'o THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA LOS ANGELES U. C. L, A. THE ELEMENTARY PRINCIPLES OF GENERAL BIOLOGY THE MACMILLAN COMPANY NEW YORK • BOSTON • CHICAGO DALLAS ATLANTA • SAN FRANCISCO MACMILLAN & CO., LIMITED LONDON • BOMBAY • CALCUTTA MELBOURNE THE MACMILLAN CO. OF CANADA, LTD. TORONTO THE ELEMENTARY PRINCIPLES OF GENERAL BIOLOGY BY JAMES FRANCIS ABBOTT PROFESSOR OF ZOOLOGY IN WASHINGTON UNIVERSITY Wefa gotfe THE MACMILLAN COMPANY 1920 All rights reserved COPYRIGHT, 1914, BY THE MACMILLAN COMPANY. Set up and electrotyped. Published January, 1914. Nortoooti -|fh«5S J. 8. Gushing Co. - Berwick & Smith Co. Norwood, Mass., U.S.A. > 19X0 " BEFORE the great problems [of Biology], the cleft between Zoology and Botany fades away, for the same problems are common to the twin sciences. When the zoologist becomes a student not of the dead but of the living, of the vital processes of the cell rather than of the dry bones of the body, he becomes once more a physiologist and the gulf between these two disciplines disappears. When he becomes a physiologist, he becomes, ipso facto, a student of chemistry and physics." D'ARCY THOMPSON, — " Magnalia Naturae." 626599 PREFACE IN this book I have endeavored to present in an elementary way some of the fundamental generali- zations that are the product of modern research in biology. The artificial division between the study of plants and that of animals is one that is becom- ing increasingly difficult to maintain, inasmuch as some biological principles are best illustrated by phenomena in the plant world, others by those of the animal world. I have tried, therefore, to utilize both aspects of the subject and to draw my illustra- tive material impartially from both kingdoms. The practice that insists upon the student getting his knowledge of natural science at first hand needs nowadays no justification. The laboratory method of study has shown itself to be not only the best means of acquiring a concrete and accurate knowl- edge of the science studied but also a primary pre- requisite for those habits of thought that are essential to what has come to be known as the " scientific method." Nevertheless in Biology the field is so broad and so varied that the student is very likely to lose sight of the fundamental principles that underlie all living nature. Moreover, these princi- ples do not grow out of the laboratory work so obviously nor are they so easily demonstrated by viii PREFACE experiment as is the case with such sciences as chemistry and physics. This book is accordingly planned to supply a background for a laboratory course in Biology and to supplement the facts ac- quired in such a course, the exact nature of which will depend upon the convictions or preliminary training of the individual instructor. On the other hand, it is believed that the general reader also will find here a simple statement of the fundamentals of General Biology, a subject that is becoming increasingly important in our everyday life. In covering so much ground I have been compelled to condense many subjects to paragraphs that might well have deserved whole chapters to themselves. The wide-awake teacher, I think, will have no diffi- culty in amplifying those portions that he esteems most important or in which he is most interested. I am conscious, too, of the fact that many generali- zations have been stated in a much less cautious way than would have been the case if condensation had not seemed so essential a feature. But, apart from this, I think that it is preferable, pedagogically, that a student should get a few clean-cut funda- mental ideas which perhaps require subsequent qual- ification than that he should have vague notions in which exceptions to rules figure as largely as the rules themselves. For instance, it is best that he should acquire the fact that the division of chromo- somes in mitosis is equal and that in consequence the number of chromosomes in an individual or a species is constant, leaving any consideration of the PREFACE ix accessory chromosome, important as it may be, to a time when the former concept shall have taken firm root. A chapter on Animal Behavior was projected but was abandoned when it was found that its inclusion would have increased the size of the volume unduly. For the same reason no apology need be offered for the constant reference by name without comment to the various groups of animals and plants. The first-hand knowledge of the types in the laboratory will have supplied the descriptive details for which there is no room in the present work, although text- figures have been freely used to illustrate the forms mentioned. In such a book as the present one, little can be claimed for originality except the manner of pre- senting the subject. I have sought counsel and criticism in those fields in which my personal knowl- edge is least dependable, and I hope that such errors as may have crept in* will not be significant ones. I am particularly indebted to Professor George T. Moore, Director of the Missouri Botanical Gardens, who read the whole book in manuscript, and to Pro- fessor Walter E. Garrey, who read the proof of the first four chapters. Acknowledgments are also due to the following for the use of cliches or permission to copy figures: to Herr Gust a v Fischer, Jena, for permission to use figures 7, 13, 34, 55, 60, and 82 ; to Messrs. Henry Holt and Co., for the use of fig- ures 8, 22, 49, 92, 94, 100, 106, and 112; to Messrs. Ginn and Co., for the use of figures 31 and 103; to x PREFACE the American Book Co., for the use of figures 2, 52, 97, and 109; to Messrs. G. P. Putnam's Sons, for the use of figure 16; to Messrs. D. Appleton and Co., for the use of figures 19, 58, 62, 67, 70, 90, 93, 95, 96, and 113; to Messrs. Longmans, Green, and Co., for the use of figure 23 ; to Messrs. P. Blakis- ton's Son and Co., for the use of figure 64 ; to Pro- fessor C. B. Davenport and the Editors of the Popular Science Monthly, for the use of figure 71 ; to Professor Davenport and Messrs. John Wiley and Sons, for the use of figure 73 ; to the Columbia University Press, for the use of figure 110; to Pro- fessor John Schaffner, for permission to copy figure 66; to Professor C. C. Curtis, for the use of figures 51 and 107 ; to the Editors of the American Review of Reviews, for the use of figure 108 ; and to Sir E. Ray Lankester, for permission to copy figure 112. For photographs from which were made figures 85, 98, and 99 I am indebted to the kindness of Profes- sor S. M. Coulter. All the other illustrations, with the exception of figures 6, 10, 24, 25, 37, 40, 42-44, 50, 54, 56, 57, 63, 74, 77; 88, 89, and 114, are from publications of The Macmillan Co. J. F. A. JANUARY, 1914. CONTENTS CHAPTER PAGE I. LIVING SUBSTANCE 1 Living and Non-living 2 Life and Death. Elemental Death .... 4 Chemistry of Protoplasm 8 Proteins, Fats, and Carbohydrates .... 10 Physical Structure of Protoplasm 14 Organization of Protoplasm 17 The Cell 18 II. THE PRIMARY FUNCTIONS OP THE ORGANISM 26 Cellular and Non-cellular Organization .... 26 Functions of a Free Cell 28 Locomotion, Ingestion, Digestion, Egestion, Assim- ilation, Irritability, Reproduction ... 29 Specialization in Locomotor Organs .... 32 Specialization in Conducting Organs .... 39 Secretion 41 Specialization in Digestion ...... 42 Summary ......... 45 Specialization and Differentiation .... 45 Tissues, Organs, Systems 46 Homology and Analogy 47 [II. METABOLISM 48 Oxidation 48 Conservation of Energy ...... 50 Chemical Synthesis in the Organism .... 52 Photosynthesis 54 Production of Fats and Proteins .... 55 Dissimilation . 56 Metabolism in Animals . 57 Foods in General . ....... 60 xi xii CONTENTS CHAPTER PAGE Fate of Foods in Higher Animals . .... 61 Role of Oxygen in Metabolism . . . . , 62 Aerobic and Anaerobic Forms ..... 63 Combustion and Respiration . . . . .64 Poisons and Antiseptics 66 Cycle of the Elements in Organic Nature ... 68 The Nitrogen Cycle 71 Destruction of Organisms 72 Putrefactive Organisms . ... 73 Denitrification and Nitrogen Fixation ... 75 Nature of Energy Transformed 77 Movement 77 Heat 79 Electricity 79 Light . . .80 Enzymes and Enzymatic Action .... 82 Internal Secretions and Hormones .... 86 IV. GROWTH 90 Cumulative Integration 91 Amitosis and Mitosis . 92 Abnormal Mitosis 97 Nature of the Centrosome 98 Influence of External Conditions on Growth . . . 100 Light and Heat 101 Chemical Agents 102 V. TISSUE DIFFERENTIATION FOR SPECIFIC FUNCTIONS . . 104 Differentiation in Animals .... .104 Alimentary System 104 Sensory Organs, — Cephalization .... 107 Skeletal Structures . . . . . . .110 Endoskeleton 110 Exoskeleton 112 Muscular System . . . . . . .113 Circulatory System . .' . . . . .114 Excretory Organs . 117 Differentiation in Plants 121 Plant Movement 123 CONTENTS xiii Supporting Structures 123 Circulatory System 126 Alimentary System 126 VI. ONTOGENESIS 129 Biogenesis and A biogenesis 130 Reproduction as a Growth Process . . . .181 Fission in Metazoa 132 Fission in Lower Plants . . . . '_ * . 135 Temporary Budding . . " . . .136 Permanent Budding 139 Spore Formation . . . . ~ • . . 140 Sexual Reproduction . . . ... . .141 Total Conjugation 142 Isogamy . . . 142 Anisogamy 144 Sexual Differentiation . . . • . .147 Partial Conjugation «• 151 Cytoplasmic Conjugation (Plastogamy) . . 151 Nuclear Conjugation (Karyogamy) . . . 152 Nuclear Phenomena of Zygosis in Animals . . 155 Cleavage 158 Gastrulation . . . . . . .161 Further Differentiation . ... 162 Conjugation in Protozoa 164 Parthenogenesis 167 Artificial Parthenogenesis 169 Alternation of Generations in Animals . . 171 Sexual Reproduction in Plants .... 174 Liverworts and Mosses .... 175 Ferns 176 Seed Plants 177 Germination of the Megaspore . . . 179 Germination of the Microspore . . .179 Parthenogenesis in Plants .... 182 Apogamy 183 The Probable Evolution of the Plant World . .183 Morphogenesis 185 Regeneration . . 185 xiv CONTENTS CHAPTER PAGE Regulation 187 Heteromorphosis 189 Theories of Morphogenesis 190 Preformation 191 Epigenesis 192 " Weismannism " . . . . . . . 193 Vitalism and Mechanism . . ... .194 Summary 196 VII. VARIATION AND HEREDITY 198 Variation 198 The Law of Frequency of Error . . . .200 Types of Variation Curves 202 Asymmetrical Variation .... 204 Discontinuous Variation .... 204 Mutations 207 Correlated Variability 209 Effect of Life Conditions on Variation . . .211 Causes of Variation ...... 212 Heredity 214 Heredity and Inheritance 214 Individual Heredity and Racial Heredity . .216 Galton's Law of Ancestral Inheritance . .217 Filial Regression 219 Effect of Selection in Heredity . . . .220 Pure Lines 221 Unit Characters and Mendelian Inheritance . 223 Sex-limited Inheritance . . . .233 Economic Aspects of the Subject . . .234 The Inheritance of Disease . . . .236 The Inheritance of Defects . . . .239 Eugenics 240 VIH. ORGANIC RESPONSE 242 Environment 243 The Usual Conditions of Environment . . 244 Temperature 244 Light .245 Chemical Environment 245 CONTENTS xv • CHAPTER PAOE Nature of Organic Response 246 Electric Response 247 Individual Response to Unsymmetrical Stimuli . 248 Adaptive Response 253 Immunity 255 Morphogenetic Response 256 Non-adaptive Morphogenetic Response . . 257 Influence of Food 258 General Adaptation 259 Some Types of Adaptation 260 Aquatic Organisms 260 Aerial Adaptations 262 Subterranean Adaptations . . . .263 Protective Adaptations 266 Protective Coloration . . . .267 Specific Resemblance .... 268 Aggressive Resemblance .... 269 Mimicry 269 The Care of the Young .... 272 Environmental Adaptations of Plants . . . 274 Adaptations for Seed Dispersal . . .278 Associations of Animals ....... 279 Commensalism ........ 280 Parasitism in Protozoa 283 Parasitism in Worms 285 Parasitism in Insects 287 Sacculina 289 Associations among Plants 290 Lichens . .291 Parasitism in Plants 292 Associations of Plants and Animals .... 293 Grafts . 294 IX. SPECIES AND THEIR ORIGIN 296 Meaning of Species 296 Polymorphism 301 Elementary Species and Linnsean Species . . 305 CONTENTS PAG The Origin of Species 30 Evidence for the Evolution of Species in the Past . 30 History of the Elephant 30 Vestigial Structures 31 "Darwinism" 31 Lamarck's Theory .31 Critique of the Darwinian Theory . . . .31 Critique of the Lamarckian Theory . . .32 Conclusion . 32 THE ELEMENTARY PRINCIPLES OF GENERAL BIOLOGY GENERAL BIOLOGY CHAPTER I LIVING SUBSTANCE BIOLOGY, the "science of life," includes in its broadest aspects the investigation of all that per- tains to the structure and functions of living things. The observing and recording of the wonderful variety of Nature will always have a fascination not only for the poet, but for the scientist as well. But the latter is more especially concerned with the meaning, the analysis, or the explanation of natural phenomena. Philosophy tells us that science can never hope to get the ultimate explanation of anything which it observes. All that it can do is to reduce the complexities to simpler expression, to find the common denominator for things that seem .at first glance unrelated, in the same way that the mathematician by processes of factoring reduces elaborate and complex algebraic expressions to simple statements of relation. And, just as in mathematics, the greater the number of variables we have to deal with, the more involved and difficult becomes our computation, so in physical and biological science the greater the number of 2 GENERAL BIOLOGY unknown factors there may be, the greater becomes our difficulty in reducing them to fundamental principles. This is why biology is so strikingly an "inexact" science in comparison with physics or inorganic chemistry. Yet, it is not necessary even for the physicist or the chemist to know what is the ultimate nature of matter or force or electricity or atoms in order to study such things and formulate general laws based on such observation ; nor is it necessary for the biologist to concern himself with the meaning or nature of life in order to find out what principles govern in the world of living things. The study and comparison of the structures of plants and animals, of their methods of growth and reproduction, their relation to each other and the world about them, has revealed the fact that there is an underlying unity in nature that makes it pos- sible for us to sum up our observations in general principles, incompletely understood, of course, but more or less applicable to all living things. The consideration of these general principles forms the basis for a General Biology in the sense in which it will be taken in the present work. Although we shall not attempt to elucidate life in any philosophical sense, it is of interest, notwith- standing, to discover at the start just how much science can tell us of the nature of life, or of living things as a whole. Living and Non-living. — If a biologist should ask the average layman whether he could tell the LIVING SUBSTANCE 3 difference between something alive and something that is not, he would hardly be taken seriously. Yet, if such a layman should be pressed to define just what he meant by "being alive," he might be hard put. It might be assumed that some charac- teristic chemical compounds are to be found in liv- ing matter which are absent in non-living matter. But thousands of exact chemical analyses have been made of every sort of living thing and no ele- ment or compound has ever been found which is essentially different from what may exist in the non-living world. Long ago a distinction used to be made between "organic" and "inorganic" substances, — the former being the product of living "organisms." But such a distinction has broken down. It is possible to synthesize substances in the test tube, identical in chemical composition with those formed in Nature's laboratory, — the tissue of plant and animal. Indeed, the ability to artificially reproduce natural products in this way has proved of great value commercially, and arti- ficially synthesized indigo, camphor, etc., now supplement in large measure Nature's meager store of such things. Nor is it easier to discover any unique physical phenomena in living things. So far as we can observe, — and the more our observations are extended, the more is the conclusion confirmed, — living matter obeys the same physical laws that obtain in the rest of the universe. Again, living things grow : but so do crystals and clouds. They 4 GENERAL BIOLOGY reproduce themselves, but, as we shall see later, this is but a discontinuous form of growth, and may be paralleled, perhaps, in other " inorganic " bodies. Life and Death. — If we find it so difficult to point to any one thing as the touchstone of living matter contrasted with non-living matter, what shall we say of the difference between that which is alive and that which has been, but is no longer, — in other words between living matter and dead matter ? A turtle may justly be called a dead turtle if we cut off its head, yet, if we cut out the heart of such a decapitated turtle and suspend it on hooks in a moist chamber, wet with a weak solution of common salt, such a heart will go on beating rhythmically for days. So long as it beats we are forced to consider the substance composing it as living matter. We must make a distinction, then, between general lije and death, which affects the whole or- ganism and elemental life and death, which affects only the elements or tissues. This distinction is much more apparent in animals than in plants on account of the greater degree of specialization in the former. Ordinarily, decay and disintegration in the tissues promptly follow general death, but experimentally we may avoid this contingency if we exclude bacterial invasion,1 and such a piece of tissue may be kept passively " alive " for a consider- able interval of time, regaining its functions when replaced in a living organism. In this way sections 1 See Chapter IV. LIVING SUBSTANCE 5 of blood vessels and other organs have been cut out and later replaced in the same or other animals without injury. By keeping 'such a tissue at a trifle above freezing point the period of suspended vitality may be extended to weeks or months.1 Recent experimenters have shown that not only may the pieces of excised tissue be kept passively alive, but that under proper conditions they will sprout and grow like so many plant cuttings. It is only necessary that they be surrounded by a nutritive medium drawn from the same animal from which they came, and that they be kept free from all bac- teria. If the turtle heart in the experiment described should after a while cease to beat, but later begin to do so again, we would of course say that, like the excised tissues just described, it was still alive during its period of inactivity, although our only knowledge of its being alive is derived from its subsequent beating. For, we say, our idea of life, however vague it may be, does not admit of discontinuity. Once alive, always alive, until dead. The experimental physiologist is not so sure of that. Such a suspended heart muscle of the turtle will not beat except in the presence of salt (or some sodium compound). But in pure salt solution it stops beating. The pure salt acts as a poison. We might now consider the muscle dead, were it not 1 We find an analogous instance in Nature in the fact that many seeds will retain their vitality unimpaired for years, until proper condi- tions of warmth and moisture cause them to sprout and grow. 6 GENERAL BIOLOGY for the fact that if we add a little calcium chloride to the salt solution, the heart begins to beat again. The calcium has neutralized the ill effect of the sodium. If, then, contracting is any criterion of whether the heart is alive or not, its life would seem FIG. 1. — A tardigrade (Macrobiotus) : a, in the creeping active con- dition ; 6, dried, in the state of apparent death. to depend upon the presence or absence of something wholly outside the living matter itself. Under perfectly natural conditions such a state of "ceasing to live " and then resuming life again is not uncommon. Some animals that live in shallow puddles exposed to the chance of drying up are capa- LIVING SUBSTANCE 7 ble of drying up also, and remaining in an apparently lifeless condition for long periods of time, blown about in the dust by the winds (fig. 1). Falling in a favorable spot where there is sufficient water, they " come to life again " and resume their activity as if it had never ceased. The excessively minute " germs " of bacteria keep the race in existence in this way. In seeking an analogy to these phenomena a German scientist, Preyer, has compared the plant or animal to a clock, which goes through its charac- teristic movements so long as the energy in its mainspring lasts. It may be stopped and remain so until its pendulum is set swinging again, in which case it may be compared with a fertile seed. But if its mainspring be broken or if it run down, even though externally it be just the same in appearance, it no longer " goes." Some such a difference as this may exist, not to push the comparison too far, between an organism in which life is merely suspended and one that is dead.1 Our search, therefore, for the answer to the ancient 1 Waller has shown that in such forms of living substance as nerves, which do not contract or give any visible evidence of life or death, it is possible by galvanometric test to show that a "live" nerve deflects the needle, whereas a "dead" one does not; in other words that the electric response is a very delicate and accurate sign of life. By this means he claims to have been able to mark the "beginning of life" in an incubating hen's egg. A Hindu physiologist, Professor Bose, has claimed, on the basis of very careful experimental work, that this electrical sign of life is dependent upon the "molecular mobility" of the matter, and that it disappears when "molecular fixation" or strains ensue. Herein may be, possibly, the simple difference between living and dead matter. 