which seems so still in
+the house on a summer's day, is really travelling faster than a rifle
+bullet does at the beginning of its journey. It collides with another
+molecule every twenty-thousandth of an inch of its journey. It is turned
+from its course 5,000,000,000 times in every second by collisions. If we
+could stop the molecules of hydrogen gas, and utilise their energy, as
+we utilise the energy of steam or the energy of the water at Niagara, we
+should find enough in every gramme of gas (about two-thousandths of a
+pound) to raise a third of a ton to a height of forty inches.
+
+I have used for comparison the speed of a rifle bullet, and in an
+earlier generation people would have thought it impossible even to
+estimate this. It is, of course, easy. We put two screens in the path of
+the bullet, one near the rifle and the other some distance away. We
+connect them electrically and use a fine time-recording machine, and the
+bullet itself registers the time it takes to travel from the first to
+the second screen.
+
+Now this is very simple and superficial work in comparison with the
+system of exact and minute measurements which the physicist and chemist
+use. In one of his interesting works Mr. Charles R. Gibson gives a
+photograph of two exactly equal pieces of paper in the opposite pans of
+a fine balance. A single word has been written in pencil on one of these
+papers, and that little scraping of lead has been enough to bring down
+the scale! The spectroscope will detect a quantity of matter four
+million times smaller even than this; and the electroscope is a million
+times still more sensitive than the spectroscope. We have a
+heat-measuring instrument, the bolometer, which makes the best
+thermometer seem Early Victorian. It records the millionth of a degree
+of temperature. It is such instruments, multiplied by the score,
+which enable us to do the fine work recorded in these pages.
+
+[Illustration: _Reproduced from "The Forces of Nature" (Messrs.
+Macmillan)._
+
+A SOAP BUBBLE
+
+The iridescent colours sometimes seen on a soap bubble, as in the
+illustration, may also be seen in very fine sections of crystals, in
+glass blown into extremely fine bulbs, on the wings of dragon-flies and
+the surface of oily water. The different colours correspond to different
+thicknesses of the surface. Part of the light which strikes these thin
+coatings is reflected from the upper surface, but another part of the
+light penetrates the transparent coating and is reflected from the lower
+surface. It is the mixture of these two reflected rays, their
+"interference" as it is called, which produces the colours observed. The
+"black spots" on a soap bubble are the places where the soapy film is
+thinnest. At the black spots the thickness of the bubble is about the
+three-millionth part of an inch. If the whole bubble were as thin as
+this it would be completely invisible.]
+
+
+§ 3
+
+THE DISCOVERY OF X-RAYS AND RADIUM
+
+The Discovery of Sir Wm. Crookes
+
+But these wonders of the atom are only a prelude to the more romantic
+and far-reaching discoveries of the new physics--the wonders of the
+electron. Another and the most important phase of our exploration of the
+material universe opened with the discovery of radium in 1898.
+
+In the discovery of radio-active elements, a new property of matter was
+discovered. What followed on the discovery of radium and of the X-rays
+we shall see.
+
+As Sir Ernest Rutherford, one of our greatest authorities, recently
+said, the new physics has dissipated the last doubt about the reality of
+atoms and molecules. The closer examination of matter which we have been
+able to make shows positively that it is composed of atoms. But we must
+not take the word now in its original Greek meaning (an "indivisible"
+thing). The atoms are not indivisible. They can be broken up. They are
+composed of still smaller particles.
+
+The discovery that the atom was composed of smaller particles was the
+welcome realisation of a dream that had haunted the imagination of the
+nineteenth century. Chemists said that there were about eighty different
+kinds of atoms--different kinds of matter--but no one was satisfied with
+the multiplicity. Science is always aiming at simplicity and unity. It
+may be that science has now taken a long step in the direction of
+explaining the fundamental unity of all the matter. The chemist was
+unable to break up these "elements" into something simpler, so he called
+their atoms "indivisible" in that sense. But one man of science after
+another expressed the hope that we would yet discover some fundamental
+matter of which the various atoms were composed--_one primordial
+substance from which all the varying forms of matter have been evolved
+or built up_. Prout suggested this at the very beginning of the century,
+when atoms were rediscovered by Dalton. Father Secchi, the famous Jesuit
+astronomer said that all the atoms were probably evolved from ether; and
+this was a very favoured speculation. Sir William Crookes talked of
+"prothyl" as the fundamental substance. Others thought hydrogen was the
+stuff out of which all the other atoms were composed.
+
+The work which finally resulted in the discovery of radium began with
+some beautiful experiments of Professor (later Sir William) Crookes in
+the eighties.
+
+It had been noticed in 1869 that a strange colouring was caused when an
+electric charge was sent through a vacuum tube--the walls of the glass
+tube began to glow with a greenish phosphorescence. A vacuum tube is one
+from which nearly all the air has been pumped, although we can never
+completely empty the tube. Crookes used such ingenious methods that he
+reduced the gas in his tubes until it was twenty million times thinner
+than the atmosphere. He then sent an electric discharge through, and got
+very remarkable results. The negative pole of the electric current (the
+"cathode") _gave off rays which faintly lit the molecules of the thin
+gas in the tube_, and caused a pretty fluorescence on the glass walls of
+the tube. What were these Rays? Crookes at first thought they
+corresponded to a "new or fourth state of matter." Hitherto we had only
+been familiar with matter in the three conditions of solid, liquid, and
+gaseous.
+
+Now Crookes really had the great secret under his eyes. But about twenty
+years elapsed before the true nature of these rays was finally and
+independently established by various experiments. The experiments proved
+"that the rays consisted of a stream of negatively charged particles
+travelling with enormous velocities from 10,000 to 100,000 miles a
+second. In addition, it was found that the mass of each particle was
+exceedingly small, about 1/1800 of the mass of a hydrogen atom, the
+lightest atom known to science." _These particles or electrons, as they
+are now called, were being liberated from the atom._ The atoms of matter
+were breaking down in Crookes tubes. At that time, however, it was
+premature to think of such a thing, and Crookes preferred to say that
+the particles of the gas were electrified and hurled against the walls
+of the tube. He said that it was ordinary matter in a new
+state--"radiant matter." Another distinguished man of science, Lenard,
+found that, when he fitted a little plate of aluminum in the glass wall
+of the tube, the mysterious rays passed through this as if it were a
+window. They must be waves in the ether, he said.
+
+[Illustration: _From "Scientific Ideas of To-day_."
+
+DETECTING A SMALL QUANTITY OF MATTER
+
+In the left-hand photograph the two pieces of paper exactly balance. The
+balance used is very sensitive, and when the single word "atoms" has
+been written with a lead pencil upon one of the papers the additional
+weight is sufficient to depress one of the pans as shown in the second
+photograph. The spectroscope will detect less than one-millionth of the
+matter contained in the word pencilled above.]
+
+[Illustration: _Reproduced by permission of X-Rays Ltd._
+
+THIS X-RAY PHOTOGRAPH IS THAT OF A HAND OF A SOLDIER WOUNDED IN THE
+GREAT WAR
+
+Note the pieces of shrapnel which are revealed.]
+
+[Illustration: _Photo: National Physical Laboratory._
+
+AN X-RAY PHOTOGRAPH OF A GOLF BALL, REVEALING AN IMPERFECT CORE]
+
+[Illustration: _Reproduced by permission of X-Rays Ltd._
+
+A WONDERFUL X-RAY PHOTOGRAPH
+
+Note the fine details revealed, down to the metal tags of the bootlace
+and the nails in the heel of the boot.]
+
+
+§ 4
+
+The Discovery of X-rays
+
+So the story went on from year to year. We shall see in a moment to what
+it led. Meanwhile the next great step was when, in 1895, Röntgen
+discovered the X-rays, which are now known to everybody. He was
+following up the work of Lenard, and he one day covered a "Crookes tube"
+with some black stuff. To his astonishment a prepared chemical screen
+which was near the tube began to glow. _The rays had gone through the
+black stuff; and on further experiment he found that they would go
+through stone, living flesh, and all sorts of "opaque" substances._ In a
+short time the world was astonished to learn that we could photograph
+the skeleton in a living man's body, locate a penny in the interior of a
+child that had swallowed one, or take an impression of a coin through a
+slab of stone.
+
+And what are these X-rays? They are not a form of matter; they are not
+material particles. X-rays were found to be a new variety of _light_
+with a remarkable power of penetration. We have seen what the
+spectroscope reveals about the varying nature of light wave-lengths.
+Light-waves are set up by vibrations in ether,[2] and, as we shall see,
+these ether disturbances are all of the same kind; they only differ as
+regards wave-lengths. The X-rays which Röntgen discovered, then, are
+light, but a variety of light previously unknown to us; they are ether
+waves of very short length. X-rays have proved of great value in many
+directions, as all the world knows, but that we need not discuss at this
+point. Let us see what followed Röntgen's discovery.
+
+ [2] We refer throughout to the "ether" because, although modern
+ theories dispense largely with this conception, the theories of
+ physics are so inextricably interwoven with it that it is necessary,
+ in an elementary exposition, to assume its existence. The modern
+ view will be explained later in the article on Einstein's Theory.
+
+While the world wondered at these marvels, the men of science were
+eagerly following up the new clue to the mystery of matter which was
+exercising the mind of Crookes and other investigators. In 1896
+Becquerel brought us to the threshold of the great discovery.
+
+Certain substances are phosphorescent--they become luminous after they
+have been exposed to sunlight for some time, and Becquerel was trying to
+find if any of these substances give rise to X-rays. One day he chose a
+salt of the metal uranium. He was going to see if, after exposing it to
+sunlight, he could photograph a cross with it through an opaque
+substance. He wrapped it up and laid it aside, to wait for the sun, but
+he found the uranium salt did not wait for the sun. Some strong
+radiation from it went through the opaque covering and made an
+impression of the cross upon the plate underneath. Light or darkness was
+immaterial. The mysterious rays streamed night and day from the salt.
+This was something new. Here was a substance which appeared to be
+producing X-rays; the rays emitted by uranium would penetrate the same
+opaque substances as the X-rays discovered by Röntgen.
+
+
+Discovery of Radium
+
+Now, at the same time as many other investigators, Professor Curie and
+his Polish wife took up the search. They decided to find out whether
+the emission came from the uranium itself or _from something associated
+with it_, and for this purpose they made a chemical analysis of great
+quantities of minerals. They found a certain kind of pitchblende which
+was very active, and they analysed tons of it, concentrating always on
+the radiant element in it. After a time, as they successively worked out
+the non-radiant matter, the stuff began to glow. In the end they
+extracted from eight tons of pitchblende about half a teaspoonful of
+something _that was a million times more radiant than uranium_. There
+was only one name for it--Radium.
+
+That was the starting-point of the new development of physics and
+chemistry. From every laboratory in the world came a cry for radium
+salts (as pure radium was too precious), and hundreds of brilliant
+workers fastened on the new element. The inquiry was broadened, and, as
+year followed year, one substance after another was found to possess the
+power of emitting rays, that is, to be radio-active. We know to-day that
+nearly every form of matter can be stimulated to radio-activity; which,
+as we shall see, means that _its atoms break up into smaller and
+wonderfully energetic particles which we call "electrons."_ This
+discovery of electrons has brought about a complete change in our ideas
+in many directions.