8 GENERAL BIOLOGY conundrum, " What is life ? " so long as we attempt to solve it by processes of analysis, leads us up a blind alley no matter what clue we follow. Yet, in spite of our difficulty in defining living matter, we recognize the existence of it as something real and not imaginary, and when we compare the in- numerable kinds of living things from the standpoint of their physical and chemical composition, we find that they all have much in common ; that life, whatever its metaphysical aspects, has also a mate- rial basis, a " life stuff " or living substance. This living substance has received various names, but that which is most commonly used is Protoplasm.1 This is what Huxley called in enduring phrase " the physical basis of life." Chemistry of Protoplasm. — In studying the physics and chemistry of protoplasm we find that it is exceedingly complex. But its complexity arises from the almost infinite combinations and permutations of a very limited series of chemical elements. Carbon, hydrogen, oxygen, nitrogen, — these we find in all protoplasm, and they constitute its bulk. Sulphur and phosphorus are also always present, but in very much smaller quantities, and usually chlorine, potassium, sodium, magnesium, calcium, and iron as well. Many other elements occur normally, though rarely, in the protoplasm of certain animals and plants. Iodine occurs in sea- 1 Bioplasm, a term used by many English authors, is perhaps prefer- able, but protoplasm is firmly established in the literature of biologv LIVING SUBSTANCE 9 weeds and in the thyroid gland of certain animals. Zinc and manganese seem to be normal constituents of the tissues of some mollusks. In other words, protoplasm is not a definite chemical substance or compound, like quartz or salt or starch, but is some- times one thing, sometimes another. Rather, it is a mixture of various things, all of them, however, of an infinite complexity of mutual relations, — " a mixture, but certainly no jumble." The word "protoplasm," then, is a sort of group name covering a multitude of different sorts of such chemical mix- tures, as many as there are different manifestations of life phenomena. It has been just said that the bulk of all kinds of protoplasm is made up of carbon, hydrogen, oxygen, and nitrogen. These exist in elaborate combina- tions, of which the carbon atom seems, as a rule, to form the heart or foundation. The older "or- ganic chemistry," or the chemistry of organisms and their products, has become the " chemistry of the carbon compounds." An exception must be made to the statement that the combinations of the four elements cited are always complex. One of the simplest of all compounds, water (H2O), is a necessary constit- uent of living matter. The percentage of water in all protoplasm is high. Muscles are three fourths water, even bones, nearly one fourth, and in the jellyfishes of the open sea, that which is not water is but one per cent or less of the total bulk. With these facts in mind, one is inclined to think of living 10 GENERAL BIOLOGY organisms as liquids that contain solid matter rather than as solids with a percentage of liquids. The large amount of water in protoplasm is a very im- portant and significant feature of its make-up, since it affords a means for the transfusion of sub- stances from one part of the animal or plant to another, and gives the organism a certain necessary plasticity as well.1 The combinations of carbon (C), oxygen (O), hydrogen (H), and nitrogen (N) that make up the bulk of protoplasm fall into three great groups or classes, the proteins or albumens, the carbohydrates (sugar and starches), and the fats. These three groups are much more easily described than defined. The Proteins are found in all protoplasm and are indispensable to the processes of life. They consti- tute a large and diverse group differing widely one from another, but all sharing certain group charac- 1 Recent advances in physical chemistry have thrown much light on the physical and chemical processes of protoplasm. It has been dis- covered that, in great dilution, many chemical substances tend to dis- sociate into ions, as e.g. NaOH into Na and OH, each ion consisting, in the current explanation, of an atom or atomic group bearing a charge of negative or positive electricity. As a rule, it is only in this dissociated condition that atoms are active in combining with one another. It has been found by experiment that the effect of certain salts on protoplasm (for instance the poisonous action of the heavy metals) is in direct pro- portion to the ionization of the salts. It has been shown also that cer- tain salts are absolutely essential to life processes, although the amount required may be very minute. A fresh-water crustacean, Gammarus, according to W. Ostwald, inevitably dies if placed in absolutely pure distilled water, but will live indefinitely if a trace of common salt (NaCl) be added. The amount necessary is only eight ten-thousandths of a gram (twelve thousandths of a grain, Troy) to a liter of water. LIVING SUBSTANCE 11 teristics. Among these are the invariable presence of nitrogen along with the carbon, hydrogen, and oxygen, and a very large and complex molecule, which is always laevo-rotatory, i.e. turns the rays of polarized light to the left, and contains sometimes thousands of atoms.1 About half the weight is car- bon and 15 per cent to 18 per cent nitrogen. A giant protein molecule is not exactly a unit in itself, but is usually an aggregation of smaller atomic groups or other protein molecules weakly held to- gether by the bond of " chemical affinity," just as a village of five hundred inhabitants may be thought of as made up not exactly of that many individuals, but of one hundred families composed in turn of an average of five persons each. The atomic groups may freely break away from the protein molecule or be added to it, and in this instability lies the great significance of the presence of the proteins in all living matter. (See chapter on Metabolism.) As examples of nearly pure protein may be mentioned lean meat fiber and white of egg (albumen). J. Loeb and Pauli have called attention to the strong probability that the proteins in animal proto- plasm are united with the ionized 2 inorganic salts to form "ion-protein compounds." This hypothesis accounts for many otherwise inexplicable phenom- ena, and explains the importance of even extremely minute quantities of certain salts in life processes. 1 An analysis of the blood pigment of the horse has yielded the fol- lowing formula : 2 See page 10. 12 GENERAL BIOLOGY The Carbohydrates (as also the Fats) lack the nitrogen characteristic of protein. They contain only carbon, hydrogen, and oxygen, the latter two always present in the proportion found in water, twice the number of hydrogen atoms as of oxygen. They are simpler in structure than the proteins, but like them may be combined into molecular aggre- gates of higher degrees of complexity. The simple sugars or monosaccharids, of which dextrose or glucose is the most familiar, have the formula C6Hi2O6. By combining two molecules of a monosaccharid with the loss of a molecule of water l a disaccharid may be formed, of which the most familiar example is cane sugar (sucrose). Under the influence of yeast a monosaccharid will break up into carbon dioxide and alcohol, a process known as fermenta- tion. By continuing the addition of monosaccharid molecules, one to another, each time with the loss of a molecule of water, more complex sugars, the polysaccharids, are formed. These are the starches and dextrines, and, most familiar of all, cellulose and woody fiber. The ^carbohydrates in general are more abundant in plants than in animals, al- though one of them, glycogen, which is found abun- dantly in the liver and muscles of higher animals, is, of very great importance in animal nutrition. The Fats contain the same elements found in the carbohydrates, C, H, and O, but in different proportions and arrangements. In every case they are the result of the combination of an acid with ' C,H120, + C6H120, = LIVING SUBSTANCE 13 glycerine. The acids, with few exceptions, belong to the *' fatty-acid " series. Three molecules of fatty acid, not necessarily of the same kind, combine with one of glycerine to form a fat. The cleavage and recombination of these two component parts of a fat (the fatty-acid element and the glycerine) is FIG. 2. — Milk under the microscope, showing the nature of an emulsion. The spheres are fat globules, the dark rods lactic-acid- forming bacteria. (From " Elements of Biology," copyright, 1907, by George William Hunter. — Permission of the American Book Co., publishers.) easily accomplished. When the fatty- acid portion combines with an alkali, it forms a soap. Fats are insoluble in water, but readily soluble in ether, ben- zine, etc., — a fact made use of in the cleaning of clothing. They may be shaken up in alkaline water until the particles become very finely divided and remain in suspension, forming an emulsion. The most familiar example of such an emulsion is milk. 14 GENERAL BIOLOGY Physical Structure of Protoplasm. — Proteins are not soluble in water in the usual sense, that is, they do not make a clear solution as do sugar and salt. They do absorb a great quantity of water, however, and swell up enormously. In the presence of large amounts of water they may become very finely divided and form permanent aqueous suspensions, which differ from true solu- tions in that they will not diffuse through vegetable parchment or animal membranes. Such substances are usually known as " colloids," in contrast to ** crystalloids " or substances which do" diffuse through such membranes. Another characteristic of colloids is their property of coagulating or " set- ting," a familiar example of which may be observed in the hardening of the white of an egg in the pro- cess of boiling. Such coagulation may be produced by heat, electric currents, dehydration, and chemical reagents. Certain classes of colloids, like the egg albumen, are unalterable when once coagulated, and are known as irreversible. Others may be brought back to the fluid state any number of times. Such substances, of which gelatin is an example, are called reversible. The essential difference between " solu- tions " of colloids and of crystalloids consists in the fact that the particles of the former are larger and are enveloped in a film of water, whereas true solution involves the separation of the crystalloid into its molecules or even into its ions, in which con- dition the particles obey the law of gases and the solution exerts pressure (osmotic pressure) in all LIVING SUBSTANCE 15 directions. The colloidal nature of the proteins, therefore, is probably to be attributed to the great size of the molecules of which they are composed, aggregates which in fact are not true molecules but composites of other smaller aggregates. Chemists refer to this welding together of molecular aggre- gates as polymerization. As we shall see later, the process of animal digestion involves merely the breaking up of these aggregates into others of lesser degree, small enough to diffuse through the lining membranes of the alimentary canal. Protoplasm, being composed largely of proteins, is thus colloidal in its physical make-up. But the examination of living protoplasm with the high powers of the microscope reveals a structure much more complex than may be found in a mere lump of non-living colloid. Living substance has a characteristic physical structure of its own, to ex- plain which several theories have been advanced. According to one, the essential basis of protoplasmic structure is granular, and granules are certainly to be found in protoplasm. Others find fibrils like detached threads, others see a skein or reticulum in the meshes of which more watery substances are held. The view first advocated by Biitschli is, however, the one most commonly held by biologists. According to this theory, protoplasm has the struc- ture of a foam in which the denser parts surround the lighter as the film of water does the air in soap bubbles. Perhaps a more accurate object of com- parison would be a fine emulsion. In an 'emulsion, 16 GENERAL BIOLOGY instead of a gas and liquid (as in soapsuds), we have two liquids of different densities and qualities, form- ing what is known as a diphasic system. According to Biitschli protoplasm is such an emulsion, com- posed on the one hand of substances insoluble in water which are highly viscous or sticky, and on the other hand of a watery medium supporting the various-sized particles of the former. Owing to the fact that the refractive indices of the different components of protoplasm are nearly the same, it is very difficult to see its structure in the unaltered living state. Biologists have recourse, therefore, to " fixing " or coagulating the protoplasm with various poisons and dyeing it with aniline or other colors. Under such circumstances protoplasm appears to have a skein or net structure. But this has been interpreted as being merely the appearance of particles caught and held by surface tension — the films of the bubbles, so to speak, which when set and viewed in connection with those of adjacent bubbles appear to form a continuous layer or thread. This view is justified by the fact that artificial emul- sions, subjected to the same processes of fixation and staining employed in the study of protoplasm, show a strikingly similar appearance to that of living substance. More recent investigations, in- volving the dissection of living protoplasm under the highest powers of the microscope, seem to point to the conclusion that Biitschli's conception may require certain modification. The physical " phases " in which protoplasm consists appear to be all col- LIVING SUBSTANCE 17 loidal, differing only in the amount of water which they absorb, an amount which may be rapidly altered under varying circumstances. Organization of Protoplasm. — Not only is pro- toplasm, chemically considered, a mixture of a great variety of substances, but this aggregate of ma- terials composing a given mass of living matter in one kind of animal or plant differs from that of another. Moreover, the same mass of protoplasm is constantly changing with regard to the substances composing it, thus making impossible anything like a fixed and definite picture of its exact composi- tion. But it is also true that it is not so much the substances composing it, as the relations, both physi- cal and chemical, that these bear to one another that determines the character of the protoplasm. A comparison may make this clearer. A watch is a delicately constructed and complicated mechanism of many parts which by their action in moving the hands in certain fixed relations of time and space enables us thereby to tell the time of day. It is possible for any one to take such an instrument apart and make a little pile of the wheels, screws, and springs, but when he has done so, the mass of metal and jewels that he holds in his hand is no longer a watch and cannot be made to serve its original purpose. The very apparent reason is that the inherent quality of a watch, by virtue of which it is a watch, that is, a timekeeper, is involved not only in the parts composing it, but in their relations to 18 GENERAL BIOLOGY each other as well, and such a mechanism will run correctly only when every part is in its proper place and adjusted carefully to every other part with reference to the joint action of the whole. It is so with protoplasm. However we may de- fine life it is certainly true that the property of protoplasm, by virtue of which it is " living matter," is bound up in the interrelations of the various parts composing it. And just as a machine must be properly assembled, .to use a technical phrase, so protoplasm does not exist as living substance except it be organized . Destroy or fundamentally alter this or- ganization and, although we may get the same chem- ical analysis of the material and the same weight of substance, it is no longer alive. But it must be noted that the comparison with the watch cannot be pushed too far, for, unlike a rigid mechanism, protoplasm is extremely plastic and capable of adjusting itself to very wide ranges of structural alteration without ceasing to be alive. Indeed the living organism is constantly so adjusting itself in response to external conditions, — a phenomenon which many hold to be the fundamental and charac- teristic fact of life itself. The Cell. —The difference between an oak leaf and a waxen image of one is not alone a difference in the chemical substances composing the two. Should we cut a thin slice from the wax leaf and examine it under a microscope we could distinguish nothing to mar its homogeneity. Should we do LIVING SUBSTANCE 19 the same with the leaf itself we would find at once that the leaf bears to its waxen counterfeit the same relation that a mosaic figure does to a photograph. Instead of being homogeneous in structure it is composed of innumerable little units, — the aggre- gate of which makes up the mass of the leaf. These structural units of organization of proto- plasm are called cells. The word owes its deriva- FIQ. 3. — Section of a leaf, showing its cellular composition : a, a breathing pore or stoma ; b, upper layer of " palisade cells " which con- tain most of the chlorophyll ; c, epidermal cell. (Bailey.) tion to the fact that the discoverer of the first cells described found, in examining a thin slice of cork, that the cork was made up of little boxes like the cells of the honeycomb. Similar observations were made later on a great variety of tissues until it was established that all plants and animals are composed of cells as structural units in much the same way that a house is built of individual bricks or stones. Of course the shapes and structures of these units vary greatly in accordance with the kinds of tissues in which they are found or the activities they 20 GENERAL BIOLOGY reveal, but there are certain structures that are common to all cells, and they may be considered fundamental in cell organization. Postponing for a moment the consideration of the outer limit of the FIG. 4. — Diagram of a composite cell. M, metaplasmic bodies; v, vacuoles ; pi, plastids (chloroplasts) ; c, attraction sphere inclosing a double centrosome ; nu, true nucleolus ; chr, chromatin network ; /, linin network ; k, karyosome or chromatin nucleolus. Vacuoles are especially characteristic of plant cells. The chloroplasts are found only in plant cells'. They are capable of independent growth and reproduction. It is these that give the green color to leaves. — (From Wilson.) cell or cell-wall, we find that all cells agree in having the protoplasm composing them differentiated into two parts, — a more or less central one, — the nucleus, often rounded in outline and somewhat dense in con- sistency, which is surrounded by a less dense area, — LIVING SUBSTANCE 21 the cytoplasm. Nucleus plus cytoplasm together make up the cell. Bounding the cytoplasm there may be a definite, often thickened cell-wall. This is especially charac- teristic of plant tissues, in which the cell- wall becomes very thick and rigid through the deposit of various carbohydrates (pectin, cellulose) . It is the latter (or its derivative, lignin) that gives its characteristic rigidity and hardness to wood. In animals, on the other hand, in only one rather obscure group 1 does cellulose occur, and it is the exception rather than the rule for the cell-wall to attain any considerable thickness or prominence. In many animal cells there is no cell- wall at all, — the viscosity and surface ten- sion of the mass of protoplasm holding it together. The nucleus and cytoplasm are found by delicate tests to be chemically different, the former combining more readily with basic and the latter with acid sub- stances. The cytoplasm often contains vacuoles filled with a watery fluid or with different sorts of non-living substances, such as crystals of silica (that give the knife-edge to certain kinds of grass), chlorophyll bodies (that give the green color to plant leaves), starch grains (as in the potato), yolk granules (such as make up the bulk of the yellow of a hen's egg), and various other substances. Such substances, being non-living material, manu- factured by the cell, are called metaplasm in contra- distinction to the living protoplasm. In the cytoplasm is also found the centrosome, 1 The Ascidians or "Sea Squirts." 22 GENERAL BIOLOGY a structure that appears only at certain periods of the cell's activity and is either invisible or non- FIG. 5. — Various kinds of cells : A, female germ-cell, ovum of the cat ; B, male germ-cell, spermatozoon of a snake, Coluber ; C, ciliated epithelium from the digestive tract of a mollusk, Cyclas ; D, cartilage of a squid ; E, striated muscle fiber from an insect larva, Corydalis cornutus; F, a nerve cell from the cerebellum of man. — (Dahlgren and Kepner.) existent throughout the greater part of cell life. (See under Cell Division, Chapter IV.) The nucleus (sometimes called karyoplasm in LIVING SUBSTANCE 23 contrast to cytoplasm), owing to the fact that it has about the same refractive index as the cyto- plasm, is usually almost or quite invisible in living cells, and must be fixed and stained before it can be easily seen. Its outline is always sharply distinct from the adjacent cytoplasm, and often a limiting membrane seems to be present, though the presence of the latter in all living cells is not definitely estab- lished. Within the nucleus the protoplasm is further dif- ferentiated into two substances, — one that stains very readily with most dyes, and for that reason is called chromatin, and another (the linin) that stains with great difficulty, and looks like a sort of network or scaffolding supporting the chromatin. Both chromatin and linin are surrounded by a watery transparent fluid sometimes termed hyaloplasm. In many cells, especially egg-cells of animals, a nucleolus is prominent, — a rounded aggregation of chromatin material which is found to be chem- ically different from the true chromatin, but the nature and function of which has never been clearly understood. The nucleus owes its acid nature to the chromatin, which is largely made up of some form of nucleic acid, a complex substance having a characteristic percentage of phosphorus. When fixed and stained, the chromatin usually appears in the form of gran- ules of either large or minute dimensions, sometimes arranged in the form of a skein or network, with " knots " at the intersections. Only in cells that GENERAL BIOLOGY o are in process of reproduction is there any definite- ness of form, shape, or number to the chromatin aggregates. Usually we find only one nucleus in a cell, but sometimes many nuclei occur, scattered through the cytoplasm. In plants the cell- wall is an important part of the cell, since it affords the necessary rigidity and strength to \\ the plant, funo- tioning in this way much as the skeleton does in animals. But we find that the pres- ence or absence of a cell-wall is conditioned largely if not. en- tirely by such a demand. In tis- sues where there are strains and stresses to be borne, cell-walls develop in response to such stimuli, but where they are not necessary they do not appear, or only partially develop. Such tissues, however, are found to have as many nuclei as if they were cut up by cell-walls into individual cells. They con- sist, as it were, of a mass of cells "run together," and for that reason are called coenocytes or syncytia (singular, syncytium). The presence of the 'nucleus m. B FIG. 6. — Sections of the outer epithelium (skin) of a creeping Ctenophore (Caeloplana) : A, a ciliated region in which the cells are pro- vided with locomotor organs in the form of vibratile cilia, a gland-cell at the left ; B, a non-ciliated region of the same epithelium ; the cell-walls are barely indicated (except in the gland cells) ; nuclei are, however, abundant. LIVING SUBSTANCE 25 seems to be necessary to the cytoplasm in order for it to properly carry out its functions, for we find that a fragment of a cell, provided it has a bit of nuclear matter included, will continue to live, whereas a bit of cytoplasm without a nucleus soon disintegrates and dies. The nucleus, in other words, is essential for the life of the cell, and, accordingly, since the presence or absence of a cell-wall is not a determin- ing character, we may consider the essential features of a cell to be a mass of protoplasm dominated by a nucleus. The cell is thus a dynamic or functional unit rather than a static or structural one. On this account the term energid has been used in place of the less accurate word " cell." CHAPTER II PRIMARY FUNCTIONS OF THE ORGANISM WE have seen that protoplasm, although it may be resolved into a mixture of various complex chem- ical substances with a more or less definite physical structure, does not exist as protoplasm per se, but is always organized in certain definite relations, one part to another and to the environment of the whole. The unit of this organization is the cell. The individual organism usually consists of many cells linked together in a complex whole, but the individual may also subsist in a .single cell. In the latter case we speak of the individual as a Protozoan (or Protophyte, if a plant), and in the former as a Metazoan (Metaphyte, if a plant). As a rule the one-celled organisms (sometimes spoken of collec- tively as Protista) are simpler both in structure and in function than those of many cells. Such, how- ever, is not always the case. The most complex of the Protozoa is as specialized in organization and functions, if not more so, than the simplest of the Metazoa. It would probably be more accurate to speak of the Protozoa as non-cellular rather than one- celled, since the differences between the two groups are qualitative rather than quantitative, i.e. are not based on the number of cells present. So from PRIMARY FUNCTIONS OF THE ORGANISM 27 another point of view we may consider the protozoan individual to be comparable to the metazoan indi- vidual, with the difference that for purposes of utility the body of the latter is subdivided into a great number of dynamic centers (cells), whereas that of the former is not so divided. FIG. 7. — A white blood corpuscle (leucocyte) of the frog sketched at frequent intervals ; from n to m the temperature was gradually raised, then lowered (n to p).^(From Hertwig, after Engelmann.) In the metazoan body, however, some of the cells live a relatively free and independent existence, and we will begin our study of the phenomena of cell specialization with such a cell. In the blood stream of most animals are to be found 28 GENERAL BIOLOGY free cells, " corpuscles," floating in the liquid plasma. In vertebrates the majority of these cells are of definite shape and are the carriers of the charac- teristic red pigment of blood. Such corpuscles are also found in a few worms and other lower organisms. In addition to these there occurs in nearly all meta- zoans in which there is any blood or body-fluid another sort of free cell, leucocytes or amoebocytes, distinguished from the former by the lack of red pigment and especially by the absence of any defi- nite shape or bodily outline. If we examine the blood of an earthworm or a crayfish with a microscope, we may study these cells with comparative ease. If the plasma containing such cells be kept slightly warm, these leucocytes will be found to change form continually. Short processes flow out from the cell- body in different directions, and the rest of the pro- toplasm appears to flow or be pulled along after them. In this way the cell is able to progress slowly over the slide of the microscope .or over the walls of the blood-vessels in which it normally occurs. In other words the cell possesses the function of locomotion. The lobelike processes (called pseudopodia or " false feet ") are protruded at any part of the cell-body or on several parts at the same time. This function of locomotion is therefore unlocalized. The surface of the cell appears to be somewhat sticky (viscous) and retains a hold on solid objects. When the cell is creeping, a pseudopodium sticks in this way to something solid, the protoplasm then contracts, and the rest of the cell is pulled along with a flowing PRIMARY FUNCTIONS OF THE ORGANISM 29 movement. The actual basis for these creeping movements is thus the contractility of the protoplasm constituting the cell. In the course of these creeping or " amoeboid " movements the leucocyte may encounter a bit of worn-out tissue or a vagrant bacterium. It then throws out pseudopodia on both sides of the object, which flow around it, meet beyond, and thus swallow into the body of the cell the bac- terium or tissue-frag- ment, together with a little drop of the fluid in which both are floating. (The bacterium has been ingested.} While in- side the cell-body of the leucocyte, certain changes are induced in such a particle by products secreted by the leucocyte that tend to dissolve or digest it. That part which cannot be so digested is removed by the reversal of the process employed in swallowing it, i.e. the leucocyte creeps on and leaves it behind, — it is B FIG. 8. — Phagocytes (leucocytes) from the ccelomic fluid of the earth- worm : A, agglomeration of phagocytes surrounding a foreign body ; B, single leucocyte, with vacuoles. — (From Sedg- wick and Wilson, after Metchnikoff .) 30 GENERAL BIOLOGY egested. The digested material is later built up into the substance of the cell protoplasm by the process of assimilation. These three functions, — ingestion, digestion, and egestion, — like that of locomotion, are not localized, but may take place at any part of the body. If bacteria J be introduced into the body of an animal at any point, the leucocytes will soon be found gathered in great numbers at the same point. It has been shown that this gathering is due to a mechanical, i.e. not purposive, " attraction " exerted on the cells by the chemical substances produced and excreted by the bacteria. In other words, the gathering of the former is a response to an altered condition of the medium in which they exist. In the same way the cells will move from a cool region to one of greater warmth, and on a culture-slide, through which runs a weak current of electricity, may be caused to gather at the negative pole. A dead leucocyte will not respond to any of these " stimuli." The response is therefore a function of living matter and is spoken of as a result of its irritability. Irritability has been defined as " the capacity of living substance of reacting to changes of environment by changes, in the equilibrium of its matter and its energy." All of the above phenomena may be observed also in Amoeba proteus, a free-living cell found in slime and stagnant water. Amceba is an independent organism, whereas the leucocyte is one cell out of a ' " ". pus-forming staphylococci. PRIMARY FUNCTIONS OF THE ORGANISM 31 myriad composing an organism. Both, however, exhibit these primitive functions inherent in living matter. At certain times Amoeba secretes about itself a tough protecting membrane or cyst which is the product of protoplasmic activity. Within this FIG. 9. — Amoeba proteus (much enlarged) : 1, nucleus; 2, contractile vacuole ;• 3, pseudopodia ; 4, food vacuoles ; 5, grains of sand. — (From Shipley and McBride, after Gruber.) cyst the living substance tides over unfavorable periods, reproducing later in a characteristic way to be described in another connection. Under favor- able circumstances Amoeba has been observed to divide in two half-cells by a simple constriction and severance of the cell. This process is preceded by a division of the nucleus, so that each " daughter 32 GENERAL BIOLOGY cell " resulting from such a cleavage has half the nuclear material of the original parent cell. In this way the number of individuals is greatly in- creased, and the continuity of existence of the race of Amoebae insured. Such a function of reproduc- tion also accounts for the great number of leucocytes within the blood stream of an animal. Although the details of the process are not known it is probable that they are essentially similar to what has just been described for Amoeba. These, then, seem to be primary functions of living matter: contractility, assimilation, irritability, se- cretion, and reproduction. Of nutrition and the phe- nomena concerned with it we shall have more to say in the next chapter. Specialization in Locomotor Organs. — In but few cells do we find the elementary functions of living matter so evenly balanced and so little specialized as in the leucocyte and Amoeba. The common form of Amoeba just described is called Amoeba pro- FIG. 10. — Amoeba angulata (semi- t^US because of the diagrammatic), showing the antenna- fact that jt constantly like pseudopodium, a, which vibrates back and forth in the positions marked changes its shape, the production of pseudo- podia being urilocalized, as we have seen. In another species of Amoeba, Amoeba angulata, there has been PRIMARY FUNCTIONS OF THE ORGANISM 33 described a more or less permanent pseudopodium which extends from one edge of the cell-body freely into the water " and waves back and forth, serving as a sort of feeler or antenna." l In yet another form, Masti- gamceba, there is a per- manent lash or flagellum projecting from one por- tion of the body, the rest of the creature re- taining " amoeboid " movements. In the group of Protozoa called " Flagellata," the amoe- boid habit is not found, but the animal moves very swiftly by the lashing of one or more permanent whiplike fla- gella. In these three types We See three dlf- ferent grades of Special- , Mastigamcebaaspera: f, flagellum; p, pseudopodium. — (From Calkins, after Schultze.) 1 In the comparisons that follow, the reader must not understand that one type has been transformed into another in any way whatever. The different steps have been arranged side by side much as one might form an exhibit of different models of the telephone or the phonograph from the first crude type to the modern improved machine. In one sense, though not in a. material or genetic sense, the perfected phono- graph has been derived from the earlier model. In the case of specializa- tion of cells, however, as we shall see from the consideration of differ- entiation in development there is often a very direct genetic relationship between cells of a specialized type and those of the most generalized types. D 34 GENERAL BIOLOGY ization, in each one of which, compared with Amoeba proteus, there has been brought about a very much greater efficiency in move- ment through a con- centration of effort and the localiza- tion of the organ of locomotion. In another group of the Protozoa, not only the function of movement, but also that of ingestion has become more or less specialized. In the Ciliates, of which Paramecium is a typical though not the most generalized example, the animal swims rapidly by means of bristle-like cilia developed all over the cell-body. These are practically all alike, and consequently the movements of the animal are very uniform and cir- cumscribed. The cilia along the oral groove are Fio. 12. — Paramecium viewed from the oral surface: L, left side; R, right side; an, excretory area ("anus"); ec, ectosarc ; f.v., food vacuoles ; g, gullet ; m, mouth ; ma, macronucleus ; mi, micro- nucleus ; o.g., oral groove ; p, cuticle ; tr., trichocyst layer. — (From Jennings.) PRIMARY FUNCTIONS OF THE ORGANISM 35 somewhat heavier and are employed to force food particles into the so-called gullet to a point where they are ingested. Thus, in comparison with Amoeba, localization is found not only in the function of locomotion, but also in that of ingestion. In a relative of Paramecium, Stylonychia (see fig. 13), the cilia are themselves differentiated in various regions of the body for different functions. FIG. 13. — Stylonychia creeping on its oral ("lower") surface viewed from the left side: 1, anterior cirri ; 2, adoral zone of. membranelles ; 3, anterior branch of the pulsating vacuole (4) ; 5, dorsal cilia ; 6, pos- terior branch ; 7, caudal cirri ; 8, posterior cirri ; 9, ventral cirri. — (From Lang, after Biitschli and Schewiakoff.) Although those on the upper side have almost disap- peared, yet Stylonychia is more active and has a greater variety of movement than Paramecium. This is due to the fact that the cilia on the lower side of the cell-body are fused together in places to form stout, elastic, hooklike " cirri," shaped and inserted in such a variety of ways as to enable Stylonychia to crawl and jump. Rows of ordinary cilia enable it to swim. In addition to these forms of cilia, there occurs in the oral groove a series of platelike mem- 36 GENERAL BIOLOGY branes, formed by the fusion of many cilia, that beat with the motion of a fan and drive the food down the gullet with force and precision. This function is further subserved by undulating membranes (see fig. 13) likewise formed of fused cilia. Each of these types of locomotor organs has a special function to perform. In some way there has come about a division oj labor among the cilia in different parts of the body, one group of cilia performing one function, another group another; and in proportion to the extension of this division of labor there has arisen a corresponding efficiency of action of each part. Along with this physiological specialization of function