+
+So, instead of atoms being indivisible, they are actually dividing
+themselves, spontaneously, and giving off throughout the universe tiny
+fragments of their substance. We shall explain presently what was later
+discovered about the electron; meanwhile we can say that every glowing
+metal is pouring out a stream of these electrons. Every arc-lamp is
+discharging them. Every clap of thunder means a shower of them. Every
+star is flooding space with them. We are witnessing the spontaneous
+breaking up of atoms, atoms which had been thought to be indivisible.
+The sun not only pours out streams of electrons from its own atoms, but
+the ultra-violet light which it sends to the earth is one of the most
+powerful agencies for releasing electrons from the surface-atoms of
+matter on the earth. It is fortunate for us that our atmosphere absorbs
+most of this ultra-violet or invisible light of the sun--a kind of light
+which will be explained presently. It has been suggested that, if we
+received the full flood of it from the sun, our metals would
+disintegrate under its influence and this "steel civilisation" of ours
+would be impossible!
+
+But we are here anticipating, we are going beyond radium to the
+wonderful discoveries which were made by the chemists and physicists of
+the world who concentrated upon it. The work of Professor and Mme. Curie
+was merely the final clue to guide the great search. How it was followed
+up, how we penetrated into the very heart of the minute atom and
+discovered new and portentous mines of energy, and how we were able to
+understand, not only matter, but electricity and light, will be told in
+the next chapter.
+
+
+THE DISCOVERY OF THE ELECTRON AND HOW IT EFFECTED A REVOLUTION IN IDEAS
+
+What the discovery of radium implied was only gradually realised. Radium
+captivated the imagination of the world; it was a boon to medicine, but
+to the man of science it was at first a most puzzling and most
+attractive phenomenon. It was felt that some great secret of nature was
+dimly unveiled in its wonderful manifestations, and there now
+concentrated upon it as gifted a body of men--conspicuous amongst them
+Sir J. J. Thomson, Sir Ernest Rutherford, Sir W. Ramsay, and Professor
+Soddy--as any age could boast, with an apparatus of research as far
+beyond that of any other age as the _Aquitania_ is beyond a Roman
+galley. Within five years the secret was fairly mastered. Not only were
+all kinds of matter reduced to a common basis, but the forces of the
+universe were brought into a unity and understood as they had never been
+understood before.
+
+[Illustration: ELECTRIC DISCHARGE IN A VACUUM TUBE
+
+The two ends, marked + and -, of a tube from which nearly all air has
+been exhausted are connected to electric terminals, thus producing an
+electric discharge in the vacuum tube. This discharge travels straight
+along the tube, as in the upper diagram. When a magnetic field is
+applied, however, the rays are deflected, as shown in the lower diagram.
+The similarity of the behaviour of the electric discharge with the
+radium rays (see diagram of deflection of radium rays, _post_) shows
+that the two phenomena may be identified. It was by this means that the
+characteristics of electrons were first discovered.]
+
+[Illustration: THE RELATIVE SIZES OF ATOMS AND ELECTRONS
+
+An atom is far too small to be seen. In a bubble of hydrogen gas no
+larger than the letter "O" there are billions of atoms, whilst an
+electron is more than a thousand times smaller than the smallest atom.
+How their size is ascertained is described in the text. In this diagram
+a bubble of gas is magnified to the size of the world. Adopting this
+scale, _each atom_ in the bubble would then be as large as a tennis
+ball.]
+
+[Illustration: IF AN ATOM WERE MAGNIFIED TO THE SIZE OF ST. PAUL'S
+CATHEDRAL, EACH ELECTRON IN THE ATOM (AS REPRESENTED BY THE CATHEDRAL)
+WOULD THEN BE ABOUT THE SIZE OF A SMALL BULLET]
+
+[Illustration: ELECTRONS STREAMING FROM THE SUN TO THE EARTH
+
+There are strong reasons for supposing that sun-spots are huge
+electronic cyclones. The sun is constantly pouring out vast streams of
+electrons into space. Many of these streams encounter the earth, giving
+rise to various electrical phenomena.]
+
+
+§ 5
+
+The Discovery of the Electron
+
+Physicists did not take long to discover that the radiation from radium
+was very like the radiation in a "Crookes tube." It was quickly
+recognised, moreover, that both in the tube and in radium (and other
+metals) the atoms of matter were somehow breaking down.
+
+However, the first step was to recognise that there were three distinct
+and different rays that were given off by such metals as radium and
+uranium. Sir Ernest Rutherford christened them, after the first three
+letters of the Greek alphabet, the Alpha, the Beta, and Gamma rays. We
+are concerned chiefly with the second group and purpose here to deal
+with that group only.[3]
+
+ [3] The "Alpha rays" were presently recognised as atoms of helium
+ gas, shot out at the rate of 12,000 miles a second.
+
+The "Gamma rays" are _waves_, like the X-rays, not material particles.
+They appear to be a type of X-rays. They possess the remarkable power of
+penetrating opaque substances; they will pass through a foot of solid
+iron, for example.
+
+The "Beta rays," as they were at first called, have proved to be one of
+the most interesting discoveries that science ever made. They proved
+what Crookes had surmised about the radiations he discovered in his
+vacuum tube. But it was _not_ a fourth state of matter that had been
+found, but a new _property_ of matter, a property common to all atoms of
+matter. The Beta rays were later christened Electrons. They are
+particles of disembodied electricity, here spontaneously liberated from
+the atoms of matter: only when the electron was isolated from the atom
+was it recognised for the first time as a separate entity. Electrons,
+therefore, are a constituent of the atoms of matter, and we have
+discovered that they can be released from the atom by a variety of
+agencies. Electrons are to be found everywhere, forming part of every
+atom.
+
+"An electron," Sir William Bragg says, "can only maintain a separate
+existence if it is travelling at an immense rate, from one
+three-hundredth of the velocity of light upwards, that is to say, at
+least 600 _miles a second, or thereabouts_. Otherwise the electron
+sticks to the first atom it meets." These amazing particles may travel
+with the enormous velocity of from 10,000 to more than 100,000 miles a
+second. It was first learned that they are of an electrical nature,
+because they are bent out of their normal path if a magnet is brought
+near them. And this fact led to a further discovery: to one of those
+sensational estimates which the general public is apt to believe to be
+founded on the most abstruse speculations. The physicist set up a little
+chemical screen for the "Beta rays" to hit, and he so arranged his tube
+that only a narrow sheaf of the rays poured on to the screen. He then
+drew this sheaf of rays out of its course with a magnet, and he
+accurately measured the shift of the luminous spot on the screen where
+the rays impinged on it. But when he knows the exact intensity of his
+magnetic field--which he can control as he likes--and the amount of
+deviation it causes, and the mass of the moving particles, he can tell
+the speed of the moving particles which he thus diverts. These particles
+were being hurled out of the atoms of radium, or from the negative pole
+in a vacuum tube, at a speed which, in good conditions, reached nearly
+the velocity of light, i.e. nearly 186,000 miles a second.
+
+Their speed has, of course, been confirmed by numbers of experiments;
+and another series of experiments enabled physicists to determine the
+size of the particles. Only one of these need be described, to give the
+reader an idea how men of science arrived at their more startling
+results.
+
+Fog, as most people know, is thick in our great cities because the
+water-vapour gathers on the particles of dust and smoke that are in the
+atmosphere. This fact was used as the basis of some beautiful
+experiments. Artificial fogs were created in little glass tubes, by
+introducing dust, in various proportions, for supersaturated vapour to
+gather on. In the end it was possible to cause tiny drops of rain, each
+with a particle of dust at its core, to fall upon a silver mirror and be
+counted. It was a method of counting the quite invisible particles of
+dust in the tube; and the method was now successfully applied to the new
+rays. Yet another method was to direct a slender stream of the particles
+upon a chemical screen. The screen glowed under the cannonade of
+particles, and a powerful lens resolved the glow into distinct sparks,
+which could be counted.
+
+In short, a series of the most remarkable and beautiful experiments,
+checked in all the great laboratories of the world, settled the nature
+of these so-called rays. They were streams of particles more than a
+thousand times smaller than the smallest known atom. The mass of each
+particle is, according to the latest and finest measurements 1/1845 of
+that of an atom of hydrogen. The physicist has not been able to find any
+character except electricity in them, and the name "electrons" has been
+generally adopted.
+
+
+The Key to many Mysteries
+
+The Electron is an atom, of disembodied electricity; it occupies an
+exceedingly small volume, and its "mass" is entirely electrical. These
+electrons are the key to half the mysteries of matter. Electrons in
+rapid motion, as we shall see, explain what we mean by an "electric
+current," not so long ago regarded as one of the most mysterious
+manifestations in nature.
+
+"What a wonder, then, have we here!" says Professor R. K. Duncan. "An
+innocent-looking little pinch of salt and yet possessed of special
+properties utterly beyond even the fanciful imaginings of men of past
+time; for nowhere do we find in the records of thought even the hint of
+the possibility of things which we now regard as established fact. This
+pinch of salt projects from its surface bodies [i.e. electrons]
+possessing the inconceivable velocity of over 100,000 miles a second, a
+velocity sufficient to carry them, if unimpeded, five times around the
+earth in a second, and possessing with this velocity, masses a thousand
+times smaller than the smallest atom known to science. Furthermore,
+they are charged with negative electricity; they pass straight through
+bodies considered opaque with a sublime indifference to the properties
+of the body, with the exception of its mere density; they cause bodies
+which they strike to shine out in the dark; they affect a photographic
+plate; they render the air a conductor of electricity; they cause clouds
+in moist air; they cause chemical action and have a peculiar
+physiological action. Who, to-day, shall predict the ultimate service to
+humanity of the beta-rays from radium!"
+
+
+§ 6
+
+THE ELECTRON THEORY, OR THE NEW VIEW OF MATTER
+
+The Structure of the Atom
+
+There is general agreement amongst all chemists, physicists, and
+mathematicians upon the conclusions which we have so far given. We know
+that the atoms of matter are constantly--either spontaneously or under
+stimulation--giving off electrons, or breaking up into electrons; and
+they therefore contain electrons. Thus we have now complete proof of the
+independent existence of atoms and also of electrons.
+
+When, however, the man of science tries to tell us _how_ electrons
+compose atoms, he passes from facts to speculation, and very difficult
+speculation. Take the letter "o" as it is printed on this page. In a
+little bubble of hydrogen gas no larger than that letter there are
+_trillions_ of atoms; and they are not packed together, but are
+circulating as freely as dancers in a ball-room. We are asking the
+physicist to take one of these minute atoms and tell us how the still
+smaller electrons are arranged in it. Naturally he can only make mental
+pictures, guesses or hypotheses, which he tries to fit to the facts, and
+discards when they will _not_ fit.
+
+At present, after nearly twenty years of critical discussion, there are
+two chief theories of the structure of the atom. At first Sir J. J.
+Thomson imagined the electrons circulating in shells (like the layers of
+an onion) round the nucleus of the atom. This did not suit, and Sir E.
+Rutherford and others worked out a theory that the electrons circulated
+round a nucleus rather like the planets of our solar system revolving
+round the central sun. Is there a nucleus, then, round which the
+electrons revolve? The electron, as we saw, is a disembodied atom of
+electricity; we should say, of "negative" electricity. Let us picture
+these electrons all moving round in orbits with great velocity. Now it
+is suggested that there is a nucleus of "positive" electricity
+attracting or pulling the revolving electrons to it, and so forming an
+equilibrium, otherwise the electrons would fly off in all directions.
+This nucleus has been recently named the proton. We have thus two
+electricities in the atom: the positive = the nucleus; the negative =
+the electron. Of recent years Dr. Langmuir has put out a theory that the
+electrons do not _revolve round_ the nucleus, but remain in a state of
+violent agitation of some sort at fixed distances from the nucleus.
+
+[Illustration: PROFESSOR SIR J. J. THOMSON
+
+Experimental discoverer of the electronic constitution of matter, in the
+Cavendish Physical Laboratory, Cambridge. A great investigator, noted
+for the imaginative range of his hypotheses and his fertility in
+experimental devices.]
+
+[Illustration: _From the Smithsonian Report_, 1915.
+
+ELECTRONS PRODUCED BY PASSAGE OF X-RAYS THROUGH AIR
+
+A photograph clearly showing that electrons are definite entities. As
+electrons leave atoms they may traverse matter or pass through the air
+in a straight path The illustration shows the tortuous path of electrons
+resulting from collision with atoms.]
+
+[Illustration: MAGNETIC DEFLECTION OF RADIUM RAYS
+
+The radium rays are made to strike a screen, producing visible spots of
+light. When a magnetic field is applied the rays are seen to be
+deflected, as in the diagram. This can only happen if the rays carry an
+electric charge, and it was by experiments of this kind that we obtained
+our knowledge respecting the electric charges carried by radium rays.]
+
+[Illustration: _Reproduced by permission of "Scientific American."_
+
+PROFESSOR R. A. MILLIKAN'S APPARATUS FOR COUNTING ELECTRONS]
+
+But we will confine ourselves here to the facts, and leave the
+contending theories to scientific men. It is now pretty generally
+accepted that an atom of matter consists of a number of electrons, or
+charges of negative electricity, held together by a charge of positive
+electricity. It is not disputed that these electrons are in a state of
+violent motion or strain, and that therefore a vast energy is locked up
+in the atoms of matter. To that we will return later. Here, rather, we
+will notice another remarkable discovery which helps us to understand
+the nature of matter.
+
+A brilliant young man of science who was killed in the war, Mr. Moseley,
+some years ago showed that, when the atoms of different substances are
+arranged in order of their weight, _they are also arranged in the order
+of increasing complexity of structure_. That is to say, the heavier the
+atom, the more electrons it contains. There is a gradual building up of
+atoms containing more and more electrons from the lightest atom to the
+heaviest. Here it is enough to say that as he took element after
+element, from the lightest (hydrogen) to the heaviest (uranium) he found
+a strangely regular relation between them. If hydrogen were represented
+by the figure one, helium by two, lithium three, and so on up to
+uranium, then uranium should have the figure ninety-two. This makes it
+probable that there are in nature ninety-two elements--we have found
+eighty-seven--and that the number Mr. Moseley found is the number of
+electrons in the atom of each element; that is to say, the number is
+arranged in order of the atomic numbers of the various elements.
+
+
+§ 7
+
+The New View of Matter
+
+Up to the point we have reached, then, we see what the new view of
+Matter is. Every atom of matter, of whatever kind throughout the whole
+universe, is built up of electrons in conjunction with a nucleus. From
+the smallest atom of all--the atom of hydrogen--which consists of one
+electron, rotating round a positively charged nucleus, to a heavy
+complicated atom, such as the atom of gold, constituted of many
+electrons and a complex nucleus, _we have only to do with positive and
+negative units of electricity_. The electron and its nucleus are
+particles of electricity. All Matter, therefore, is nothing but a
+manifestation of electricity. The atoms of matter, as we saw, combine
+and form molecules. Atoms and molecules are the bricks out of which
+nature has built up everything; ourselves, the earth, the stars, the
+whole universe.
+
+But more than bricks are required to build a house. There are other
+fundamental existences, such as the various forms of energy, which give
+rise to several complex problems. And we have also to remember, that
+there are more than eighty distinct elements, each with its own definite
+type of atom. We shall deal with energy later. Meanwhile it remains to
+be said that, although we have discovered a great deal about the
+electron and the constitution of matter, and that while the physicists
+of our own day seem to see a possibility of explaining positive and
+negative electricity, the nature of them both is unknown. There exists
+the theory that the particles of positive and negative electricity,
+which make up the atoms of matter, are points or centres of disturbances
+of some kind in a universal ether, and that all the various forms of
+energy are, in some fundamental way, aspects of the same primary entity
+which constitutes matter itself.
+
+But the discovery of the property of radio-activity has raised many
+other interesting questions, besides that which we have just dealt with.
+In radio-active elements, such as uranium for example, the element is
+breaking down; in what we call radio-activity we have a manifestation of
+the spontaneous change of elements. What is really taking place is a
+transmutation of one element into another, from a heavier to a lighter.
+The element uranium spontaneously becomes radium, and radium passes
+through a number of other stages until it, in turn, becomes lead. Each
+descending element is of lighter atomic weight than its predecessor. The
+changing process, of course, is a very slow one. It may be that all
+matter is radio-active, or can be made so. This raises the question
+whether all the matter in the universe may not undergo disintegration.
+
+There is, however, another side of the question, which the discovery of
+radio-activity has brought to light, and which has effected a revolution
+in our views. We have seen that in radio-active substances the elements
+are breaking down. Is there a process of building up at work? If the
+more complicated atoms are breaking down into simpler forms, may there
+not be a converse process--a building up from simpler elements to more
+complicated elements? It is probably the case that both processes are at
+work.
+
+There are some eighty-odd chemical elements on the earth to-day: are
+they all the outcome of an inorganic evolution, element giving rise to
+element, going back and back to some primeval stuff from which they
+were all originally derived infinitely long ago? Is there an evolution
+in the inorganic world which may be going on, parallel to that of the
+evolution of living things; or is organic evolution a continuation of
+inorganic evolution? We have seen what evidence there is of this
+inorganic evolution in the case of the stars. We cannot go deeply into
+the matter here, nor has the time come for any direct statement that can
+be based on the findings of modern investigation. Taking it altogether
+the evidence is steadily accumulating, and there are authorities who
+maintain that already the evidence of inorganic evolution is convincing
+enough. The heavier atoms would appear to behave as though they were
+evolved from the lighter. The more complex forms, it is supposed, have
+_evolved_ from the simpler forms. Moseley's discovery, to which
+reference has been made, points to the conclusion that the elements are
+built up one from another.
+
+
+§ 8
+
+Other New Views
+
+We may here refer to another new conception to which the discovery of
+radio-activity has given rise. Lord Kelvin, who estimated the age of the
+earth at twenty million years, reached this estimate by considering the
+earth as a body which is gradually cooling down, "losing its primitive
+heat, like a loaf taken from the oven, at a rate which could be
+calculated, and that the heat radiated by the sun was due to
+contraction." Uranium and radio-activity were not known to Kelvin, and
+their discovery has upset both his arguments. Radio-active substances,
+which are perpetually giving out heat, introduce an entirely new factor.
+We cannot now assume that the earth is necessarily cooling down; it may
+even, for all we know, be getting hotter. At the 1921 meeting of the
+British Association, Professor Rayleigh stated that further knowledge
+had extended the probable period during which there had been life on
+this globe to about one thousand million years, and the total age of
+the earth to some small multiple of that. The earth, he considers, is
+not cooling, but "contains an internal source of heat from the
+disintegration of uranium in the outer crust." On the whole the estimate
+obtained would seem to be in agreement with the geological estimates.
+The question, of course, cannot, in the present state of our knowledge,
+be settled within fixed limits that meet with general agreement.
+
+[Illustration: MAKING THE INVISIBLE VISIBLE
+
+Radium, as explained in the text, emits rays--the "Alpha," the "Beta"
+(electrons), and "Gamma" rays. The above illustration indicates the
+method by which these invisible rays are made visible, and enables the
+nature of the rays to be investigated. To the right of the diagram is
+the instrument used, the Spinthariscope, making the impact of radium
+rays visible on a screen.
+
+The radium rays shoot out in all directions; those that fall on the
+screen make it glow with points of light. These points of light are
+observed by the magnifying lens.
+
+A. Magnifying lens. B. A zinc sulphite screen. C. A needle on whose
+point is placed a speck of radium.
+
+The lower picture shows the screen and needle magnified.]
+
+[Illustration: THE THEORY OF ELECTRONS
+
+An atom of matter is composed of electrons. We picture an atom as a sort
+of miniature solar system, the electrons (particles of negative
+electricity) rotating round a central nucleus of positive electricity,
+as described in the text. In the above pictorial representation of an
+atom the whirling electrons are indicated in the outer ring. Electrons
+move with incredible speed as they pass from one atom to another.]
+
+[Illustration: ARRANGEMENTS OF ATOMS IN A DIAMOND
+
+The above is a model (seen from two points of view) of the arrangement
+of the atoms in a diamond. The arrangement is found by studying the
+X-ray spectra of the diamond.]
+
+As we have said, there are other fundamental existences which give rise
+to more complex problems. The three great fundamental entities in the
+physical universe are matter, ether, and energy; so far as we know,
+outside these there is nothing. We have dealt with matter, there remain
+ether and energy. We shall see that just as no particle of matter,
+however small, may be created or destroyed, and just as there is no such
+thing as empty space--ether pervades everything--so there is no such
+thing as _rest_. Every particle that goes to make up our solid earth is
+in a state of perpetual unremitting vibration; energy "is the universal
+commodity on which all life depends." Separate and distinct as these
+three fundamental entities--matter, ether, and energy--may appear, it
+may be that, after all, they are only different and mysterious phases of
+an essential "oneness" of the universe.
+
+
+§ 9
+
+The Future
+
+Let us, in concluding this chapter, give just one illustration of the
+way in which all this new knowledge may prove to be as valuable
+practically as it is wonderful intellectually. We saw that electrons are
+shot out of atoms at a speed that may approach 160,000 miles a second.
+Sir Oliver Lodge has written recently that a seventieth of a grain of
+radium discharges, at a speed a thousand times that of a rifle bullet,
+thirty million electrons a second. Professor Le Bon has calculated that
+it would take 1,340,000 barrels of powder to give a bullet the speed of
+one of these electrons. He shows that the smallest French copper
+coin--smaller than a farthing--contains an energy equal to eighty
+million horsepower. A few pounds of matter contain more energy than we
+could extract from millions of tons of coal. Even in the atoms of
+hydrogen at a temperature which we could produce in an electric furnace
+the electrons spin round at a rate of nearly a hundred trillion
+revolutions a second!