there has developed the corresponding modification of structure which we have called dif- ferentiation. Physiological specialization and mor- phological differentiation are thus two connected consequences that result from a division of labor among the parts of an organism. The fundamental function involved in the examples just described is, however, in each case the contractility of the protoplasm. It is in the method or the physical basis of utilizing this function that efficiency is attained ; just as the same amount of current from the same wire will produce a brighter light in a tungsten filament than in a carbon incandescent lamp. The limits of the specialization of contractile organs are apparently soon reached. In some Protista, however, there has been a differentiation of contractile substance within the cell-body, a PRIMARY FUNCTIONS OF THE ORGANISM 37 localization of the function of contractility to certain regions of the cell. In Vorticella, a ciliate protozoan that is usually rooted plantlike to a fixed base, the long stalk which the cell-body develops (see fig. 14) is very contractile, extending the vorticella-bell to a considerable dis- tance and then sud- denly pulling it back. During this move- ment the stem coils and uncoils like a spring, owing to the presence in it of a very contractile fiber of differentiated pro- toplasm. The stem contains little else than this contractile fiber (myoneme, of authors), and we may say that the sole func- tion of the stem, aside from that of support- ing the "bell," is to withdraw it out of danger or ex- tend it into an area of greater food supply. In return for this the bell eats and digests and reacts for both. Here is a division of labor that has proceeded so far that a different kind of protoplasmic substance has FIG. 14. — Vorticella: a, extended; 6, contracted ; c, the stalk more highly magnified, showing the contractile fiber which is not seen in a and b. 38 GENERAL BIOLOGY been segregated from the rest, and in this area, one primary function of living substance has been emphasized to the practical exclusion of the others. If a cell-wall should form across the base of the vorticella-bell, and if both cells should then remain together, we should be justified in speaking of the FIG. 15. — Types of muscle-cells: A and B, smooth muscle-cells ; C, two fragments of cross-striated muscle fibers (cells); at the left above, the end of a fiber. Note the numerous nuclei. — (Verworn.) contractile member of this two-celled organism as a muscle-cell. We know of no instance of such a development in Vorticella or any other protozoan, but in the Metazoa the segregation of the contractile function of protoplasm into a special area and the differentiation of this area into contractile muscle- cells is almost universal. Such a cell is in turn differentiated with respect to the nature of the con- PRIMARY FUNCTIONS OF THE ORGANISM 39 traction which it is of advantage to the organism to have produced. In the vertebrates the tissues that carry out slow, rhythmic contractions, such as those of the intestinal walls, are made up of small, narrow, spindle-shaped cells (fig. 15) with a single nucleus and a delicate longitudinal striation. The skeletal muscles and those throughout the animal series in which rapid or " voluntary " movement is produced are very highly differentiated (fig. 15 c). In the specialization of their substance along the line of contractility they have lost the function of food- taking, and to a great degree, though not entirely, that of conduction and general irritability. Specialization in Conducting Organs. — A par- ticular kind of irritability, however, has come into play in connection with another sort of cells called nerves. Nerve-cells represent another line of spe- cialization. Here the function of specific irritability and conduction has been developed, until, in com- pensation, the functions of contractility and nutri- tion have entirely disappeared. An experiment of Professor J. Loeb's demon- strates in an interesting way the performance of the same function by a highly specialized tissue and by a less specialized one. One of the group of degen- erate animals called Tunicates is provided with two siphons (fig. 16) or passages, through one of which the water passes into the saclike body and through the other of which it flows out. Midway between the two is situated the nervous system, 40 GENERAL BIOLOGY • consisting of a single large " ganglion " or nerve-knot, from which ramify nerves in all directions. In Ciona, a member of this group, if one of these siphons is touched with a needle, both of them contract almost simultaneously. If, however, the ganglion be snipped out with a pair of scissors and one of the siphons be touched with a needle, the one stim- ulated will contract at once, but only after a considerable interval does the other siphon like- wise contract. There has been a conduc- tion of the stimulus in both cases. In the first instance the nervous system afforded so perfect FIG. 16. — C-iona intestinalis, a Tuni- * j cate : a and 6, the two siphons ; c, foot ; a means Of COndUC- d, location of the ganglion. — (From tioil that the COntraC- Loeb.) tion of both siphons occurred almost together. In the second experi- ment, however, the only path of conduction lay through the intervening muscle fibers, specialized along the line of contractility but not that of con- duction. Hence the conduction was very imper- fectly and slowly carried out. PRIMARY FUNCTIONS OF THE ORGANISM 41 Secretion. — The majority of cells retain in greater or less degree the primary function of protoplasm called secretion. By virtue of its chemical and physical organization living matter not only builds non-living food substance into itself to form new protoplasm (nutrition), but it reverses the process and forms from its living substance non-living prod- ucts or secretions. Certain types of cells are spe- cialized in this direction to a great degree, and we speak of them as secretory or gland cells. But many animal cells and nearly all plant cells secrete a denser sub- stance about themselves, the cell-wall, and further, a cement substance which binds the cells together into a unified mass. Cellular secretions are thus of two sorts, " permanent " secretions that remain in place after being formed, and secretions which, when elaborated, are passed out of the cell-body to other parts of the organism. In plant tissues the cell-wall is usually greatly thickened and strengthened by the secretion of cellulose, a derivative of which gives wood its hard quality. In animal tissues the cell- wall is not usually so thickened, but a similar result is obtained by the development of intercellular substances. These give the connective tissues their characteristic structure and qualities. In the latter the cells are specialized in the production of secre- tions until the functions of contractility, irritability, and conduction have almost if not entirely dis- appeared, and the intercellular substance is produced in such quantities as to outbulk many times the living cells themselves. According to the nature 42 GENERAL BIOLOGY of this intercellular substance the connective tissue is described as white fibrous tissue, yellow elastic tissue, cartilage, bone, etc. Strikingly dissimilar as these types of tissue are, they not only are ex- amples of the same sort of protoplasmic activity, but all of them in development have been shown to be derived from an original, much more generalized type of primitive connective tissue cell. In the other phase of secretory activity mentioned, the production of secretions which are freely trans- ported to other parts of the organism, we have a very easily apprehended example of the division of labor between the parts. Specialization in Digestion. — Amoeba, when it has ingested a particle of food-substance, digests it by dissolving it so that it can be assimilated into the protoplasm. It does so by means of minute quan- tities of a substance which it secretes that acts chemically upon the food. (See Chapter III.) This process of digestion, like that of ingestion, may take place in all parts of the cell-body, i.e. it is not localized. In the complex structures composing the body of a higher animal it is obvious that it would be impossible for food to come in contact with every part of the body so that each cell could attend to its own function of digestion. Accordingly, we find that the principle of the physiological division of labor comes into play in very simple and otherwise quite generalized forms (such as Hydra}. The secretion of digestive fluids and the function of alimentation PRIMARY FUNCTIONS OF THE ORGANISM 43 in general becomes restricted first to a layer of cells (in Hydra) and then to localized regions of this layer (digestive portion of the alimentary canal), the specific secretions for different kinds of food eventually being segregated in higher animals in different areas of the canal (stomach, duodenum, FIG. 17. — Transverse section of Hydra, showing the coelenteric cavity and the two layers of the body wall.— (Shipley and McBride.) etc.)- Such an adaptation makes for much greater efficiency. However, in some parasitic worms, such as the tapeworm, which lies in the alimentary canal of its host and, so to speak, does not have to exert itself to get or digest food, the localized apparatus for alimentation has disappeared, and the digested food supplied by the host is absorbed directly through the body- wall of the worm. In connection with the specific character of the 44 GENERAL BIOLOGY secretions of gland cells has arisen the necessity for the production of large quantities of the secretion at one time in a limited space. The natural extension of the secreting area is ordinarily quickly limited by the need for concentrating the secretion at a certain point. In consequence the secretory surface is increased by folding and by sinking below the FIG. 18. — Diagram of the formation of glands by the sinking in of a secretory area of the epithelium : 1 , simple tubular gland ; 2 and 3, branched tubular glands ; 4 and 5, simple alveolar glands ; 6, branched alveolar gland. — (Hertwig.) surface of the cell-layer (epithelium) of which it is a part, so that a maximum amount of secreting surface is produced in a minimum of space. In this way is developed a gland (see fig. 18), the common channel for the substance secreted being the duct. All organisms possess the power of reproducing themselves, otherwise the species of which they are members would become extinct. In the higher plants and animals this, like the other properties PRIMARY FUNCTIONS OF THE ORGANISM 45 of living matter, has become the special function of a certain class of cells, the so-called germ-cells: In these cells as in other classes we can trace the gradual increase of efficiency and certainly of action of the function through a division of labor. Summary. — We have seen that the organism, whether simple or complex, may be looked upon as a machine that does certain things ; in other words, " works." The nature of the work that the protoplas- mic machine does sets it off from all others as some- thing unique and (at present at least) inimitable. No machine that man has ever made reproduces itself, repairs itself, or automatically adjusts itself to chang- ing external conditions. These primary functions, however, are carried out with varying degrees of perfection by different sorts of organisms. As a rule the organisms of comparatively complex struc- ture perform these functions as a unit more efficiently than those of a lower grade of organization. This is due to the fact, as we have seen, that the functions are performed by various parts of the whole organism particularly adapted for the purpose. Physiological specialization and structural differentiation come into play side by side as the result of a division of labor. An organism in which this phenomenon has been extensively developed is spoken of as specialized. One, such as Amoeba, in which there is little or no specialization, is called generalized. As a rule this is the criterion by which we estimate the position of a plant or animal in the scale of life. 46 GENERAL BIOLOGY A " low " form is a generalized one, a " higher " form is one that is specialized. This " criterion of perfection " confronts the student of nature at every turn, but the method by which such a condition comes about is the great central problem of biology as yet unsolved. Specialization af- fects particularly the cells, since it is these that are structurally dif- ferentiated. In proportion, how- ever, to the degree of their specializa- tion they lose their own independent self-sufficient iden- tity and become FIG. 19. — Diagrams of wings, showing merged with homology and analogy : a, wing of a fly ; , b, wing of a bird ; c, wing of a bat. c is the Others into ag- homologue of 6, a is an analogue. — (Jordan crre£ates of Cells and Kellogg.) 6 . oi similar func- tion. These aggregates we have already referred to as tissues. Again, for the better carrying out of the various functions of the whole organism, different tissues are combined into organs. The stomach, which not only digests food, but also kneads it and breaks it up, is made up, in addition to secretory tissue, of muscular and connective tissues as well. Again, PRIMARY FUNCTIONS OF THE ORGANISM 47 organs of allied and connected functions are com- bined into systems, some of which, like the nervous and circulatory systems, pervade the whole organism. It is of great interest and value to compare animals and plants with respect to the degree of specializa- tion of their parts ; for such a comparison often re- veals relationships. In making such comparisons it is sometimes found that organs carrying out the same function, such as the wing of a bird and that of a butterfly, are of a very diverse origin and structure. On the other hand, the wing of a bird and the foreleg of a dog, in spite of the apparently very different functions which each performs, have each the same origin relative to the rest of the body and the same general internal structure. Such a similarity we call homology, and we speak of the two parts as homologous, whereas the similarity of function be- tween the wing of a bird and of a butterfly we speak of as analogy, and the parts as analogous. CHAPTER III METABOLISM Oxidation. — It is known that oxygen exists in two forms, ordinary atmospheric oxygen and ozone, the molecule of the former consisting of two atoms (written O2), and that of the other of three (O3). During thunderstorms ozone is often formed from oxygen by condensation, through the action of electricity. Both gases are chemically active in combining with other elements or compounds, a process known as oxidation. The activity of ozone, however, is much greater than that of ordinary oxygen. It gives up its extra atom of O with facility, and is, therefore, spoken of as less stable than the latter. But the resulting product of oxidation by either oxygen or ozone is exactly the same. The only difference between the two must be the way in which the atoms of O are combined to form the molecule. In oxidation there is an evolution of heat, which is the release of the intrinsic energy of combination in the oxygen or ozone molecule. Measurements have shown that in the oxidation of finely divided platinum by ozone, some 72,400 calories 1 more of heat per gram is evolved than in the corresponding oxidation by ordinary oxygen. This figure must represent the additional amount of 1 A calorie is the measure of heat required to raise one cubic centimeter of water one degree. 48 METABOLISM 49 energy locked up in the ozone molecule in comparison with the oxygen molecule. When we speak of this energy as being locked up or " latent," we have exhausted our knowledge of it. We know that it is there, holding the atoms together by what we call " chemical affinity," and that, when the atoms are released from their bonds, this energy becomes evident to our senses ; that is, it becomes kinetic, and assumes various forms, such as heat, light, elec- tricity, or motion. Two points of great significance must be noted in the illustration just given ; first, the fact that the more complex substance (O3) has latent a great deal more of the energy of combination than the simpler one (O2) ; and secondly, that the former breaks down or gives up its latent energy more readily than the latter. Few atomic combinations are as simple as oxygen and ozone and at the same time are so readily disrupted ; indeed, among the more complex substances it is a general rule that the greater the complexity of the molecule, the greater the amount of its potential energy and the greater its instability. The storing up of the potential energy of " chem- ical affinity " is well illustrated by a so-called endo- thermic compound such as acetylene, — C^R^ '- Heat of combustion of C2 = 2 X 96,980 cal. = 193,960 cal. Heat of combustion of H2 = 2 X 34,960 cal. = 69,920 cal. Total 263,880 cal. Heat of combustion of C2H2 (acetylene) . = 310,600 cal. Difference . * 46,720 cal. 50 GENERAL BIOLOGY That is, the energy that binds together the hydro- gen and carbon in the form of acetylene is nearly one sixth greater than the sum total of the intrinsic energy, measured by the heat evolved in combus- tion, of the same amount of either element taken separately.1 Conservation of Energy. — In the above example the potential energy is released as heat, but it is conceivable, on the basis of other experiments, that this energy, released by the breaking down of acet- ylene, might be employed at once in building up some other chemical compound, and thus be non- evident to us except from its end-result. At any rate the decomposition and recombination (analysis and synthesis) of any chemical compound may be repeated indefinitely, the same amount of energy being released or absorbed at each change. All the forms of energy known to us may be transformed one into another in' this way. Gravitation acting on the molecules of water in a brook may be caused to drive a waterwheel. The wheel, by its motion, in addition to producing heat by friction, may also run a dynamo, 'The above figures refer to the heat of combustion of one gram each of carbon, hydrogen, and acetylene. If equivalent amounts of sub- stance be taken the difference is even more striking. The molecular weight of acetylene is 26 and every gram of acetylene contains twenty- four twenty-sixths of a gram of carbon (mol. wt. of C = 12) and two twenty-sixths of a gram of hydrogen (H = 1) ; twenty-four twenty- sixths of 193,960 cal. is 179,040 cal. and two twenty-sixths of 69,960 cal. is 5,378 cal, ; the sum of these is 184,418 cal. or 126,182 cal. less than the heat of combustion of one gram of acetylene. METABOLISM 51 the electricity generated by which affords us light, heat, and such chemical analyses and syntheses as are involved in cooking. In all these transformations of one kind of energy into another, though much is wasted in manipulation, none is lost ; in other words all can be accounted for. This conception, that the sum of energy in the universe is constant and merely changes its form from one kind to another, is one of the great generalizations of natural science, and is known as the " Law of the Conservation of Energy." Chemistry teaches us also that the more complex " organic " compounds are built up of relatively few kinds of atoms; but these are combined and recombined into groups of higher and higher orders, an absorption of energy taking place with every combination, until a huge aggregate results, the potential energy of which is enormous. Thus, a mole- cule of a simple sugar, dextrose (CeHisOe), may be combined with another molecule of a similar sugar to form a new sugar of a higher order, cane-sugar or sucrose (C^H^On).1 In this reaction a mole- cule of water is subtracted. More molecules may be added to the combination over and over again, like keys on a keyring, if the molecule of water is each time removed. With every such combination there is an addition to the amount of potential energy accumulated in the molecule, and of course this energy must be supplied from without. Usually its source is the heat of the alcohol or gas flame which 1 C6H1206 + CsHx^e = CuH^On + H2O. 52 GENERAL BIOLOGY the chemist supplies. Furthermore, if a compound sugar, like the cane-sugar just described, should be broken up, it would not resolve itself into its ultimate components (atoms) at once, but the line of cleavage would occur first at the point where the two larger groups had joined. In other words, the affinity of the two simple sugars for one another is much weaker than the affinities of their constituent atoms for each other. In general, the simplest compounds, such as CO2, H2O, NH3, etc., are bound together by very strong chemical affinity and require much force to disrupt them, whereas the chemical affinity binding together very complex organic substances is usually so weak that these molecules often appear to disintegrate spontaneously, for which reason they are spoken of as unstable. These facts, as we shall see, have an important -bearing on the utilization of food substances by plants and animals. Chemical Synthesis in the Organism. — Green plants not only require the normal conditions of heat and moisture demanded by all living things, but they, require sunlight as well, else they grow pale and sickly. This is true even of those plants, like ferns, that thrive best in the shade. The position and attitude of every leaf on a tree is' ad- justed to receive the maximum amount of sunshine. If we inclose a leaf in a glass tube filled with CO2, and then expose it to the sunshine, after several hours we will find by tests that a large part or all METABOLISM 53 of the CO2 has disappeared and has been replaced by an equal volume of oxygen. The total volume of the gas has not been altered, but we find that the carbon has disappeared from the tube. And since this change will not- take place in the dark, even if the other conditions be similar, we conclude that the sunlight has supplied the neces- sary energy. What, then, has become of the carbon ? Green plants al- ways have a certain amount of food ma- terial stored up in the leaves, usually in the form of Starch. FIG. 20.