+
+Every man asks at once: "Will science ever tap this energy?" If it does,
+no more smoke, no mining, no transit, no bulky fuel. The energy of an
+atom is of course only liberated when an atom passes from one state to
+another. The stored up energy is fortunately fast bound by the electrons
+being held together as has been described. If it were not so "the earth
+would explode and become a gaseous nebula"! It is believed that some day
+we shall be able to release, harness, and utilise atomic energy. "I am
+of opinion," says Sir William Bragg, "that atom energy will supply our
+future need. A thousand years may pass before we can harness the atom,
+or to-morrow might see us with the reins in our hands. That is the
+peculiarity of Physics--research and 'accidental' discovery go hand in
+hand." Half a brick contains as much energy as a small coal-field. The
+difficulties are tremendous, but, as Sir Oliver Lodge reminds us, there
+was just as much scepticism at one time about the utilisation of steam
+or electricity. "Is it to be supposed," he asks, "that there can be no
+fresh invention, that all the discoveries have been made?" More than one
+man of science encourages us to hope. Here are some remarkable words
+written by Professor Soddy, one of the highest authorities on
+radio-active matter, in our chief scientific weekly (_Nature_, November
+6, 1919):
+
+ The prospects of the successful accomplishment of artificial
+ transmutation brighten almost daily. The ancients seem to have had
+ something more than an inkling that the accomplishment of
+ transmutation would confer upon men powers hitherto the prerogative
+ of the gods. But now we know definitely that the material aspect of
+ transmutation would be of small importance in comparison with the
+ control over the inexhaustible stores of internal atomic energy to
+ which its successful accomplishment would inevitably lead. It has
+ become a problem, no longer redolent of the evil associations of the
+ age of alchemy, but one big with the promise of a veritable physical
+ renaissance of the whole world.
+
+If that "promise" is ever realised, the economic and social face of the
+world will be transformed.
+
+Before passing on to the consideration of ether, light, and energy, let
+us see what new light the discovery of the electron has thrown on the
+nature and manipulation of electricity.
+
+
+WHAT IS ELECTRICITY?
+
+The Nature of Electricity
+
+There is at least one manifestation in nature, and so late as twenty
+years ago it seemed to be one of the most mysterious manifestations of
+all, which has been in great measure explained by the new discoveries.
+Already, at the beginning of this century, we spoke of our "age of
+electricity," yet there were few things in nature about which we knew
+less. The "electric current" rang our bells, drove our trains, lit our
+rooms, but none knew what the current was. There was a vague idea that
+it was a sort of fluid that flowed along copper wires as water flows in
+a pipe. We now suppose that it is _a rapid movement of electrons from
+atom to atom_ in the wire or wherever the current is.
+
+Let us try to grasp the principle of the new view of electricity and see
+how it applies to all the varied electrical phenomena in the world about
+us. As we saw, the nucleus of an atom of matter consists of positive
+electricity which holds together a number of electrons, or charges of
+negative electricity.[4] This certainly tells us to some extent what
+electricity is, and how it is related to matter, but it leaves us with
+the usual difficulty about fundamental realities. But we now know that
+electricity, like matter, is atomic in structure; a charge of
+electricity is made up of a number of small units or charges of a
+definite, constant amount. It has been suggested that the two kinds of
+electricity, i.e. positive and negative, are right-handed and
+left-handed vortices or whirlpools in ether, or rings in ether, but
+there are very serious difficulties, and we leave this to the future.
+
+ [4] The words "positive" and "negative" electricity belong to the
+ days when it was regarded as a fluid. A body overcharged with the
+ fluid was called positive; an undercharged body was called negative.
+ A positively-electrified body is now one whose atoms have lost some
+ of their outlying electrons, so that the positive charge of
+ electricity predominates. The negatively-electrified body is one
+ with more than the normal number of electrons.
+
+
+§ 10
+
+What an Electric Current is
+
+The discovery of these two kinds of electricity has, however, enabled us
+to understand very fairly what goes on in electrical phenomena. The
+outlying electrons, as we saw, may pass from atom to atom, and this, on
+a large scale, is the meaning of the electric current. In other words,
+we believe an electric current to be a flow of electrons. Let us take,
+to begin with, a simple electrical "cell," in which a feeble current is
+generated: such a cell as there is in every house to serve its electric
+bells.
+
+In the original form this simple sort of "battery" consisted of a plate
+of zinc and a plate of copper immersed in a chemical. Long before
+anything was known about electrons it was known that, if you put zinc
+and copper together, you produce a mild current of electricity. We know
+now what this means. Zinc is a metal the atoms of which are particularly
+disposed to part with some of their outlying electrons. Why, we do not
+know; but the fact is the basis of these small batteries. Electrons from
+the atoms of zinc pass to the atoms of copper, and their passage is a
+"current." Each atom gives up an electron to its neighbour. It was
+further found long ago that if the zinc and copper were immersed in
+certain chemicals, which slowly dissolve the zinc, and the two metals
+were connected by a copper wire, the current was stronger. In modern
+language, there is a brisker flow of electrons. The reason is that
+the atoms of zinc which are stolen by the chemical leave their
+detachable electrons behind them, and the zinc has therefore more
+electrons to pass on to the copper.
+
+[Illustration: DISINTEGRATION OF ATOMS
+
+An atom of Uranium, by ejecting an Alpha particle, becomes Uranium X.
+This substance, by ejecting Beta and Gamma rays, becomes Radium. Radium
+passes through a number of further changes, as shown in the diagram, and
+finally becomes lead. Some radio-active substances disintegrate much
+faster than others. Thus Uranium changes very slowly, taking
+5,000,000,000 years to reach the same stage of disintegration that
+Radium A reaches in 3 minutes. As the disintegration proceeds, the
+substances become of lighter and lighter atomic weights. Thus Uranium
+has an atomic weight of 238, whereas lead has an atomic weight of only
+206. The breaking down of atoms is fully explained in the text.]
+
+[Illustration: _Reproduced by permission from "The Interpretation of
+Radium" (John Murray)._
+
+SILK TASSEL ELECTRIFIED
+
+The separate threads of the tassel, being each electrified with the same
+kind of electricity, repel one another, and thus the tassel branches out
+as in the photograph.]
+
+[Illustration: SILK TASSEL DISCHARGED BY THE RAYS FROM RADIUM
+
+When the radium rays, carrying an opposite electric charge to that on
+the tassel, strikes the threads, the threads are neutralised, and hence
+fall together again.]
+
+[Illustration: A HUGE ELECTRIC SPARK
+
+This is an actual photograph of an electric spark. It is leaping a
+distance of about 10 feet, and is the discharge of a million volts. It
+is a graphic illustration of the tremendous energy of electrons.]
+
+[Illustration: _From "Scientific Ideas of To-day_."
+
+ELECTRICAL ATTRACTION BETWEEN COMMON OBJECTS
+
+Take an ordinary flower-vase well dried and energetically rub it with a
+silk handkerchief. The vase which thus becomes electrified will attract
+any light body, such as a feather, as shown in the above illustration.]
+
+Such cells are now made of zinc and carbon, immersed in sal-ammoniac,
+but the principle is the same. The flow of electricity is a flow of
+electrons; though we ought to repeat that they do not flow in a body, as
+molecules of water do. You may have seen boys place a row of bricks,
+each standing on one end, in such order that the first, if it is pushed,
+will knock over the second, the second the third, and so on to the last.
+There is a flow of _movement_ all along the line, but each brick moves
+only a short distance. So an electron merely passes to the next atom,
+which sends on an electron to a third atom, and so on. In this case,
+however, the movement from atom to atom is so rapid that the ripple of
+movement, if we may call it so, may pass along at an enormous speed. We
+have seen how swiftly electrons travel.
+
+But how is this turned into power enough even to ring a bell? The actual
+mechanical apparatus by which the energy of the electron current is
+turned into sound, or heat, or light will be described in a technical
+section later in this work. We are concerned here only with the
+principle, which is clear. While zinc is very apt to part with
+electrons, copper is just as obliging in facilitating their passage
+onward. Electrons will travel in this way in most metals, but copper is
+one of the best "conductors." So we lengthen the copper wire between the
+zinc and the carbon until it goes as far as the front door and the bell,
+which are included in the circuit. When you press the button at the
+door, two wires are brought together, and the current of electrons
+rushes round the circuit; and at the bell its energy is diverted into
+the mechanical apparatus which rings the bell.
+
+Copper is a good conductor--six times as good as iron--and is therefore
+so common in electrical industries. Some other substances are just as
+stubborn as copper is yielding, and we call them "insulators," because
+they resist the current instead of letting it flow. Their atoms do not
+easily part with electrons. Glass, vulcanite, and porcelain are very
+good insulators for this reason.
+
+
+What the Dynamo does
+
+But even several cells together do not produce the currents needed in
+modern industry, and the flow is produced in a different manner. As the
+invisible electrons pass along a wire they produce what we call a
+magnetic field around the wire, they produce a disturbance in the
+surrounding ether. To be exact, it is through the ether surrounding the
+wire that the energy originated by the electrons is transmitted. To set
+electrons moving on a large scale we use a "dynamo." By means of the
+dynamo it is possible to transform mechanical energy into electrical
+energy. The modern dynamo, as Professor Soddy puts it, may be looked
+upon as an electron pump. We cannot go into the subject deeply here, we
+would only say that a large coil of copper wire is caused to turn round
+rapidly between the poles of a powerful magnet. That is the essential
+construction of the "dynamo," which is used for generating strong
+currents. We shall see in a moment how magnetism differs from
+electricity, and will say here only that round the poles of a large
+magnet there is a field of intense disturbance which will start a flow
+of electrons in any copper that is introduced into it. On account of the
+speed given to the coil of wire its atoms enter suddenly this magnetic
+field, and they give off crowds of electrons in a flash.
+
+It is found that a similar disturbance is caused, though the flow is in
+the _opposite_ direction, when the coil of wire leaves the magnetic
+field. And as the coil is revolving very rapidly we get a powerful
+current of electricity that runs in alternate directions--an
+"alternating" current. Electricians have apparatus for converting it
+into a continuous current where this is necessary.
+
+A current, therefore, means a steady flow of the electrons from atom to
+atom. Sometimes, however, a number of electrons rush violently and
+explosively from one body to another, as in the electric spark or the
+occasional flash from an electric tram or train. The grandest and most
+spectacular display of this phenomenon is the thunderstorm. As we saw
+earlier, a portentous furnace like the sun is constantly pouring floods
+of electrons from its atoms into space. The earth intercepts great
+numbers of these electrons. In the upper regions of the air the stream
+of solar electrons has the effect of separating positively-electrified
+atoms from negatively-electrified ones, and the water-vapour, which is
+constantly rising from the surface of the sea, gathers more freely round
+the positively-electrified atoms, and brings them down, as rain, to the
+earth. Thus the upper air loses a proportion of positive electricity, or
+becomes "negatively electrified." In the thunderstorm we get both kinds
+of clouds--some with large excesses of electrons, and some deficient in
+electrons--and the tension grows until at last it is relieved by a
+sudden and violent discharge of electrons from one cloud to another or
+to the earth--an electric spark on a prodigious scale.
+
+
+§ 11
+
+Magnetism
+
+We have seen that an electric current is really a flow of electrons. Now
+an electric current exhibits a magnetic effect. The surrounding space is
+endowed with energy which we call electro-magnetic energy. A piece of
+magnetised iron attracting other pieces of iron to it is the popular
+idea of a magnet. If we arrange a wire to pass vertically through a
+piece of cardboard and then sprinkle iron filings on the cardboard we
+shall find that, on passing an electric current through the wire, the
+iron filings arrange themselves in circles round it. The magnetic force,
+due to the electric current, seems to exist in circles round the wire,
+an ether disturbance being set up. Even a single electron, when in
+movement, creates a magnetic "field," as it is called, round its path.
+There is no movement of electrons without this attendant field of
+energy, and their motion is not stopped until that field of energy
+disappears from the ether. The modern theory of magnetism supposes that
+all magnetism is produced in this way. All magnetism is supposed to
+arise from the small whirling motions of the electrons contained in the
+ultimate atoms of matter. We cannot here go into the details of the
+theory nor explain why, for instance, iron behaves so differently from
+other substances, but it is sufficient to say that here, also, the
+electron theory provides the key. This theory is not yet definitely
+_proved_, but it furnishes a sufficient theoretical basis for future
+research. The earth itself is a gigantic magnet, a fact which makes the
+compass possible, and it is well known that the earth's magnetism is
+affected by those great outbreaks on the sun called sun-spots. Now it
+has been recently shown that a sun-spot is a vast whirlpool of electrons
+and that it exerts a strong magnetic action. There is doubtless a
+connection between these outbreaks of electronic activity and the
+consequent changes in the earth's magnetism. The precise mechanism of
+the connection, however, is still a matter that is being investigated.
+
+
+ETHER AND WAVES
+
+Ether and Waves
+
+The whole material universe is supposed to be embedded in a vast medium
+called the ether. It is true that the notion of the ether has been
+abandoned by some modern physicists, but, whether or not it is
+ultimately dispensed with, the conception of the ether has entered so
+deeply into the scientific mind that the science of physics cannot be
+understood unless we know something about the properties attributed to
+the ether. The ether was invented to explain the phenomena of light, and
+to account for the flow of energy across empty space. Light takes time
+to travel. We see the sun at any moment by the light that left it 8
+minutes before. It has taken that 8 minutes for the light from the
+sun to travel that 93,000,000 miles odd which separates it from our
+earth. Besides the fact that light takes time to travel, it can be shown
+that light travels in the form of waves. We know that sound travels in
+waves; sound consists of waves in the air, or water or wood or whatever
+medium we hear it through. If an electric bell be put in a glass jar and
+the air be pumped out of the jar, the sound of the bell becomes feebler
+and feebler until, when enough air has been taken out, we do not hear
+the bell at all. Sound cannot travel in a vacuum. We continue to _see_
+the bell, however, so that evidently light can travel in a vacuum. The
+invisible medium through which the waves of light travel is the ether,
+and this ether permeates all space _and all matter_. Between us and the
+stars stretch vast regions empty of all matter. But we see the stars;
+their light reaches us, even though it may take centuries to do so. We
+conceive, then, that it is the universal ether which conveys that light.
+All the energy which has reached the earth from the sun and which,
+stored for ages in our coal-fields, is now used to propel our trains and
+steamships, to heat and light our cities, to perform all the
+multifarious tasks of modern life, was conveyed by the ether. Without
+that universal carrier of energy we should have nothing but a stagnant,
+lifeless world.
+
+[Illustration: _Photo: Leadbeater._
+
+AN ELECTRIC SPARK
+
+An electric spark consists of a rush of electrons across the space
+between the two terminals. A state of tension is established in the
+ether by the electric charges, and when this tension passes a certain
+limit the discharge takes place.]
+
+[Illustration: _From "Scientific Ideas of To-day."_
+
+AN ETHER DISTURBANCE AROUND AN ELECTRON CURRENT
+
+In the left-hand photograph an electric current is passing through the
+coil, thus producing a magnetic field and transforming the poker into a
+magnet. The poker is then able to support a pair of scissors. As soon as
+the electric current is broken off, as in the second photograph, the
+ether disturbance ceases. The poker loses its magnetism, and the
+scissors fall.]
+
+We have said that light consists of waves. The ether may be considered
+as resembling, in some respects, a jelly. It can transmit vibrations.
+The waves of light are really excessively small ripples, measuring from
+crest to crest. The distance from crest to crest of the ripples in a
+pond is sometimes no more than an inch or two. This distance is
+enormously great compared to the longest of the wave-lengths that
+constitute light. We say the longest, for the waves of light differ in
+length; the colour depends upon the length of the light. Red light has
+the longest waves and violet the shortest. The longest waves, the waves
+of deep-red light, are seven two hundred and fifty thousandths of an
+inch in length (7/250,000 inch). This is nearly twice the length of
+deep-violet light-waves, which are 1/67,000 inch. But light-waves, the
+waves that affect the eye, are not the only waves carried by the ether.
+Waves too short to affect the eye can affect the photographic plate, and
+we can discover in this way the existence of waves only half the length
+of the deep-violet waves. Still shorter waves can be discovered, until
+we come to those excessively minute rays, the X-rays.
+
+
+Below the Limits of Visibility
+
+But we can extend our investigations in the other direction; we find
+that the ether carries many waves longer than light-waves. Special
+photographic emulsions can reveal the existence of waves five times
+longer than violet-light waves. Extending below the limits of visibility
+are waves we detect as heat-waves. Radiant heat, like the heat from a
+fire, is also a form of wave-motion in the ether, but the waves our
+senses recognise as heat are longer than light-waves. There are longer
+waves still, but our senses do not recognise them. But we can detect
+them by our instruments. These are the waves used in wireless
+telegraphy, and their length may be, in some cases, measured in miles.
+These waves are the so-called electro-magnetic waves. Light, radiant
+heat, and electro-magnetic waves are all of the same nature; they differ
+only as regards their wave-lengths.
+
+
+LIGHT--VISIBLE AND INVISIBLE
+
+If Light, then, consists of waves transmitted through the ether, what
+gives rise to the waves? Whatever sets up such wonderfully rapid series
+of waves must be something with an enormous vibration. We come back to
+the electron: all atoms of matter, as we have seen, are made up of
+electrons revolving in a regular orbit round a nucleus. These electrons
+may be affected by out-side influences, they may be agitated and their
+speed or vibration increased.
+
+
+Electrons and Light
+
+The particles even of a piece of cold iron are in a state of vibration.
+No nerves of ours are able to feel and register the waves they emit, but
+your cold poker is really radiating, or sending out a series of
+wave-movements, on every side. After what we saw about the nature of
+matter, this will surprise none. Put your poker in the fire for a time.
+The particles of the glowing coal, which are violently agitated,
+communicate some of their energy to the particles of iron in the poker.
+They move to and fro more rapidly, and the waves which they create are
+now able to affect your nerves and cause a sensation of heat. Put the
+poker again in the fire, until its temperature rises to 500° C. It
+begins to glow with a dull red. Its particles are now moving very
+violently, and the waves they send out are so short and rapid that they
+can be picked up by the eye--we have _visible_ light. They would still
+not affect a photographic plate. Heat the iron further, and the crowds
+of electrons now send out waves of various lengths which blend into
+white light. What is happening is the agitated electrons flying round in
+their orbits at a speed of trillions of times a second. Make the iron
+"blue hot," and it pours out, in addition to light, the _invisible_
+waves which alter the film on the photographic plate. And beyond these
+there is a long range of still shorter waves, culminating in the X-rays,
+which will pass between the atoms of flesh or stone.
+
+Nearly two hundred and fifty years ago it was proved that light
+travelled at least 600,000 times faster than sound. Jupiter, as we saw,
+has moons, which circle round it. They pass behind the body of the
+planet, and reappear at the other side. But it was noticed that, when
+Jupiter is at its greatest distance from us, the reappearance of the
+moon from behind it is 16 minutes and 36 seconds later than when the
+planet is nearest to us. Plainly this was because light took so long to
+cover the additional distance. The distance was then imperfectly known,
+and the speed of light was underrated. We now know the distance, and we
+easily get the velocity of light.
+
+No doubt it seems far more wonderful to discover this within the walls
+of a laboratory, but it was done as long ago as 1850. A cogged wheel is
+so mounted that a ray of light passes between two of the teeth and is
+reflected back from a mirror. Now, slight as is the fraction of a second
+which light takes to travel that distance, it is possible to give such
+speed to the wheel that the next tooth catches the ray of light on its
+return and cuts it off. The speed is increased still further until the
+ray of light returns to the eye of the observer through the notch _next_
+to the one by which it had passed to the mirror! The speed of the wheel
+was known, and it was thus possible again to gather the velocity of
+light. If the shortest waves are 1/67,000 of an inch in length, and
+light travels at 186,000 miles a second, any person can work out that
+about 800 trillion waves enter the eye in a second when we see "violet."
+
+
+Sorting out Light-waves
+
+The waves sent out on every side by the energetic electrons become
+faintly visible to us when they reach about 1/35,000 of an inch. As they
+become shorter and more rapid, as the electrons increase their speed, we
+get, in succession, the colours red, orange, yellow, green, blue,
+indigo, and violet. Each distinct sensation of colour means a wave of
+different length. When they are all mingled together, as in the light of
+the sun, we get white light. When this white light passes through glass,
+the speed of the waves is lessened; and, if the ray of light falls
+obliquely on a triangular piece of glass, the waves of different lengths
+part company as they travel through it, and the light is spread out in a
+band of rainbow-colour. The waves are sorted out according to their
+lengths in the "obstacle race" through the glass. Anyone may see this
+for himself by holding up a wedge-shaped piece of crystal between the
+sunlight and the eye; the prism separates the sunlight into its
+constituent colours, and these various colours will be seen quite
+readily. Or the thing may be realised in another way. If the seven
+colours are painted on a wheel as shown opposite page 280 (in the
+proportion shown), and the wheel rapidly revolved on a pivot, the wheel
+will appear a dull white, the several colours will not be seen. But
+_omit_ one of the colours, then the wheel, when revolved, will not
+appear white, but will give the impression of one colour, corresponding
+to what the union of six colours gives. Another experiment will show
+that some bodies held up between the eye and a white light will not
+permit all the rays to pass through, but will intercept some; a body
+that intercepts all the seven rays except red will give the impression
+of red, or if all the rays except violet, then violet will be the colour
+seen.