— Experiment showing the rpi f function of sunlight in the synthesis of starch in the green leaf. A slice of cork is Starch may be easily fastened over the leaf and the rest of the leaf exposed to the sunlight. The figure detected by testing to the right shows the result when the cork with a solution of is removed and the leaf dipped in solution of iodine. — (Bailey and Coleman.) iodine, which colors it a bright blue. If we keep such a plant in the dark for a while, it will exhaust this store of starch, as may be shown by the leaves giving a negative test with the iodine.1 If, however, we pin a strip of cork across part of a leaf from which the starch has been exhausted, and expose the 1 In such an experiment the chlorophyll must be dissolved out in hot alcohol before the iodine is applied, in order that its green color may not mask the starch reaction. 54 GENERAL BIOLOGY partially covered leaf to the sunshine for a while, we find, when we treat it with iodine, that, whereas the strip covered by cork gives a negative test as before, the rest of the leaf which was exposed to the sunshine turns a deep blue with the iodine. This demonstrates that starch has been actually formed in that part of the leaf which has received the sun's rays. This conclusion we can confirm with the microscope, for the starch granules may be found in the chlorophyll bodies of the cells after such ex- posure to sunlight. Out of the water absorbed by the plant, and the CO2 always present in the air, in the presence of chlorophyll and the sunshine, the plant can synthe- size starch,1 which may be conveyed (in the form of sugar) to other parts of the plant-body to be stored up as reserve food or utilized at once as a source of energy for the vital processes of the plant. The CO2 in the air is the source of the bulk of the carbon v compounds in the substance of the plant. This is shown by the fact that most plants grown in an atmosphere free from CO2 die (of starvation) even if the soil in which they are rooted be richly supplied with carbon compounds. Conversely, they will thrive in a substratum free from carbon compounds if they have access to ordinary air. Although starch is the first evident product of this process (photosynthesis), yet it is probably 1 6 C02 + 6 H2O = (CeHiiOe) + 6 Oj ... (C«H12O,)» (- n H2O glucose = (C6H100P)n. starch METABOLISM 55 only the end-product of a long series of changes. The carbohydrate food in many plants is more often in the form of a dilute solution of sugar, which, of course, is much less easily demonstrated than the solid starch. Production of Fats and Proteins. — We know :>f no other food than the starch that is synthesized from such simple chemical compounds,1 and it has been shown that the fats and proteins are produced from the starches as a foundation. The spontaneous pro- duction of fatty oils in seeds containing starch has been directly observed, and, since the fats contain no elements not also found in the carbohydrates, such transformation involves no more than a re- arrangement of these elements in the molecule. It is known that water and CO2 are produced as the result of such a change, although we have still much to learn of the intermediate steps in the process. The explanation of the origin of the proteins is much more difficult, for proteins possess, in addition to the carbon, hydrogen, and oxygen of starch, a high percent- age of nitrogen, as well as sulphur, and often phos- phorus. These latter elements are obtained from the soil in the form of nitrates (or nitrites), sulphates, and phosphates, and are absorbed through the roots of the plant in solution. The nitrates (sodium or potassium) probably enter into union with the carbohydrate radicle to produce some simple amino-acid such as asparagin (C^gNjOs). By a succession of syntheses involving 1 Other organisms recently discovered can apparently utilize carbon monoxide (coal gas), while still others may use methane (marsh gas) a« their only source of carbon. 56 GENERAL BIOLOGY the condensation of these amino-acids, other radicles or "albuminous nuclei" are welded on, combined, and recombined with the carbohydrate base, or with each other, each time with the disappearance of the poten- tial energy of chemical affinity, until the huge, com- plex, unwieldy, protein molecule results, and becomes a part of the mixture we call protoplasm. This pro- cess has been shown to take place in the green leaves, although it goes on in the dark. To summarize : The plant in its myriads of cell- laboratories is constantly carrying on a variety of chemical syntheses. First, the carbon element of the CO2 derived from the air, and the H2O absorbed from the soil, are combined in the green leaves to form carbohydrates, such as sugars and starch. These molecules may be welded together to form more complex carbohydrates, such as dextrine, cellulose, or wood fiber, or they may be combined and recombined with other elementary compounds containing nitrogen and sulphur until the complex and unstable albuminous or protein molecule results. Each step involves the change of kinetic energy into potential energy, and all this energy is derived in the first instance from sunlight. This building-up process from simple to complex is called Anabolism, and, as a consequence of this process, the plant is endowed with an immense reserve of potential energy. Dissimilation. — But the plant is all the while living, — developing new buds and leaves, maturing its fruit, secreting characteristic products, even METABOLISM 57 moving, although its movements may not always be evident. Says Huxley, referring to the constant streaming and circulation of the protoplasm in the cell : " The wonderful noonday silence of a tropical forest is after all due only to the dullness of our hearing; and could our ears catch the murmur of these tiny maelstroms as they whirl in the innumer- able myriads of living cells which constitute each tree we should be stunned as with the roar of a great city." All these phenomena, as we know, are but mani- festations of energy. What is its source? From careful experiment we have learned that it is not only the breaking down of the circulating food substances, but may be the disintegration of the protoplasm itself. Made up of complex aggregations of matter held together by the power of chemical affinity, the protoplasm is a storehouse of potential energy that may be translated into kinetic energy by the disinte- gration of the unstable compounds composing it. In proportion, then, as the plant does work of any sort, it draws on its own substances for the energy requisite. Here we have the direct reverse of the building-up process just described. The circulating food sub- stances or the living tissue itself is constantly break- ing down and as constantly being renewed. This continuous flux and flow is called Metabolism, the tearing down process, Katabolism, Metabolism in Animals. — The whole animal world is dependent upon the plant world for its existence, since even the flesh-eaters depend ultimately upon 58 GENERAL BIOLOGY the plant-eaters for food. For animals, unlike plants, are quite unable to utilize, directly, the energy of the sun's rays, and combine into sugars and starches the water and CO2 with which they are surrounded. Nor can they, like plants, utilize the nitrogen as it exists in simple combination. Nitrogen is an es- sential element of protein, and protein an essential of protoplasm, and without it the animal cannot repair the wastes of katabolism. But this nitrogen must be furnished to the animal already combined in proteins. The metabolism of animals therefore begins at a higher level than that of plants. Plants take in and assimilate gases and liquids of very simple composi- tion, whereas animals require liquid or solid food al- ready organized as fats, carbohydrates, or proteins. The latter kind of nutrition is sometimes referred to as Holozoic, the former, which is characteristic of all green (chlorophyll-bearing) plants, as Holophytic. Some groups of plants, however, show decided exceptions to such a rule. The bacteria and the fungi, for instance, lack chlorophyll and cannot manufacture starch, but must depend on other or- ganisms, either plant or animal, for their food supply. Such plants are called parasitic when they feed on living tissue, or saprophytic when they subsist on dead and decaying tissue. Even some of the higher plants, such as the dodder, a relative of the morning glory, have abandoned the independent manufacture of their own food materials and live as parasites on other plants. METABOLISM 59 FIG. 21. — Leaf of the Venus' fly-trap : A, open ; B, closed. Note the three sensitive hairs on each leaflet of A. Some plants, indeed, might be called, if not holo- zoic, at least " amphizoic," since they have de- veloped means of catching and killing living animal prey. Of such are the familiar " Venus' Fly Trap " (Dioncea), or the "Pitcher plant" (Nepenthes), FIG. 22. — Two leaves of the sundew (Drosera rotundifolia) ; the one to the right in the expanded condition, that to the left shortly after the capture of an insect ; the tentacles of the right half are bent over to bring the glandular tips in contact with the prey. Magnified 1\ times. — (From Barnes, after Kerner.) 60 GENERAL BIOLOGY etc. In the ingenious traps possessed by such plants unwary insects are caught and killed; digestive fluids there secreted dissolve the tissues of the prey, and they are absorbed precisely as they would be in the stomach of a carnivorous animal. Foods in General. — In the light of what has just been said, it will be seen that we must modify our notions of foods. It is not enough to classify foods as the scientific cook books do, — merely as carbo- hydrates, fats, and proteins. Since the sole purpose of taking food is, as we have seen, the accumulation of a store of energy, we might define a food to be anything that contains potential energy. The CO2 and H2O are foods to a green plant only when combined with the energy of sunlight. They are better called food-materials. A welsh rarebit, the food value of which is very high, which I may eat with impunity, may be "the other man's poison." But a stick of hickory that supplies the wood-boring beetle larva all the nourishment it requires, is to me useless because, for my purposes, the large amount of energy locked up in it is not available. If, however, I reduce the stick to sawdust and boil it with sulphuric acid, thereby converting it into glucose, it becomes a very good food. We must, therefore, modify our previous definition by designating as a food anything that contains available potential energy. Fate of the Foods in the Higher Animals. — In METABOLISM 61 animals the fats and carbohydrates yield a ready source of energy in the form of heat. Whether they are always directly oxidized in the animal body with- out ever having become part of the tissue itself is perhaps questionable, but this seems to be true of the carbohydrates if not of the fats. The liver functions in an important way in the carbohydrate metabolism of Vertebrates. The digested sugar is transformed into another carbohydrate called gly- cogen (" animal starch "), and stored up in the liver, and later in the muscles, in the form of granules. The glycogen is dissolved and given back to the blood stream as the body requires it between meals, or is oxidized in place, to release the energy involved in muscular work. Similarly, the fats are stored up in the different parts of the body or in special organs (the fat-body of the frog, e.g.) to be drawn on as need arises. If neither fats nor carbohydrates are available, the proteins in the blood stream or even those of the tissue itself may be broken down to supply the necessary energy. Hence, the fats and carbohydrates are often spoken of as the " protein-sparing " sub- stances. In the digestive tract of the higher animals the fats are split into their components, glycerine and fatty acid, through the action of the enzyme, lipase.1 Being absorbed in this form they are recombined in the epi- thelial cells or within the capillaries and circulate as 1 See page 83. 62 GENERAL BIOLOGY fats in the blood, or are laid down in various parts of the body, unchanged. The carbohydrates are all split into simple sugars (e.g. glucose) before being absorbed, and possibly circulate in the blood loosely combined with the serum-proteid base in the same way that oxygen combines with haemoglobin. The proteins follow a more complicated path, al- though our knowledge of them is confined almost wholly to what we know of the metabolism of warm- blooded animals. Using the digestive fluids of the alimentary canal, we can split up protein (a strip of lean meat, for example) into smaller and smaller bodies until we reach the amino-acids, which are the units out - of which the proteid molecule is built up. These are ab- sorbed through the blood-vessels of the alimentary canal and are apparently split further into urea, CO(NH2)2, on the one hand, and on the other hand a residue which is then resynthesized into complex albumens that circulate in the blood stream or are built up into the protoplasm of the tissues. Although an absolute essential for the maintenance of life, nitrogen is not ac- cumulated in the body ; the greater the amount of ni- trogenous food ingested, the greater the amount of urea eventually excreted, — a condition known as nitrogenous equilibrium. Role of Oxygen in Metabolism. — It is a matter of familiar experience that all animals require an abundant supply of oxygen in order to live. The air-breathing vertebrates are especially sensitive to the lack of this element, and if deprived of a supply of oxygen soon succumb with characteristic symptoms of asphyxiation. If we boil water and thus drive out the air dissolved in it, fishes and other aquatic animals soon die. But plants are no less dependent METABOLISM 63 upon a supply of oxygen. If a transparent cell of plant tissue, in which the protoplasm is in active streaming movement, be so mounted that the sur- rounding air can be replaced by pure hydrogen, the streaming will cease entirely until oxygen is again supplied (the hydrogen is itself inert toward pro- toplasm). Plants likewise cease to grow in the absence of oxygen. The presence of this element thus appears to be an absolute necessity for life as it exists on the earth to-day. But an important exception to this statement must be noted. A number of the lower plant forms have been found to thrive in an absence of oxygen and in- deed to refuse to grow in its presence. Such forms, which include numbers of the bacteria, are called anaerobic. Some bacteria are able to adapt them- selves to either the presence or the absence of oxygen ; others can thrive only in the absence of it. The former are termed facultative anaerobes, the latter obligate anaerobes. Among the latter are numbered some of the most dreaded disease-producing or pathogenic organisms. And in this connection their anaerobic habit is of much significance. The germ of tetanus (lockjaw), for example, is very widely distributed, and the only reason that the disease is not much more frequent than it is, is that the spores of the active agent cannot develop in the presence of oxygen, and hence only those wounds that are deep and that close over, thus excluding the air, are likely to afford sites for the development of the poison-producing bacteria. 