+
+[Illustration: _Photo: H. J. Shepstone._
+
+LIGHTNING
+
+In a thunderstorm we have the most spectacular display in lightning of a
+violent and explosive rush of electrons (electricity) from one body to
+another, from cloud to cloud, or to the earth. In this wonderful
+photograph of an electrical storm note the long branched and undulating
+flashes of lightning. Each flash lasts no longer than the one
+hundred-thousandth part of a second of time.]
+
+[Illustration: LIGHT WAVES
+
+Light consists of waves transmitted through the ether. Waves of light
+differ in length. The colour of the light depends on the wave-length.
+Deep-red waves (the longest) are 7/250000 inch and deep-violet waves
+1/67000 inch. The diagram shows two wave-motions of different
+wave-lengths. From crest to crest, or from trough to trough, is the
+length of the wave.]
+
+[Illustration: THE MAGNETIC CIRCUIT OF AN ELECTRIC CURRENT
+
+The electric current passing in the direction of the arrow round the
+electric circuit generates in the surrounding space circular magnetic
+circuits as shown in the diagram. It is this property which lies at the
+base of the electro-magnet and of the electric dynamo.]
+
+[Illustration: THE MAGNET
+
+The illustration shows the lines of force between two magnets. The lines
+of force proceed from the north pole of one magnet to the south pole of
+the other. They also proceed from the north to the south poles of the
+same magnet. These facts are shown clearly in the diagram. The north
+pole of a magnet is that end of it which turns to the north when the
+magnet is freely suspended.]
+
+
+The Fate of the World
+
+Professor Soddy has given an interesting picture of what might happen
+when the sun's light and heat is no longer what it is. The human eye
+"has adapted itself through the ages to the peculiarities of the sun's
+light, so as to make the most of that wave-length of which there is
+most.... Let us indulge for a moment in these gloomy prognostications,
+as to the consequences to this earth of the cooling of the sun with the
+lapse of ages, which used to be in vogue, but which radio-activity has
+so rudely shaken. Picture the fate of the world when the sun has become
+a dull red-hot ball, or even when it has cooled so far that it would no
+longer emit light to us. That does not all mean that the world would be
+in inky darkness, and that the sun would not emit light to the people
+then inhabiting this world, if any had survived and could keep
+themselves from freezing. To such, if the eye continued to adapt itself
+to the changing conditions, our blues and violets would be ultra-violet
+and invisible, but our dark heat would be light and hot bodies would be
+luminous to them which would be dark to us."
+
+
+§ 12
+
+What the Blue "Sky" means
+
+We saw in a previous chapter how the spectroscope splits up light-waves
+into their colours. But nature is constantly splitting the light into
+its different-lengthed waves, its colours. The rainbow, where dense
+moisture in the air acts as a spectroscope, is the most familiar
+example. A piece of mother-of-pearl, or even a film of oil on the street
+or on water, has the same effect, owing to the fine inequalities in its
+surface. The atmosphere all day long is sorting out the waves. The blue
+"sky" overhead means that the fine particles in the upper atmosphere
+catch the shorter waves, the blue waves, and scatter them. We can make a
+tubeful of blue sky in the laboratory at any time. The beautiful
+pink-flush on the Alps at sunrise, the red glory that lingers in the
+west at sunset, mean that, as the sun's rays must struggle through
+denser masses of air when it is low on the horizon, the long red waves
+are sifted out from the other shafts.
+
+Then there is the varied face of nature which, by absorbing some waves
+and reflecting others, weaves its own beautiful robe of colour. Here and
+there is a black patch, which _absorbs_ all the light. White surfaces
+_reflect_ the whole of it. What is reflected depends on the period of
+vibration of the electrons in the particular kind of matter. Generally,
+as the electrons receive the flood of trillions of waves, they absorb
+either the long or the medium or the short, and they give us the
+wonderful colour-scheme of nature. In some cases the electrons continue
+to radiate long after the sunlight has ceased to fall upon them. We get
+from them "black" or invisible light, and we can take photographs by it.
+Other bodies, like glass, vibrate in unison with the period of the
+light-waves and let them stream through.
+
+
+Light without Heat
+
+There are substances--"phosphorescent" things we call them--which give
+out a mysterious cold light of their own. It is one of the problems
+of science, and one of profound practical interest. If we could produce
+light without heat our "gas bill" would shrink amazingly. So much energy
+is wasted in the production of heat-waves and ultra-violet waves which
+we do not want, that 90 per cent. or more of the power used in
+illumination is wasted. Would that the glow-worm, or even the dead
+herring, would yield us its secret! Phosphorus is the one thing we know
+as yet that suits the purpose, and--it smells! Indeed, our artificial
+light is not only extravagant in cost, but often poor in colour. The
+unwary person often buys a garment by artificial light, and is disgusted
+next morning to find in it a colour which is not wanted. The colour
+disclosed by the sun was not in the waves of the artificial light.
+
+[Illustration: ROTATING DISC OF SIR ISAAC NEWTON FOR MIXING COLOURS
+
+The Spectroscope sorts out the above seven colours from sunlight (which
+is compounded of these seven colours). If painted in proper proportions
+on a wheel, as shown in the coloured illustration, and the wheel be
+turned rapidly on a pivot through its centre, only a dull white will be
+perceived. If one colour be omitted, the result will be one colour--the
+result of the union of the remaining six.]
+
+Beyond the waves of violet light are the still shorter and more rapid
+waves--the "ultra-violet" waves--which are precious to the photographer.
+As every amateur knows, his plate may safely be exposed to light that
+comes through a red or an orange screen. Such a screen means "no
+thoroughfare" for the blue and "beyond-blue" waves, and it is these
+which arrange the little grains of silver on the plate. It is the same
+waves which supply the energy to the little green grains of matter
+(chlorophyll) in the plant, preparing our food and timber for us, as
+will be seen later. The tree struggles upward and spreads out its leaves
+fanwise to the blue sky to receive them. In our coal-measures, the
+mighty dead forests of long ago, are vast stores of sunlight which we
+are prodigally using up.
+
+The X-rays are the extreme end, the highest octave, of the series of
+waves. Their power of penetration implies that they are excessively
+minute, but even these have not held their secret from the modern
+physicist. From a series of beautiful experiments, in which they were
+made to pass amongst the atoms of a crystal, we learned their length. It
+is about the ten-millionth of a millimetre, and a millimetre is about
+the 1/25 of an inch!
+
+One of the most recent discoveries, made during a recent eclipse of the
+sun, is that light is subject to gravitation. A ray of light from a star
+is bent out of its straight path when it passes near the mass of the
+sun. Professor Eddington tells us that we have as much right to speak of
+a pound of light as of a pound of sugar. Professor Eddington even
+calculates that the earth receives 160 tons of light from the sun every
+year!
+
+
+ENERGY: HOW ALL LIFE DEPENDS ON IT
+
+As we have seen in an earlier chapter, one of the fundamental entities
+of the universe is matter. A second, not less important, is called
+energy. Energy is indispensable if the world is to continue to exist,
+since all phenomena, including life, depend on it. Just as it is humanly
+impossible to create or to destroy a particle of matter, so is it
+impossible to create or to destroy energy. This statement will be more
+readily understood when we have considered what energy is.
+
+Energy, like matter, is indestructible, and just as matter exists in
+various forms so does energy. And we may add, just as we are ignorant of
+what the negative and positive particles of electricity which constitute
+matter really are, so we are ignorant of the true nature of energy. At
+the same time, energy is not so completely mysterious as it once was. It
+is another of nature's mysteries which the advance of modern science has
+in some measure unveiled. It was only during the nineteenth century that
+energy came to be known as something as distinct and permanent as matter
+itself.
+
+
+Forms of Energy
+
+The existence of various forms of energy had been known, of course, for
+ages; there was the energy of a falling stone, the energy produced by
+burning wood or coal or any other substance, but the essential
+_identity_ of all these forms of energy had not been suspected. The
+conception of energy as something which, like matter, was constant in
+amount, which could not be created nor destroyed, was one of the great
+scientific acquisitions of the past century.
+
+[Illustration: WAVE SHAPES
+
+Wave-motions are often complex. The above illustration shows some fairly
+complicated wave shapes. All such wave-motions can be produced by
+superposing a number of simple wave forms.]
+
+[Illustration: THE POWER OF A MAGNET
+
+The illustration is that of a "Phoenix" electric magnet lifting scrap
+from railway trucks. The magnet is 52 inches in diameter and lifts a
+weight of 26 tons. The same type of magnet, 62 inches in diameter, lifts
+a weight of 40 tons.]
+
+[Illustration: _Photo: The Locomotive Publishing Co., Ltd._
+
+THE SPEED OF LIGHT
+
+A train travelling at the rate of sixty miles per hour would take rather
+more than seventeen and a quarter days to go round the earth at the
+equator, i.e. a distance of 25,000 miles. Light, which travels at the
+rate of 186,000 miles per second, would take between one-seventh and
+one-eighth of a second to go the same distance.]
+
+[Illustration: ROTATING DISC OF SIR ISAAC NEWTON FOR MIXING COLOURS
+
+The Spectroscope sorts out the above seven colours from sunlight (which
+is compounded of these seven colours). If painted in proper proportions
+on a wheel, as shown in the coloured illustration, and the wheel turned
+rapidly on a pivot through its centre, only a dull white will be
+perceived. If one colour be omitted, the result will be one colour--the
+result of the union of the remaining six.]
+
+It is not possible to enter deeply into this subject here. It is
+sufficient if we briefly outline its salient aspects. Energy is
+recognised in two forms, kinetic and potential. The form of energy which
+is most apparent to us is the _energy of motion_; for example, a rolling
+stone, running water, a falling body, and so on. We call the energy of
+motion _kinetic energy_. Potential energy is the energy a body has in
+virtue of its position--it is its capacity, in other words, to acquire
+kinetic energy, as in the case of a stone resting on the edge of a
+cliff.
+
+Energy may assume different forms; one kind of energy may be converted
+directly or indirectly into some other form. The energy of burning coal,
+for example, is converted into heat, and from heat energy we have
+mechanical energy, such as that manifested by the steam-engine. In this
+way we can transfer energy from one body to another. There is the energy
+of the great waterfalls of Niagara, for instance, which are used to
+supply the energy of huge electric power stations.
+
+
+What Heat is
+
+An important fact about energy is, that all energy _tends to take the
+form of heat energy_. The impact of a falling stone generates heat; a
+waterfall is hotter at the bottom than at the top--the falling particles
+of water, on striking the ground, generate heat; and most chemical
+changes are attended by heat changes. Energy may remain latent
+indefinitely in a lump of wood, but in combustion it is liberated, and
+we have heat as a result. The atom of radium or of any other
+radio-active substance, as it disintegrates, generates heat. "Every hour
+radium generates sufficient heat to raise the temperature of its own
+weight of water, from the freezing point to the boiling point." And what
+is heat? _Heat is molecular motion._ The molecules of every substance,
+as we have seen on a previous page, are in a state of continual motion,
+and the more vigorous the motion the hotter the body. As wood or coal
+burns, the invisible molecules of these substances are violently
+agitated, and give rise to ether waves which our senses interpret as
+light and heat. In this constant movement of the molecules, then, we
+have a manifestation of the energy of motion and of heat.