64 GENERAL BIOLOGY Combustion and Respiration. — Since oxygen is so significant in organic life, it is important to find out what role it plays in metabolism. It was among the earlier discoveries of modern chemis- try that when anything is burned, the combustion involves a using-up of oxygen (oxidation) and will not take place in the absence of oxygen. When wood is burned, the carbon in it unites with the oxygen to form CO2, and the hydrogen to form H2O or water. It is easy to observe that in plants as well as in oxygen-breathing animals not only is O taken in, but GO2 is expelled. This interchange is known as respira- tion, and it was an obvious step to compare it with ordinary combustion, particularly as the production of the bodily heat of the higher ani- mals is unquestionably dependent upon oxida- tions. From this standpoint, the foods which the organism takes into itself were supposed to be oxidized with an evolution of energy, in the same way that the fuel burnt under an engine boiler generates steam to drive the wheels of the engine. In the burning of fuel the oxygen supplied by the draft combines directly with the fuel, but it is not difficult to show that the CO2-production of an animal or plant bears no direct relation at all to the intake of oxygen. A frog in an atmosphere of hy- drogen will continue to evolve CO2 without any possible supply of gaseous oxygen. The CO2 in such a case must have been evolved as a by- METABOLISM 65 product of changes taking place in the tissue sub- stance itself.1 These reactions, taking place constantly in the tissues, are obviously of a very different sort from the exchange of gases (breathing) to be observed in higher plants and animals. They constitute the true respiratory process. But the term respiration 1 Chemists have discovered, indeed, that dry oxygen, at low tempera- tures, is the greatest retarding agent in combustion. We are familiar with the fact that iron rusts much more quickly if wet than if dry, and if kept perfectly dry, will not rust at all. The rusting is an oxidation or slow combustion resulting from the combination of the metal with oxygen to form iron oxide. In this case the reactions are perhaps as follows (Matthews) : I. Fe + 2 H2O = Fe(OH)2 + 2H II. 4 H + O2 = 2 H2O, or 2 H + a = H2o2 Fe = FeO + HzO. In the first equation the iron combines with the water to form ferrous hydrate and hydrogen. The latter would immediately reduce the former to metallic iron again if there were not oxygen present with which it can combine to form hydrogen peroxide, which, giving up its extra atom of O, forms ferric oxide, or iron rust, and water. The oxygen acts thus not as a direct combining agent with the iron, but rather as a sort of depolarizer to take off the nascent hydrogen, and the oxidation of the iron is effected by the hydrogen peroxide. It is supposed that in animal and plant tissues much the same sort of thing takes place, but with infinitely more complicated reactions. The essential point, however, is that the oxygen does not combine directly with the carbon element of the protoplasm to form COj, but, where water enters into reactions with the substances composing the tissues, it acts as a sort of depolarizer to combine with the hydrogen liberated. The CO2 probably arises independently as a by-product in the shifting and rearrangement of various components of the sub- stances making up protoplasm. "It is a sort of receipt for a given amount of energy released by chemical decomposition." F 66 GENERAL BIOLOGY is so firmly associated with the external phenomena just mentioned that its use in connection with the oxidations and reductions taking place in tissues is apt to be misleading. For this reason the term Energesis has been proposed for tissue respiration. Since the function of the process is to release the necessary energy required in the life of the organism, this word is very appropriate and deserves wider use. Poisons. — So long as katabolism is compensated by an approximately equal anabolism, the life process proceeds normally. If the destruction of proto- plasm, however, occurs too rapidly for the con- structive changes to keep up with it, then abnormal conditions arise which soon result in the death of the tissue or of the organism. Thus, although oxidations are absolutely essential for life, excessive oxidation soon destroys the protoplasm, and certain oxidizing substances, such as potassium perman- ganate, are poisons on this account. Other sub- stances, such as the salts of the heavy metals, mercury, silver, etc., destroy the life of the proto- plasm by entering into permanent combination with substances composing it. Other more complex substances act as poisons by substituting themselves for essential parts of the protoplasm. Thus many of the proteins elaborated by plants, such as strychnine, morphine, caffeine, etc., are deadly poisons to the tissues of higher animals. The actions of these substances are, however, very diverse. Indeed, the animal organism constantly forms such protein- METABOLISM 67 like or nitrogenous compounds as by-products of normal metabolism, and these, unless removed by excretion, poison and eventually kill the organism that produces them. Another group of substances, called anaesthetics, of which chloroform and ether are the most familiar, depress the activities of protoplasm and, if not counteracted, kill it. The action of such substances is still a matter of debate, but since all of them are fat solvents, it has been supposed that their poisonous action may be exerted on the fatty components of pro- toplasm. The poisonous action of these substances is of a different character from the depressant action mentioned. If the action of the anaesthetic is not too violent or too prolonged, the protoplasm will later recover its activities and resume its functions as before. It has been shown that, whereas the action of poisons (including such substances as ether and chloroform in poisonous doses) greatly increases the permeability of the cell membrane, the merely anaesthetic effect is accompanied by a temporary decrease of permeability. Antiseptics. — Nearly all the disease and physical suffering that man is subject to is the result of the activity of microorganisms that find lodgment within the body, and, rapidly multiplying, produce poisons that affect the whole system. We combat these by the use of such poisons as have been men- tioned above, — chemicals that either inhibit the growth of the organisms or destroy them. Especially 68 GENERAL BIOLOGY useful are bichloride of mercury, alcohol, and phenol (carbolic acid). The Cycle of the Elements hi Organic Nature. — The ultimate source of the carbohydrates and fats in plants and, hence, secondarily in animals, is, as we have -seen, the carbon existing as CO2 in the atmosphere. There is reason to suppose that in geologic time past, owing probably to great volcanic activity then, the amount of CO2 in the atmosphere was much greater than it is at present.1 At any rate the proportion of CO2 in the air nowadays is sur- prisingly small, — not more than .05 per cent, of which the carbon itself, the part utilized by the plant, constitutes but a little more than one fifth. The botanists have calculated that the cellulose in a single dried tree trunk weighing 11,000 Ibs., repre- sents a carbon moiety of 5500 Ibs., and that to secure this amount of carbon such a tree must have drained more than 16,125,000 cubic yards of air of its CO2. Meteorologists calculate that, although the atmos- pheric envelope of the earth may extend (in a very tenuous state) several hundred miles above the sur- face, yet -seven eighths of it by weight lies under a height of 10.2 miles from the ground. Allowing for the diffusion of its constituents, we may estimate, for the sake of the argument, that the surface vege- tation can draw tribute of CO2 from a height of ten 1 Geologists have even ascribed the initiation of the glacial epochs to the decrease of COj in the atmosphere, consequent in part upon the great development of plant life in preceding epochs. Sec Chamberlain and Salisbury, Geology, III, p. 424. METABOLISM 69 miles. The surface of the state of Oregon is given as 94,560 square miles, that is to say, 945,600 cubic miles of air cover that densely wooded state. On the basis of the calculation made in the last paragraph this would allow but 338.1 trees of the sort mentioned to a cubic mile, or 3381 to a square mile of surface, or one tree to every 918 square yards, — a number which is probably exceeded many times in any one generation. Of course, both plants and animals " breathe out " quantities of CO2 in respiration as described in the previous section, yet such amounts must be far inadequate to make good the loss to the atmosphere through plant growth. Since we have seen that the carbon compounds in the soil cannot be utilized by the plants to build up carbohydrates, it is evident that the air must be constantly supplied with quantities of CO2 from some source. Before we follow this farther, let us consider for a moment the nitrogen balance sheet. The plant draws from the soil all the requisites for its protein, and, hence, for the bulk of its proto- plasm. The rapidity with which a crop, say of wheat, can exhaust the available and necessary mineral food in the soil has been frequently and strikingly demonstrated in America, where the originally rich virgin soils have been repeatedly " robbed " and then abandoned for other un worked fields. Only recently, when the supply of free land is reaching its end, has pressure been brought 70 GENERAL BIOLOGY to bear on the agriculturist to replace by fertilizers what his crops have drawn out of the soil. It is obvious that the materials thus removed depend largely upon the kind of plant that is grow- ing, each kind drawing out certain things for its own particular needs. Yet there are some mineral com- pounds that are demanded by all plants, the absence of which in- terferes with or prevents normal growth. These, as has been noted already, are the metal salts of phosphoric, nitric, and sulphuric acids, usually po- tassium phos- phate, potassium nitrate, and cal- cium and mag- nesium sulphates. It is possible to make a solution 1 of a mixture of these salts in which plants will thrive. The ab- sence of any of the above-mentioned elements in such a solution induces marked disturbances in the normal growth of the plant. 1 The following solution, devised by Schimper, has been much used in experimental work : calcium nitrate, 6 gm. ; potassium nitrate, 15 gm. ; magnesium nitrate, 15 gm. ; potassium phosphate, 15 gm. ; sodium nitrate, 1.5 gm. ; distilled water, 600 cc., to which is added a trace of ferric chloride FIG. 23. — Culti of hemp grown neutral solid substratum. A complete nu- trient solution has been added to /, and the plants have attained a height of 1.5 meters; a solution larking potassium nitrate has been added to the substratum in II , and only the sterile substratum placed in the pot in ///. — (From MacDougle, after Ville.) METABOLISM 71 The amount of nitrogen taken from the soil, annu- ally, by an average crop in Alsace was estimated at approximately 46 Ibs. to the acre. Less than half this amount is returned directly to the soil (mostly as volatile ammonia). Moreover, every inch of rain falling on the land and draining through it causes an additional loss of something like 2^ Ibs. of nitrogen to the acre. There must be an excess of nitrogen, therefore, in the soil over and above what the plant life demands. Here again, as was the case with the carbon, the demand would seem to greatly exceed the supply. Since man first began to develop the art of agricul- ture, he has practiced various methods of replenishing the soil from which his crops have taken their foods. Especially has he used various sorts of manures, which contain nitrates and phosphates. Modern man has added to these fertilizers various mineral substances which he quarries from the earth, such as phosphate of lime and " potash " (potassium phosphates and sulphates). The so-called " basic slag," a residue from metal smelting, which contains 12-20 per cent of phosphoric acid, is nowadays finely ground and largely used *to replenish the supply of phosphates. The weathering of the rocks also slowly adds to the soil the soluble components needed by organic life, and of course this was their original source. But whatever man can return to the soil is obviously insignificant compared with what Nature's crops remove. The huge stores of nitrogen and carbon constantly built up into GENERAL BIOLOGY plant and animal tissue must be, somehow, as constantly restored. Destruction of Organisms. — Each year sees a prodigious crop of annual plants — " weeds " in every vacant lot. Of course a large part of their bulk is water, yet even if dried out, the accumulations of a few years, if preserved, would so cover the PLANT TISSUES ATMOSPHERE ANIMAL TISSUES DECAY FIG. 24. — Diagram of the carbon cycle in organic and inorganic nature. ground as to make it impossible for other plants to struggle up from the soil to the light. The repro- ductive capacity of nearly all animals is astonishing, and the struggle for existence entails the destruction of hosts of individuals every year. Yet we do not find the surface of the earth cluttered with dead animals. Indeed we rarely see one at all, or, if we do, it is only the whitening bones which the rains are slowly wash- ing away. What, then, becomes of them ? The an- swer is at every hand. Wood is more resistant than animal tissue, and we can everywhere observe METABOLISM 73 the phenomenon of its slowly rotting away. With animal remains the process is much more rapid. In the end, however, the myriads of animal and plant individuals, after their brief existence on the earth, dissolve back again into the elements from which they were built up. " Dust to dust " is a very real and constantly recurring cycle. The com- plex substances composing living tissue by successive cleavages become resolved into H2O, CO2, NH3, and similar compounds. Were this not so, the material to build the organic world would soon be exhausted, and the earth would be so covered with the remains of animals and plants, accumulating during long periods of time, that life would be impossible. In very hot, dry climates, as in deserts, animal remains actually do dry up and " mummify," without de- caying. In the presence of moisture, however, dissolution begins as soon as life is extinct. Putrefactive Organisms. — This process of dis- solution, which is merely the cleavage of the proto- plasmic compounds into their simpler components, doubtless would go on automatically, at least to a certain point, but the process is hastened and carried to a final conclusion by the assistance of various kinds of bacteria and molds, known as the putre- factive organisms. In the light of what has just been said these obscure and unpleasant atoms of life are one of the most important agents in the econ- omy, of nature. They are the wreckers, tearing down that Nature may build herself anew. Through 74 GENERAL BIOLOGY their action, carbohydrates and fats are reduced to CO2 and H2O, and proteins are dissolved, by a long chain of reactions, among other things into NH3 and CO2. The urea excreted by animals goes the same way. By this means the rotation of the elements through organic and inorganic nature is hastened and facilitated. The ammonia in the soil is taken in hand by another group of bacteria, the nitrite bacteria, FIG. 25. — Putrefactive bacteria of various sorts: A, Bacillus urea, the agent that ferments urea into ammonium carbonate and water and eventually into carbon dioxide and ammonia ; B, Bacillus subtilis, a putre- factive organism commonly found in hay infusions ; C, the same in the inactive zooglcea condition ; D, Spirilla, up., from hay infusion ; E , Bac-t terium " termo," from fermenting infusion of peas. which oxidize it to nitrous acid (HNO2). This combines with potassium or ammonia in the soil to form potassium or ammonium nitrite. Another group then further oxidizes the nitrite into nitrate,1 and makes it available for plant food. There is a similar cycle for the sulphur and phosphorus, but these elements, although absolutely essential for organic life, are but a small fraction of the bulk 1 I. 2 NH3 + 3 02 = 2 HN02 + 2 H2O. II. 2 HN02 + O2 = 2 HNOS METABOLISM 75 of the carbon, hydrogen, and nitrogen in living things, and in consequence there is little danger of the supply ever becoming exhausted. The purpose of phosphate fertilization is to supply the demands of the nitrogen -fixing bacteria (Azotobacter) , rather than the green plants themselves, and thus to aid the process of nitrogen fixation (see below). Denitrification. — The circle, nevertheless, is not so ideal as it might seem, for there exists another group of bacteria in the soil whose special activity it is to reduce nitrates again to gaseous nitrogen, which escapes to the atmosphere and is added to the inert quantity that plants are unable to " fix " or utilize. This means a constant loss of available nitrogen, a constant deficit, so to speak, in the annual balance sheet, and when the facts first became known, considerable doubt was expressed as to the future habitability of the earth when the available nitrogen should have been reduced too far. For- tunately for our peace of mind, there have been dis- covered yet other kinds of bacteria and molds that are capable of fixing, that is, combining with the nitrogen of the soil. These seem to be everywhere present in soils and make up for the loss due to the denitrifying bacteria. It has been known for many centuries that it im- proves the land to grow crops of peas, beans, or their relatives, and plow them under as " green manure." The ancient Romans and the Chinese and Japanese carried out such practices without knowing any 70 GENERAL BIOLOGY reason for the improvement. It has been known for a long time, too, that the rootlets of leguminous plants (i.e. peas, beans, alfalfa, etc.) are beset with little nodules or tubercles, which were supposed FIG. 26. — Tubercles on roota of clover. — (Osterhout.) to be pathological growths of the nature of galls. Comparatively recently it has been demonstrated that these tubercles harbor minute bacteria endowed with the property of fixing atmospheric nitrogen, and that this is the chief reason for the value of such plants as fertilizers. We discover, therefore, that METABOLISM 77 life not only goes on upon the earth but in the earth as well, and that the soil, far from being the lifeless mass of rocks and dirt we are accustomed to con- sider it, is pulsing with life in every granule, harbor- ing a multitude of different organisms that live in darkness and for the most part without oxygen, but whose activities are so vital for the more familiar life above the ground that one could not exist without the other. Nature of the Energy Transformed. — Move- ment. — The latent energy of chemical affinity may be transformed into all the other forms of energy known. The movements of an animal are primarily brought about by the contractility of its protoplasm, which is almost always differentiated (except in Protozoa) as muscular tissue. Intracellular circulation has been previously men- tioned. This is a form of motion probably univer- sal in animals and plants. The simplest form of motion, after this, is that described in the previous chapter in connection with the movements of leucocytes, and known as amoeboid, because it is the characteristic and only form of locomotion possessed by Amceba. A similar mode of unlocalized movement is also found in many pigment cells and in the primitive connective tissue cells of the develop- ing embryo. The majority of the Protozoa move by means of special organs of locomotion, differentiated either as cilia, covering all or parts of the cell-body, or as 78 GENERAL BIOLOGY flagella, inserted at the end or side of the cell. It is supposed that both cilia and flagella are hollow extensions of the cuticle, which by a sudden contrac- tion on one side produce a resultant movement like that of a fishing pole when a fly is cast. Fixed ciliated cells are also found as components of many tissues in Metazoa, as in the nasal passages of verte- brates. By the force of the beating cilia currents FIG. 27. — The electric ray (Torpedo) ; the skin partially cut away so that the electric organ, a, is visible ; it consists of numerous polygonal columns of modified muscular tissue. — (From Verworn, after Ranvier.) of liquid are urged along over the surface of the epithelium. The most highly developed tissue of locomotion is, of course, striated muscle. When a muscle con- tracts, it shortens and thickens without changing its bulk. Movement is communicated through its fixed tendons to whatever bones or other structures it may be attached. When a muscle has made a number of contractions, it shows a marked increase in acidity. This is due to the appearance of sar- METABOLISM 79 colactic acid, which can be shown to be a product of the breaking down of substances in the muscle fiber. Heat. — During contraction the muscle also de- velops heat, and it has been estimated that the amount of energy liberated as heat is five times that utilized as mechanical energy, i.e. " work " in the ordinary sense. This is shown by the " warming up " that physical exercise brings. The contracting mus- cle liberates still a third form of energy, namely, electricity, which may be measured by a capillary electrometer. Electricity. — Both heat and electricity as accompaniments of muscular activity are, in a sense, waste products and repre- ^IG- , a phosphorescent marine sent a certain necessary loss. Protozoan; magnified so On the other hand, heat is a ^ffififiSB transformation of energy very necessary to the organism, not only in the higher animals, where a definite bodily temperature must be kept up, but in all animals and plants as well, since it is only in the presence of a cer- tain amount of heat that the necessary oxidations and reductions involved in metabolism can take place. We are all familiar with the rise of tempera- ture produced by fermenting yeast. In both animals and plants the evolution of heat is usually brought about by the cleavage of a carbohydrate. 