+
+That energy which disappears in one form reappears in another has been
+found to be universally true. It was Joule who, by churning water, first
+showed that a measurable quantity of mechanical energy could be
+transformed into a measurable quantity of heat energy. By causing an
+apparatus to stir water vigorously, that apparatus being driven by
+falling weights or a rotating flywheel or by any other mechanical means,
+the water became heated. A certain amount of mechanical energy had been
+used up and a certain amount of heat had appeared. The relation between
+these two things was found to be invariable. Every physical change in
+nature involves a transformation of energy, but the total quantity of
+energy in the universe remains unaltered. This is the great doctrine of
+the Conservation of Energy.
+
+
+§ 13
+
+Substitutes for Coal
+
+Consider the source of nearly all the energy which is used in modern
+civilisation--coal. The great forests of the Carboniferous epoch now
+exists as beds of coal. By the burning of coal--a chemical
+transformation--the heat energy is produced on which at present our
+whole civilisation depends. Whence is the energy locked up in the coal
+derived? From the sun. For millions of years the energy of the sun's
+rays had gone to form the vast vegetation of the Carboniferous era and
+had been transformed, by various subtle processes, into the potential
+energy that slumbers in those immense fossilized forests.
+
+The exhaustion of our coal deposits would mean, so far as our knowledge
+extends at present, the end of the world's civilisation. There are other
+known sources of energy, it is true. There is the energy of falling
+water; the great falls of Niagara are used to supply the energy of huge
+electric power stations. Perhaps, also, something could be done to
+utilise the energy of the tides--another instance of the energy of
+moving water. And attempts have been made to utilise directly the energy
+of the sun's rays. But all these sources of energy are small compared
+with the energy of coal. A suggestion was made at a recent British
+Association meeting that deep borings might be sunk in order to utilise
+the internal heat of the earth, but this is not, perhaps, a very
+practical proposal. By far the most effective substitutes for coal would
+be found in the interior energy of the atom, a source of energy which,
+as we have seen, is practically illimitable. If the immense electrical
+energy in the interior of the atom can ever be liberated and controlled,
+then our steadily decreasing coal supply will no longer be the bugbear
+it now is to all thoughtful men.
+
+The stored-up energy of the great coal-fields can be used up, but we
+cannot replace it or create fresh supplies. As we have seen, energy
+cannot be destroyed, but it can become _unavailable_. Let us consider
+what this important fact means.
+
+
+§ 14
+
+Dissipation of Energy
+
+Energy may become dissipated. Where does it go? since if it is
+indestructible it must still exist. It is easier to ask the question
+than to give a final answer, and it is not possible in this OUTLINE,
+where an advanced knowledge of physics is not assumed on the part of the
+reader, to go fully into the somewhat difficult theories put forward by
+physicists and chemists. We may raise the temperature, say, of iron,
+until it is white-hot. If we stop the process the temperature of the
+iron will gradually settle down to the temperature of surrounding
+bodies. As it does so, where does its previous energy go? In some
+measure it may pass to other bodies in contact with the piece of iron,
+but ultimately the heat becomes radiated away in space where we cannot
+follow it. It has been added to the vast reservoir of _unavailable_ heat
+energy of uniform temperature. It is sufficient here to say that if all
+bodies had a uniform temperature we should experience no such thing as
+heat, because heat only travels from one body to another, having the
+effect of cooling the one and warming the other. In time the two bodies
+acquire the same temperature. The sum-total of the heat in any body is
+measured in terms of the kinetic energy of its moving molecules.
+
+There must come a time, so far as we can see at present, when, even if
+all the heat energy of the universe is not radiated away into empty
+infinite space, yet a uniform temperature will prevail. If one body is
+hotter than another it radiates heat to that body until both are at the
+same temperature. Each body may still possess a considerable quantity of
+heat energy, which it has absorbed, but that energy, so far as reactions
+between those two bodies are concerned, _is now unavailable_. The same
+principle applies whatever number of bodies we consider. Before heat
+energy can be utilised we must have bodies with different temperature.
+If the whole universe were at some uniform temperature, then, although
+it might possess an enormous amount of heat energy, this energy would be
+unavailable.
+
+
+What a Uniform Temperature would mean
+
+And what does this imply? It implies a great deal: for if all the energy
+in the world became unavailable, the universe, as it now is, would cease
+to be. It is possible that, by the constant interchange of heat
+radiations, the whole universe is tending to some uniform temperature,
+in which case, although all molecular motion would not have ceased, it
+would have become unavailable. In this sense it may be said that the
+universe is running down.
+
+[Illustration: NIAGARA FALLS
+
+The energy of this falling water is prodigious. It is used to generate
+thousands of horse-power in great electrical installations. The power is
+used to drive electric trams in cities 150 to 250 miles away.]
+
+[Illustration: _Photo: Stephen Cribb._
+
+TRANSFORMATION OF ENERGY
+
+An illustration of Energy. The chemical energy brought into existence by
+firing the explosive manifesting itself as mechanical energy, sufficient
+to impart violent motion to tons of water.]
+
+[Illustration: _Photo: Underwood & Underwood._
+
+"BOILING" A KETTLE ON ICE
+
+When a kettle containing liquid air is placed on ice it "boils" because
+the ice is intensely hot _when compared with the very low temperature of
+the liquid air_.]
+
+If all the molecules of a substance were brought to a standstill, that
+substance would be at the absolute zero of temperature. There could be
+nothing colder. The temperature at which all molecular motions would
+cease is known: it is -273° C. No body could possibly attain a lower
+temperature than this: a lower temperature could not exist. Unless there
+exists in nature some process, of which we know nothing at present,
+whereby energy is renewed, our solar system must one day sink to this
+absolute zero of temperature. The sun, the earth, and every other body
+in the universe is steadily radiating heat, and this radiation cannot go
+on for ever, because heat continually tends to diffuse and to equalise
+temperatures.
+
+But we can see, theoretically, that there is a way of evading this law.
+If the chaotic molecular motions which constitute heat could be
+_regulated_, then the heat energy of a body could be utilised directly.
+Some authorities think that some of the processes which go on in the
+living body do not involve any waste energy, that the chemical energy of
+food is transformed directly into work without any of it being
+dissipated as useless heat energy. It may be, therefore, that man will
+finally discover some way of escape from the natural law that, while
+energy cannot be destroyed, it has a tendency to become unavailable.
+
+The primary reservoir of energy is the atom; it is the energy of the
+atom, the atom of elements in the sun, the stars, the earth, from which
+nature draws for all her supply of energy. Shall we ever discover how we
+can replenish the dwindling resources of energy, or find out how we can
+call into being the at present unavailable energy which is stored up in
+uniform temperature?
+
+ It looks as if our successors would witness an interesting race,
+ between the progress of science on the one hand and the depletion of
+ natural resources upon the other. The natural rate of flow of energy
+ from its primary atomic reservoirs to the sea of waste heat energy
+ of uniform temperature, allows life to proceed at a complete pace
+ sternly regulated by the inexorable laws of supply and demand,
+ which the biologists have recognised in their field as the struggle
+ for existence.[5]
+
+ [5] _Matter and Energy_, by Professor Soddy.
+
+It is certain that energy is an actual entity just as much as matter,
+and that it cannot be created or destroyed. Matter and ether are
+receptacles or vehicles of energy. As we have said, what these entities
+really are in themselves we do not know. It may be that all forms of
+energy are in some fundamental way aspects of the same primary entity
+which constitutes matter: how all matter is constituted of particles of
+electricity we have already seen. The question to which we await an
+answer is: What is electricity?
+
+
+§ 15
+
+MATTER, ETHER, AND EINSTEIN
+
+The supreme synthesis, the crown of all this progressive conquest of
+nature, would be to discover that the particles of positive and negative
+electricity, which make up the atoms of matter, are points or centres of
+disturbances of some kind in a universal ether, and that all our
+"energies" (light, magnetism, gravitation, etc.) are waves or strains of
+some kind set up in the ether by these clusters of electrons.
+
+It is a fascinating, tantalising dream. Larmor suggested in 1900 that
+the electron is a tiny whirlpool, or "vortex," in ether; and, as such a
+vortex may turn in either of two opposite ways, we seem to see a
+possibility of explaining positive and negative electricity. But the
+difficulties have proved very serious, and the nature of the electron is
+unknown. A recent view is that it is "a ring of negative electricity
+rotating about its axis at a high speed," though that does not carry us
+very far. The unit of positive electricity is even less known. We must
+be content to know the general lines on which thought is moving toward
+the final unification.
+
+We say "unification," but it would be a grave error to think that ether
+is the only possible basis for such unity, or to make it an essential
+part of one's philosophy of the universe. Ether was never more than an
+imagined entity to which we ascribed the most extraordinary properties,
+and which seemed then to promise considerable aid. It was conceived as
+an elastic solid of very great density, stretching from end to end of
+the universe, transmitting waves from star to star at the rate of
+186,000 miles a second; yet it was believed that the most solid matter
+passed through it as if it did not exist.
+
+Some years ago a delicate experiment was tried for the purpose of
+detecting the ether. Since the earth, in travelling round the sun, must
+move through the ether if the ether exists, there ought to be a stream
+of ether flowing through every laboratory; just as the motion of a ship
+through a still atmosphere will make "a wind." In 1887 Michelson and
+Morley tried to detect this. Theoretically, a ray of light in the
+direction of the stream ought to travel at a different rate from a ray
+of light against the stream or across it. They found no difference, and
+scores of other experiments have failed. This does not prove that there
+is no ether, as there is reason to suppose that our instruments would
+appear to shrink in precisely the same proportion as the alteration of
+the light; but the fact remains that we have no proof of the existence
+of ether. J. H. Jeans says that "nature acts as if no such thing
+existed." Even the phenomena of light and magnetism, he says, do not
+imply ether; and he thinks that the hypothesis may be abandoned. The
+primary reason, of course, for giving up the notion of the ether is
+that, as Einstein has shown, there is no way of detecting its existence.
+If there is an ether, then, since the earth is moving through it, there
+should be some way of detecting this motion. The experiment has been
+tried, as we have said, but, although the method used was very
+sensitive, no motion was discovered. It is Einstein who, by
+revolutionising our conceptions of space and time, showed that no such
+motion ever could be discovered, whatever means were employed, and that
+the usual notion of the ether must be abandoned. We shall explain this
+theory more fully in a later section.
+
+
+INFLUENCE OF THE TIDES: ORIGIN OF THE MOON: THE EARTH SLOWING DOWN
+
+§ 16
+
+Until comparatively recent times, until, in fact, the full dawn of
+modern science, the tides ranked amongst the greatest of nature's
+mysteries. And, indeed, what agency could be invoked to explain this
+mysteriously regular flux and reflux of the waters of the ocean? It is
+not surprising that that steady, rhythmical rise and fall suggested to
+some imaginative minds the breathing of a mighty animal. And even when
+man first became aware of the fact that this regular movement was
+somehow associated with the moon, was he much nearer an explanation?
+What bond could exist between the movements of that distant world and
+the diurnal variation of the waters of the earth? It is reported that an
+ancient astronomer, despairing of ever resolving the mystery, drowned
+himself in the sea.
+
+
+The Earth Pulled by the Moon
+
+But it was part of the merit of Newton's mighty theory of gravitation
+that it furnished an explanation even of this age-old mystery. We can
+see, in broad outlines at any rate, that the theory of universal
+attraction can be applied to this case. For the moon, Newton taught us,
+pulls every particle of matter throughout the earth. If we imagine that
+part of the earth's surface which comprises the Pacific Ocean, for
+instance, to be turned towards the moon, we see that the moon's pull,
+_acting on the loose and mobile water_, would tend to heap it up into a
+sort of mound. The whole earth is pulled by the moon, but the water
+is more free to obey this pull than is the solid earth, although small
+tides are also caused in the earth's solid crust. It can be shown also
+that a corresponding hump would tend to be produced on the other side of
+the earth, owing, in this case, to the tendency of the water, being more
+loosely connected, to lag behind the solid earth. If the earth's surface
+were entirely fluid the rotation of the earth would give the impression
+that these two humps were continually travelling round the world, once
+every day. At any given part of the earth's surface, therefore, there
+would be two humps daily, i.e. two periods of high water. Such is the
+simplest possible outline of the gravitational theory of the tides.
+
+[Illustration: THE CAUSE OF TIDES
+
+The tides of the sea are due to the pull of the moon, and, in lesser
+degree, of the sun. The whole earth is pulled by the moon, but the loose
+and mobile water is more free to obey this pull than is the solid earth,
+although small tides are also caused in the earth's solid crust. The
+effect which the tides have on slowing down the rotation of the earth is
+explained in the text.]
+
+[Illustration: _Photo: G. Brocklehurst._
+
+THE AEGIR ON THE TRENT
+
+An exceptionally smooth formation due to perfect weather conditions. The
+wall-like formation of these tidal waves (see next page also) will be
+noticed. The reason for this is that the downward current in the river
+heads the sea-water back, and thus helps to exaggerate the advancing
+slope of the wave. The exceptional spring tides are caused by the
+combined operation of the moon and the sun, as is explained in the
+text.]
+
+[Illustration: _Photo: G. Brocklehurst._
+
+A BIG SPRING TIDE, THE AEGIR ON THE TRENT]
+
+The actually observed phenomena are vastly more complicated, and the
+complete theory bears very little resemblance to the simple form we have
+just outlined. Everyone who lives in the neighbourhood of a port knows,
+for instance, that high water seldom coincides with the time when the
+moon crosses the meridian. It may be several hours early or late. High
+water at London Bridge, for instance, occurs about one and a half hours
+after the moon has passed the meridian, while at Dublin high water
+occurs about one and a half hours before the moon crosses the meridian.
+The actually observed phenomena, then, are far from simple; they have,
+nevertheless, been very completely worked out, and the times of high
+water for every port in the world can now be prophesied for a
+considerable time ahead.
+
+
+The Action of Sun and Moon
+
+It would be beyond our scope to attempt to explain the complete theory,
+but we may mention one obvious factor which must be taken into account.
+Since the moon, by its gravitational attraction, produces tides, we
+should expect that the sun, whose gravitational attraction is so much
+stronger, should also produce tides and, we would suppose at first
+sight, more powerful tides than the moon. But while it is true that the
+sun produces tides, it is not true that they are more powerful than
+those produced by the moon. The sun's tide-producing power is, as a
+matter of fact, less than half that of the moon. The reason of this is
+that _distance_ plays an enormous rôle in the production of tides. The
+mass of the sun is 26,000,000 times that of the moon; on the other hand
+it is 386 times as far off as the moon. This greater distance more than
+counterbalances its greater mass, and the result, as we have said, is
+that the moon is more than twice as powerful. Sometimes the sun and moon
+act together, and we have what are called spring tides; sometimes they
+act against one another, and we have neap tides. These effects are
+further complicated by a number of other factors, and the tides, at
+various places, vary enormously. Thus at St. Helena the sea rises and
+falls about three feet, whereas in the Bay of Fundy it rises and falls
+more than fifty feet. But here, again, the reasons are complicated.
+
+
+§ 17
+
+Origin of the Moon
+
+But there is another aspect of the tides which is of vastly greater
+interest and importance than the theory we have just been discussing. In
+the hands of Sir George H. Darwin, the son of Charles Darwin, the tides
+had been made to throw light on the evolution of our solar system. In
+particular, they have illustrated the origin and development of the
+system formed by our earth and moon. It is quite certain that, long ages
+ago, the earth was rotating immensely faster than it is now, and that
+the moon was so near as to be actually in contact with the earth. In
+that remote age the moon was just on the point of separating from the
+earth, of being thrown off by the earth. Earth and moon were once one
+body, but the high rate of rotation caused this body to split up into
+two pieces; one piece became the earth we now know, and the other became
+the moon. Such is the conclusion to which we are led by an examination
+of the tides. In the first place let us consider the energy produced by
+the tides. We see evidences of this energy all round the word's
+coastlines. Estuaries are scooped out, great rocks are gradually reduced
+to rubble, innumerable tons of matter are continually being set in
+movement. Whence is this energy derived? Energy, like matter, cannot be
+created from nothing; what, then, is the source which makes this
+colossal expenditure possible.
+
+
+The Earth Slowing down
+
+The answer is simple, but startling. _The source of tidal energy is the
+rotation of the earth._ The massive bulk of the earth, turning every
+twenty-four hours on its axis, is like a gigantic flywheel. In virtue of
+its rotation it possesses an enormous store of energy. But even the
+heaviest and swiftest flywheel, if it is doing work, or even if it is
+only working against the friction of its bearings, cannot dispense
+energy for ever. It must, gradually, slow down. There is no escape from
+this reasoning. It is the rotation of the earth which supplies the
+energy of the tides, and, as a consequence, the tides must be slowing
+down the earth. The tides act as a kind of brake on the earth's
+rotation. These masses of water, _held back by the moon_, exert a kind
+of dragging effect on the rotating earth. Doubtless this effect,
+measured by our ordinary standards, is very small; it is, however,
+continuous, and in the course of the millions of years dealt with in
+astronomy, this small but constant effect may produce very considerable
+results.
+
+But there is another effect which can be shown to be a necessary
+mathematical consequence of tidal action. It is the moon's action on the
+earth which produces the tides, but they also react on the moon. The
+tides are slowing down the earth, and they are also driving the moon
+farther and farther away. This result, strange as it may seem, does not
+permit of doubt, for it is the result of an indubitable dynamical
+principle, which cannot be made clear without a mathematical discussion.
+Some interesting consequences follow.
+
+Since the earth is slowing down, it follows that it was once rotating
+faster. There was a period, a long time ago, when the day comprised only
+twenty hours. Going farther back still we come to a day of ten hours,
+until, inconceivable ages ago, the earth must have been rotating on its
+axis in a period of from three to four hours.
+
+At this point let us stop and inquire what was happening to the moon. We
+have seen that at present the moon is getting farther and farther away.
+It follows, therefore, that when the day was shorter the moon was
+nearer. As we go farther back in time we find the moon nearer and nearer
+to an earth rotating faster and faster. When we reach the period we have
+already mentioned, the period when the earth completed a revolution in
+three or four hours, we find that the moon was so near as to be almost
+grazing the earth. This fact is very remarkable. Everybody knows that
+there is a _critical velocity_ for a rotating flywheel, a velocity
+beyond which the flywheel would fly into pieces because the centrifugal
+force developed is so great as to overcome the cohesion of the molecules
+of the flywheel. We have already likened our earth to a flywheel, and we
+have traced its history back to the point where it was rotating with
+immense velocity. We have also seen that, at that moment, the moon was
+barely separated from the earth. The conclusion is irresistible. In an
+age more remote the earth _did_ fly in pieces, and one of those pieces
+is the moon. Such, in brief outline, is the tidal theory of the origin
+of the earth-moon system.
+
+
+The Day Becoming Longer
+
+At the beginning, when the moon split off from the earth, it obviously
+must have shared the earth's rotation. It flew round the earth in the
+same time that the earth rotated, that is to say, the month and the day
+were of equal length. As the moon began to get farther from the earth,
+the month, because the moon took longer to rotate round the earth, began
+to get correspondingly longer. The day also became longer, because the
+earth was slowing down, taking longer to rotate on its axis, but the
+month increased at a greater rate than the day. Presently the month
+became equal to two days, then to three, and so on. It has been
+calculated that this process went on until there were twenty-nine days
+in the month. After that the number of days in the month began to
+decrease until it reached its present value or magnitude, and will
+continue to decrease until once more the month and the day are equal. In
+that age the earth will be rotating very slowly. The braking action of
+the tides will cause the earth always to keep the same face to the moon;
+it will rotate on its axis in the same time that the moon turns round
+the earth. If nothing but the earth and moon were involved this state of
+affairs would be final. But there is also the effect of the solar tides
+to be considered. The moon makes the day equal to the month, but the sun
+has a tendency, by still further slowing down the earth's rotation on
+its axis, to make the day equal to the year. It would do this, of
+course, by making the earth take as long to turn on its axis as to go
+round the sun. It cannot succeed in this, owing to the action of the
+moon, but it can succeed in making the day rather longer than the month.
+
+Surprising as it may seem, we already have an illustration of this
+possibility in the satellites of Mars. The Martian day is about one
+half-hour longer than ours, but when the two minute satellites of Mars
+were discovered it was noticed that the inner one of the two revolved
+round Mars in about seven hours forty minutes. In one Martian day,
+therefore, one of the moons of Mars makes more than three complete
+revolutions round that planet, so that, to an inhabitant of Mars, there
+would be more than three months in a day.
+
+
+BIBLIOGRAPHY
+
+ ARRHENIUS, SVANTE, _Worlds in the Making_.
+ CLERK-MAXWELL, JAMES, _Matter and Motion_.
+ DANIELL, ALFRED, _A Text-Book of the Principles of Physics_.
+ DARWIN, SIR G. H., _The Tides_.
+ HOLMAN, _Matter, Energy, Force and Work_.
+ KAPP, GISBERT, _Electricity_.
+ KELVIN, LORD, _Popular Lectures and Addresses_. Vol. i. _Constitution
+ of Matter._
+ LOCKYER, SIR NORMAN, _Inorganic Evolution_.
+ LODGE, SIR OLIVER, _Electrons_ and _The Ether of Space_.
+ PERRIN, JEAN, _Brownian Movement and Molecular Reality_.
+ SODDY, FREDERICK, _Matter and Energy_ and _The Interpretation of Radium_.
+ THOMPSON, SILVANUS P., _Light, Visible and Invisible_.
+ THOMSON, SIR J. J., _The Corpuscular Theory of Matter_.
+
+
+
+
+
+End of the Project Gutenberg EBook of The Outline of Science, Vol. 1 (of 4), by
+J. Arthur Thomson
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