80 GENERAL BIOLOGY Static electricity is probably always developed by both animals and plants, as a universal concomitant of vital activity, but in some forms, such as the electric eel and electric ray (Torpedo), an arrange- ment of muscle fibers, in the structure of a galvanic pile, permits of the accumulation of a charge, so that such an animal can give a severe shock. FIG. 29. — Phosphorescence in Noctiluca. A portion of the body is represented, with numerous scintillating dots. — (From Calkins, after Quatrefages.) Light. — The energy transformations of metabo- lism also occasionally take the form of light. This is often called phosphorescence, owing to the fact that the light usually resembles the glow of phos- phorus. As a matter of fact it has nothing to do with phosphorus, which in its free and luminous state is an active poison to all living protoplasm. Phosphorescence is a special characteristic of many minute organisms of the sea and of the bacteria that develop in decaying wood and fish. Some of the more complex animals are provided with special light-giving organs that flash in the dark like torches. Certain insects show a remarkable development in METABOLISM 81 this direction. The most familiar example is that of the firefly, which has light-giving organs at the base of the abdomen. In the depths of the sea many of the fishes that inhabit those abysses have similar light-producing spots distributed over the body in characteristic ways. One of the species even develops such an organ at the end of a fila- ment like a pendent incandescent lamp. FIG. 30. — Lantern fish (Linophryne lucifer) . The phosphorescent bulb doubtless functions as a lure to entice other fishes within reach of the jaws. The body is distended with a large fish that has been swallowed. — (After Collet.) It has been found that the efficiency of the firefly's light is practically 100 per cent, that is, none of the energy is lost as heat; whereas in an ordinary in- candescent lamp but 3? per cent is utilized as light, and in an arc but 15 per cent, the rest of the energy being wasted as heat. In the insects mentioned above, the glow is produced by the oxidation of some specially secreted substance, probably fatty in nature, and the flashes of the firefly correspond with the intervals of taking in air through the res- piratory tubes. GENERAL BIOLOGY In addition to the foregoing forms of energy protoplasm utilizes its latent energy to produce the characteristic products that have already been described. These include, besides the food reserves, such as fat, starch granules, egg- yolk, etc., minute quantities of other substances (zymogens and hormones) which enable the cell to accomplish its multitudinous re- actions with the minimum ex- penditure of en- ergy. Enzymes and Enzymotic Reac- tions.— If starch be taken into the mouth or placed in a test tube with saliva, it is acted upon by the saliva and its mole- cules are split into their sugar elements, in which form they may be absorbed through the lining wall of the digestive tube. The chemist can also boil FIG. 31. — The firefly; the one on the ground shows the phosphorescent organs (the three white segments of the abdomen). — (From Linville and Kelly.) METABOLISM 83 the starch in any strong acid and produce the same change. In this case, however, a strong reagent and a high degree of heat are necessary. The mildly alkaline saliva at ordinary temperature thus seems to be able to accomplish the same result with a minute fraction of the expenditure necessary in the second experiment. The prompt and effective action of the saliva is due to the presence therein of a minute quantity of a substance called ptyalin, one of a large group of somewhat similar substances which we call enzymes. A similar enzyme, called pepsin, splits up proteins in the stomach ; another, lipase, splits fats, in various parts of the body; another clots blood, and so on. None of these enzymes has been isolated in a pure state, and we know little of their composition, al- though it has been surmised that they are allied to the proteins. They are certainly not alive in the usual sense of the word, for they may be precipitated without injury with absolute alcohol and other reagents. They are, however, " destroyed by boiling and do not " work " well below a certain minimum temperature. Most of them need a special environ- ment (e.g. hydrochloric acid for pepsin) to work best, and each of them is specific in its action ; that is, affects only one kind of substance. The most remarkable thing about them, however, is that although a minute quantity will produce a relatively enormous effect, the enzyme does not ap- pear to be used up at all, and the process may go on indefinitely, provided the products of its action are 84 GENERAL BIOLOGY removed as soon as formed. Some of these enzymes are reversible, splitting and synthesizing the same substance, as conditions differ. In these processes, enzymes seem to follow the same " law of mass action " as well as the equations of reaction- velocity in relation to temperature that have been worked out in inorganic chemistry. Within the cell, through the aid of enzymes there present, analysis and synthesis may take place side by side, and by a series of such changes, oxida- tion following reduction in continuous sequence, the most complex molecules may be built up, the energy of one disruptive process being utilized to combine the components into a molecule of a higher order. Oxidizing enzymes or " oxidases " seem to be present in all protoplasm, and owing to their presence the necessary oxidations take place in the organism very rapidly at a comparatively low temperature. The seat of these oxidations, and hence of the oxi- dative enzymes, appears to be in the nucleus. A distinction was formerly made between the so-called " unorganized ferments," such as pepsin, and the " organized ferments," such as the active principle of yeast, which was supposed to require the presence of a living cell in order to work. It has been found possible, however, to crush all life out of the yeast cells and, by filtering the extract, to get a solution which is as active a fermentative agent as living yeast. The distinction between the two sorts of enzymes then falls to the ground. The only difference seems to be that some enzymes METABOLISM 85 work within the cell and others without. The idea that the action of enzymes is physical and mechani- cal is strengthened by the fact that finely divided platinum (platinum black) will produce catalytic effects similar to those produced by oxidizing en- zymes. The most wonderful feature of all, perhaps, is the fact that the protoplasm makes its own enzymes, since they are, of course, the secreted products of the activity of the living substance, developed in just the places and apparently at just the time when needed. Since the action of enzymes is mechanical, the question has often arisen, — How is it controlled ? Why, for example, does not the stomach digest itself? We hardly know enough about enzymes to answer such a question in detail, but we have learned that many enzymes when produced are incapable of performing their offices until supplemented by another element, usually produced in a different region. The pancreatic secretion has no power to digest proteins until it has been " activated " by the secretion of the lining wall of the intestine, a secretion induced by the flow of acid liquid from the stomach. The action is due to the presence of a complementary body, enter okinase, which ap- parently combines with the zymogen (or enzyme- former), called trypsinogen, to form trypsin, the proteid-cleaving enzyme of the pancreatic secretion. Similarly it has been discovered that the muscles cannot reduce the glycogen which is necessary as 86 GENERAL BIOLOGY a source of their activity, without the presence of an activating substance, also formed in the pancreas and transported to the muscles in the blood. It seems likely that the majority of enzymes are thus compounded of a zymogen, secreted in the cells in the form of granules, and an activator with which it must unite before becoming capable of its specific action. In the stomach of warm-blooded animals the activator of the pepsin is hydrochloric acid, which is only excreted under the stimulus of the presence of food. But there seems to be also an antipepsin formed, which neutralizes the enzyme and prevents self-digestion. Internal Secretions and Hormones. — The cells and tissues of an organism, either singly or grouped in glands, produce a variety of secretions, such as starch, cellulose, egg-yolk, silica crystals, lime, mucus, zymogen granules, etc. These products are usually visible, and, being often extruded from the cells in which they are formed, they are spoken of as external secretions. By means of experi- ment it has been demonstrated that in addition to the external secretion of certain organs there is also an external secretion, so called, which, instead of collecting in ducts and being thus transported away from its source of origin, passes directly into the blood stream. Many " ductless " glands, such as the thyroid, have a large blood supply, which takes up such an internal secretion, but other glands, such as the pancreas, in addition to the evident external METABOLISM 87 secretion (the "pancreatic juice," with its various digestive enzymes), have been shown by experiment to develop important internal secretions as well. Thus it has been found that total extirpation of the pancreas produces the unexpected result of inaugurating glycosuria (diabetes), — a condition in which the kidneys constantly eliminate sugar from the blood. If, however, only a small portion of pancreatic tissue be left, no diabetes or only a very mild form results. It is evident that the exist- ence of this sort of diabetes is dependent upon the absence of pancreatic tissue. Even when the pan- creas has been removed, if a small part be grafted in where the blood can come in contact with it, no diabetes follows, a result that indicates not only that the ordinary intestinal secretion has nothing to do with it, but also that it is necessary for the blood to flow in contact with the pancreatic tissue. For this and other reasons, it has been concluded that the pancreas supplies to the circulating blood an important internal secretion, which, in some way at present unknown, controls the utilization of sugar in the animal organism, and in the absence of which this sugar passes out of the body unchanged. The adrenals, ductless glands attached to the kidneys, have been shown, in much the same way, to produce a substance, the presence of which in the blood rapidly increases the blood pressure and produces a strong contraction of the peripheral blood-vessels. This substance has recently been isolated from the extract of the gland, and is found 88 GENERAL BIOLOGY to be a white, crystalline, somewhat bitter powder, to which the name adrenalin has been given. Prac- tical use of this substance has been made in sur- gery. By its application small hemorrhages may be entirely done away with, particularly in delicate operations on the nose and eye. There has been synthesized recently a substance similar to adrenalin, if not identical with it, which produces the same effect. Another ductless gland, the thyroid, by its se- cretion, influences the normal phenomena of differ- entiative growth in the higher animals. When the thyroid is diseased, the whole system is affected. In extreme cases degenerative conditions known as " cretinism " and myxedema result. If the gland be extirpated in a very young animal, death in- evitably follows, but if small pieces be introduced elsewhere in the body by grafting, or especially if an extract of the gland be fed, the evil results dis- appear, or are greatly mitigated. The extract has been shown to owe its efficacy to the presence therein of a chemical compound (thyroiodin) containing a high percentage of iodine. Another somewhat sim- ilar ductless gland, the thymus, is also found in the throat. If tadpoles be fed thyroid, their metamor- phosis is greatly hastened, and they turn into tiny frogs before they have had time to grow to normal size. On the other hand, if fed thymus, differen- tiation is inhibited and growth accelerated, with the result that they grow into large tadpoles, but do not metamorphose at all. METABOLISM 89 The flow of half-digested food into the intestine mixed with hydrochloric acid (chyme) that is poured stimulates the cells of the lining membrane of the latter to produce a substance called secretin, which, passing into the blood, is carried to the .pancreas. Here it stimulates the excretion of pancreatic juice, which flows out into the intestine, apparently in direct response to the inflow of food, but really in response to another sort of stimulus. A chemical excitant like secretin, or thyroiodin, or the other products just described, has been called a hormone, and it is likely that the number and importance of such substances will be greatly increased by further investigation. They are the products of proto- plasmic activity, like the zymogens, but, unlike them, they are not destroyed by boiling, even in hydrochloric acid. CHAPTER IV GROWTH IN all normal plants and animals, anabolism nearly always tends to exceed katabolism, with a consequent increase in the bulk of the living sub- stance. When this increase in volume is permanent, we call it growth. Such changes are to be distin- guished from temporary changes in size or form due to the rapid imbibition of water or the evolution of gases. They are also to be distinguished from dif- ferentiating changes such as occur in development. The latter may or may not be accompanied by the increase in mass called growth. In plants, growth is a phenomenon which generally continues as long as the organism lives. In animals, it is a special feature of the earlier period of the organism's life, and then usually comes 4o an end. At that time a balance of metabolism is struck, after which the energies of the organism are directed, not toward increase in size, but toward reproduction. For this reason the period of greatest growth is coincident with immaturity. The enormous disproportion in the amount of growth in the earliest stages of exist- ence is illustrated by some calculations of Professor Hertwig. He estimates the volume of the human ovum at .004 cubic millimeter, whereas that of the 90 GROWTH 91 child at birth is from three million to four million cubic millimeters, an increase of one billion times. Yet from the first year to the twentieth the ratio of increase is figured at only one to sixteen. Since the organism is composed of cells, it is obvious that to accomplish this growth the cells themselves must increase either in size or in number. It was early discovered that both these changes take place, and that the latter seems to be consequent upon the former. We have seen that the nucleus " domi- nates " the rest of the cell, as it were, and that with- out the presence of a small portion of nuclear matter the normal changes of metabolism in the cytoplasm cannot go on. This influence of the nucleus appears to have rather narrow limits, and if the cell gets to be too large, portions of it may get out of the range of the nuclear influence. Sometimes this is avoided by the fragmenting of the nucleus, the parts being distributed about the cell; but this occurs in only a few kinds of cells. Normally, when the bulk of the cell has increased by growth to the natural limit, the nucleus divides into two halves that move apart and divide the original cell between them, thus making two new cells, separated by a newly formed cell- wall. Occasionally the cell- wall does not form, in which case we have a syncytium resulting. When the two daughter-cells have grown to the size of the original mother-cell, the process is repeated, and so on, the number of cells in a given tissue increasing with the growth of the tissue, but the average size of the cells themselves remaining nearly constant. 92 GENERAL BIOLOGY Since the cell gets its food by absorption, it is also of advantage to divide in this way in order to increase the absorptive surface; for whereas solids vary as the cubes of a dimension, surfaces vary only as the squares of a dimension. To use a concrete illustration, the combined surface of two halves of an orange cut in two in the middle is greater than that of the original orange by just the added areas of each cut surface, the cubic content remaining the same. Mitosis. — The direct cell division just described, in which the nucleus simply cuts in two and the two halves move apart while the cytoplasm cleaves between them to form two new cells, is sometimes observed, but is by no means the usual method. It will be remembered that the content of a cell is normally very heterogeneous, so that a division plane passed through the middle would result in producing two dissimilar halves, and if this were repeated a number of times, the various elements of nucleus and cytoplasm being segregated each time, the resulting cells would soon lose all real resemblance to one another, and the tissue which they compose would lose its homogeneous character. We know that this does not happen. Instead of this direct method of division we find that another sort of cell cleavage usually takes place, to which the name of mitosis1 has been given. This process is an extremely complicated one, and it will be more 1 The direct cell division just described is called amitosis. GROWTH CutiS. FIG. 41. — Diagrammatic transverse section through the shell of the tortoise. On the right side the bony shields have been removed. The endoskeleton is cross-lined, the exoskeleton is dotted. Sp.C, spinal chord ; Cap, head of rib. — (From Gadow.) Muscular System. — The tissues involved in loco- motion and other movements act by their power of contraction. In order to bring about orderly move- ments they must be attached to rigid supports. In the vertebrates, these are the bones of the skeleton, to which the muscles are firmly attached. The bones themselves are covered with a tough skin of 114 GENERAL BIOLOGY connective tissue, the periosteum. This is continu- ous with a dense inelastic tendon that spreads over and permeates the muscle bundles, forming the perimysium. Muscle and bone are thus most intimately bound together into a unit. In inverte- brates with a chitin- ous exoskeleton the muscles are usually attached to the hard shell or to internal plates that arise from the shell. In forms like the worms, in which the whole body contracts strongly, the muscles are to be found in sheets, usu- ally running in differ- ent directions, and exerting a reciprocal action on one another. In the very lowest and simplest forms they occur as isolated fibers. In such cases they represent the first step in the specialization of the contractile function common to all protoplasm. Circulatory Systems. — In most of the lower animals, particularly those protected by a tough or a hard outer covering, the tissues of the interior are very soft and loosely held together. The organs are, FIG. 42. — Diagrammatic cross-sec- tion through the thorax of an insect, showing the nature of the musculature and the manner of the insertion of the muscles to the exoskeleton : ht, heart ; n, nerve cord ; wg, root of wing ; Ig, root of leg ; ap, apodeme or chitinous brace ; Im, longitudinal muscles of the " back " ; m, dorso-ventral muscles oper- ating wings and legs. — (After Graber.) TISSUE-DIFFERENTIATION 115 as it were, awash in the body fluids. The alimentary canal occupies a relatively large part of the interior space, and, in consequence, the digested food is absorbed and passed on by diffusion to all parts of the body. Likewise the waste-products of metabol- ism find their way without difficulty to the special organs that provide for their elimination. In the larger animals, and particularly in those with rela- tively solid tissues, with a great development of muscles and an alimentary canal of small dimensions compared with the whole body cavity, it would be impossible for the digested food to find its way to the places where it is most needed, particularly to the skeletal muscles. Appropriate to the need for trans- porting such substances long distances from the alimentary canal and also of transporting the meta- bolic waste-products from the seat of their produc- tion to the excretory organs, there is a system of tubes, the circulatory system, which performs the same function carried out by a railroad system in a thickly settled community; that is, it transports substances from the region where they are produced to the region where they are utilized. The medium of transportation in the case of the organism is the blood. Thus, not only is the digested food carried from the intestine to the tissues which are to be fed, or to organs that serve as storehouses, but the oxygen taken in in respiration is supplied to all the tissues, the various hormones are transported from one place to another, and the waste products of katabolism are drained off to the excretory organs by 116 GENERAL BIOLOGY the same route. Such a system in its most elemen- tary form may be found in the insects. Hero it consists of a simple dorsal tube open at both ends, which by its alternate contractions and expansions keeps the body fluids " stirred up " and provides for FIG. 43. — Diagrams of the circulation of the frog (A) and of the lobster (J5), the former illustrating a closed system, the second, an "open" system: L, left auricle; R, right auricle; V, ventricle; 1, arterial branch to the head ; 2, arterial branch to the rest of the body ; 3, arterial branch to the lung (P) ; 4, pulmonary vein returning aerated blood from the lung to the left auricle ; 5 and 6, venous trunks return- ing blood from the head and body to the right auricle ; H, heart sending out arterial blood both to the anterior and to the posterior part of the body ; s, sinuses, in which the blood accumulates to be returned through the gills (G) to the pericardia! chamber (p.c.), whence it finds its way to the interior of the heart through the ostia (o) . an indefinite circulation of substances. In the sides of the tube are mouthlike apertures provided with valves, called ostia. Through these the body fluids can enter when the tube expands, but, owing to the valves, they can leave only through the open ends. The necessity for a complicated set of blood tubes in these animals is obviated by the develop- TISSUE-DIFFERENTIATION 117 ment of a remarkable system of air-tubes, the trachea, which open to the exterior and, permeating the insect body in every direction, permit a direct supply of atmospheric oxygen to get to all the tissues. The lobster presents a next higher step in differ- entiation and specialization. Instead of the elongate dorsal tube there is a boxlike heart, likewise pro- vided with ostia, but opening also into a number of blood tubes (arteries) that carry the blood away from the heart. These arteries divide and subdivide until finally they empty their contents into the open spaces between the muscles and other organs, called sinuses. From these spaces the blood seeps back and, after traversing the gills, reenters the heart through the ostia. Thus a circuit is completed, definite through part of its extent and indefinite through the rest. In vertebrates the blood not only leaves the heart by way of arteries, but is returned to it by another system of tubes called veins, the two systems being connected by much finer vessels called capillaries. Thus the entire circuit is com- pleted within definite channels. Such a system is called a " closed " system in contrast to the " open " system of the lobster. Excretory Organs. — The waste-products men- tioned above are not only of no use to the organism, but, on the ether hand, in most cases, are active poisons, whose ill effects are evident if they be ever so slightly concentrated. It is of the highest impor- tance to the animal that these substances be elimi- 118 GENERAL BIOLOGY nated as soon after their formation as possible. Even in the least differentiated of the Protozoa special FIG. 44. — Diagram of various types of excretory organs: A, Par- amecium with two pulsating vacuolcs, the upper one is at the close of cycle, the lower one at the beginning; B, flamc-ctll of a "flatworm"; /I, the wisp of cilia, the vibration of which urges the excreted matter down the duct ; n, nucleus ; gr, granules of waste products ; C, simple nephridium of Polygordius ; d, septum ; st, nephrostome or ciliated funnel ; 077, external orifice ; D, nephridium of a highly specialized annelid worm, the blood vessels in black ; E, nephridial element of vertebrate kidney ; g, glomerulus ; cap, capsule of urinary tubule ciliated in its lower portion, blood-vessels in black. structures are found which provide for such elimina- tion. In such organisms there is usually a spot on TISSUE-DIFFERENTIATION 119 the outer margin of the cell-body toward which the dissolved " waste-products " concentrate. These form a gradually enlarging bubble which finally ruptures and squeezes its contents out through the cell- wall to the exterior. In most Protozoa this excretory function is definitely localized, and the formation and extrusion - of the excreted drop of fluid follows a regular rhythm. Such an organ is called a " contractile vacuole." With the enlarge- ment of the body and the multiplication of the cellu- lar units composing it, it becomes impossible for such excreta to find their way to any particular spot, or even to diffuse through the tissues with sufficient rapidity to prevent the ill effects mentioned above ("intoxication"). Definite channels are found in most Metazoa through which these products pass. In the tapeworm and its allies these tubes are very delicate and permeate the body in all directions, opening to the exterior in pores. Each tube is thought to be the differentiation of a single cell and each terminates (or rather, more accurately, origi- nates) in a so-called " flame-cell." This cell has a wisp of cilia projecting into the cavity of the tube, by the flickering movement of which, suggestive of a candle flame, the absorbed fluids are urged down the tube and ultimately to the exterior. In the great group of the Annelids, of which the earthworm is the most familiar example, the body is composed of a great number of segments arranged like a row of pill-boxes strung on a tube, the tube being the alimentary canal. Each segment, al- 120 GENERAL BIOLOGY though a component part of a single organism, has a certain structural individuality of its own, and is separated from its neighbors on either side by a partition wall, the septum. The inclosed space, the " body-cavity," is filled with a watery fluid and is lined with a network of blood-vessels through whose walls the circulating waste-products transfuse into this fluid. In each segment there is found an open-mouthed, trumpet-shaped tube, fringed with strong cilia, which passes through the septum into the cavity of the segment just behind, and after a more or less convoluted course opens directly through the body-wall of that segment to the exterior. The action of the cilia creates a current in the fluid which passes down the tube, and drains off to the exterior the liquid contents of the body-cavity, laden with wastes. Such an excretory apparatus is of very widespread occurrence in various groups of animals and is called a nephridium (plural, nephridia) . Such a nephridium is found only in the most simply organized annelids. In the higher annelids we find the mechanism much improved by the elimi- nation of unnecessary steps and the securing of greater economy of energy and increased efficiency. Thus in the earthworm, branches of the blood-vessels surround portions of the nephridial tube, and the major portion of the excreted substances transfuse directly through the walls of the blood-vessels into the cavity of the nephridium, instead of transfusing first into the body-cavity The ciliated funnel in such a case may become almost (though not entirely) TISSUE-DIFFERENTIATION 121 functionless. This is much the same improvement that would be effected in the transport of grain to ships if, instead of unloading the sacks of wheat from the cars to the dock and then again to the ship, an arrangement were made whereby the grain could be poured directly from the cars into the hold. In the vertebrates a comparable relation of blood system and excretory tubules is found. Instead of being distributed throughout the body, these tubules are concentrated into a single organ, the kidney; the blood-vessels supplying the kidney develop tiny knots of capillaries called glomeruli (Latin, glomeru- lus, " little ball "), which afford a very large surface in a small space and thus permit the diffusion of the maximum of waste substances in a minimum of time. In the higher vertebrates there is no trace of the openings of these tubules into the body-cavity. In lower forms, however, such as the frog, they may be observed on the surface of the kidney although they are functionless. II. DIFFERENTIATION IN PLANTS Plants differ strikingly from animals in the emphasis which evolution has laid upon various functions common to both. Animals are, for the most part, actively moving creatures, seeking food in various places, and consequently endowed with elaborate systems of differentiated protoplasm in the form of muscles and sense-organs. In plants, on the other hand, the assimilative (" vegetative ") function is predominant, and the manifold differ- 122 GENERAL BIOLOGY entiations which we find are nearly all in the line of structures for elaborating, taking up, and storing foods, and of supporting the plant-body (omitting, of course, all consideration of the reproductive function, which will be discussed in a later chap- ter). Accordingly, we find little trace of nervous system or muscular system. Yet some plants, like the Mimosa or sensitive plant, react to stimuli with a definiteness that is comparable to a nervous reflex in animals. The leaf of the " Venus' flytrap," when an unwary insect touches it, springs shut with such suddenness and vigor as to catch the prey and hold it fast. This action is brought about by three sensitive spines which may therefore be held to be analogous to animal sense-organs. A number of plants have the habit of folding their leaves at night, in " sleep," the stimulation being the change from daylight to darkness. Moreover stimuli are trans- mitted (usually slowly, to be sure) from one por- tion of the plant to another. Plant tissues may be anesthetized, and when stimulated, they show " fatigue." We may conclude, therefore, that al- though the basis of the mechanism may be entirely different from that of animals, yet, in so far as the plant tissue is differentiated sufficiently to receive stimuli from without and transmit them to other parts of the plant-body, there to bring about an appropriate response, it may be asserted that plant j possess a rudimentary nervous system. They diffe.1 from most animals in lacking any sort of a coordinat- TISSUE-DIFFERENTIATION 123 ing or central nervous system ; that is, the impulse is conveyed directly from a more or less diffusely sensitive area to the tissue which reacts. Plant Movement. — We ordinarily think of plants as rooted fast in the ground. Nevertheless, apart from the lower forms, mostly unicellular, which move as do many Protozoa by means of the lashing of flagella, a little observation will show that most plants execute movements differing in intensity from the sharp folding of the leaflets of the sensitive plant to the gradual circling of the sunflower head as it follows the sun from east to west during the day, or the twisting of the climbing tendril. No such special- ized tissue as muscle is to be found in plants, but all such movements are to be attributed to changes in the water content of masses of spongy tissue, such that when the cells are full of water, the resistance of the walls, or turgor of the tissue, makes it stiff and erect, whereas the withdrawal of the water from the tissues causes the leaves or other parts to become flaccid or droop. A difference in the amount of turgor on opposite sides of the stem will thus cause the stem to bend to one side or the other according to which side is under the greater tension. Sudden movements are brought about by rapid alterations in the turgor in localized areas under the influence of external stimuli. Supporting Structures. — In animals, as we have seen, the skeleton consists of a framework, either 124 GENERAL BIOLOGY external or internal, to which the softer tissues are attached or by which they are inclosed. This skeleton, whether of chitin, or bone, or merely of connective tissue, owes its rigidity to the deposit among the living cells of a non-living intercellular substance; the cells themselves have very thin walls. In plants, on the other hand, we find that the intercellular substance is laid down in the form of dense cell-walls of the cells themselves, and that nearly the whole tissue of the stem or root is in one sense skeleton. This intercellular substance is a complex carbohydrate called cellulose, or a deriva- tive of cellulose, lignin, and although it forms the cell-walls it is, of course, not living substance itself, any more than the plates of lime in bone. Not all the cells are uniformly developed in this manner. In most plant.tissues there has arisen a differentiation of the supporting or " mechanical " tissue which frequently occurs in bundles or strands of fibers, constituting a sort of internal skeleton. Hemp and flax are abundantly supplied with these fibers, which provide us writh linen, hempen cord, etc. On the out- side of roots and stems, particularly of the larger plants (trees), the cell- walls become enormously thickened, with an accompanying diminution of the protoplasmic substance, to form bark OK cork. The cork is impervious to water and may be com- pared with a secreted exoskeleton, like chitin, which protects the softer living portion beneath. In trees the whole central part of the stem is composed of solid supporting tissue (wood), the living portion of TISSUE-DIFFERENTIATION 125 FIG. 45. — Stereogram of the leaf of an Iris : e, epidermis ; c, cuticle ; p, palisade cells filled with chlorophyll-bodies ; col, collecting cells ; conv, conveying cells which assist in the transfer of the synthesized sugars from their place of origin to the veins ; sh, conducting sheath of vein ; h, wood of vein ; b, bast or woody fibrous portion of vein ; a, air- space ; s, stoma ; g, guard cells. — (Osterhout.) 126 GENERAL BIOLOGY the trunk being confined to the comparatively narrow layer between this and the bark. In the leaves the skeleton assumes the form of branching " ribs " or " veins." In some groups, e.g. the grasses, these are single and unbranched ; in others they form a network. Circulatory System. — In plants, as in animals, in those species which are of such a size that substances do not diffuse readily through their tissues, there is a system of tubes which permit the circulation of liquids from one place to another. The movement of the circulating liquids is due to mechanical agencies external to the protoplasm. There is, of course, nothing corresponding to a heart. The tubes, which are of various sorts and positions, often occur in bundles. They are formed by the coalescence of cells, end to end, and the subsequent dying out of the living contents. The prime factor in causing the ascent of water in the plant tissue is the " transpira- tion " or evaporation of water from the leaves. Just how this operates is still a complicated and un- solved problem. A result of such movements of liquid is the transportation of food substances to various portions of the plant-body. Alimentary System. — Algae derive their food from the surrounding medium, and their tissues are cor- respondingly soft and permeable. There is for this reason no localized area for the alimentary processes. In land plants, on the other hand, there is a twofold TISSUE-DIFFERENTIATION 127 source of food, the soil and the air. The plant can take in food from the soil only in solution, and such dissolved substances can transfuse only through very thin cell- walls. Accordingly the roots of such plants are covered with delicate threadlike processes called root-hairs, each of which is composed of a single thin-walled cell and is, indeed, merely an extension of a cell of the skin cf the rootlet itself. These root- hairs are quite short-lived and are to be found therefore only in the youngest root-branches. The organs for taking in food from the air are the green leaves. The upper and lower surfaces of the leaf are usually made up of rather stiff cells, forming a cuticle, in which are found numerous mouthlike openings between the cells, called stomata, that lead to air-spaces within the leaf. The body of the leaf is made up of numerous irregular or elongate cells, loosely packed together, and crowded with green chloroplasts or chlorophyll bodies, the means whereby the carbon dioxide of the air is fixed and converted into carbohydrate (see Chapter II). The substances taken in through the roots, and the sugars and starches formed in the leaves, are dis- tributed throughout the. plant-body by means of the circulatory system mentioned above. In lower plants with relatively undifferentiated tissues such a distribution of substances must take place by direct transfusion through the cell-walls themselves. As in animals, however, food is not taken in continuously in higher plants, at least not in the leaves. The formation of sugar depends upon sunlight, and ceases, 128 GENERAL BIOLOGY of course, at night. During the day the sugar is usually converted into starch and stored up in the leaves as such. During the night, however, this excess is converted again into sugar and carried away to nourish the plant elsewhere. As in animals, various parts of the plant may serve as storehouses. Accumulations of food are usually found in seeds, where their presence is of manifest advantage to the developing plant. Under the influence of certain fungi the underground stems of certain plants (e.g. the potato) thicken up and accumulate starch. Such accumulations of " reserve " food may not always be starch. They may be sugar (sugar beet) or fat (cotton seed) . Some plants, including the whole group of fungi (molds, etc.), draw their food supply from other plants or animals. As they get their food at second- hand, already elaborated, they lack or have lost the special structures by means of which other plants manufacture their own food. CHAPTER VI ONTOGENESIS THERE is no reason, d priori, why the individual plant or animal, barring accident, should not live forever. We can conceive of a perfectly balanced metabolism in which the up-building or tissue- repairing process would exactly balance the dis- ruptive or tearing-down process. As a matter of fact we know of no form, even among the simplest, of which this is true. In all, katabolism after a certain time tends to exceed anabolism. In spite of Nature's wonderful recuperative powers, the living organism, like a machine, tends to wear out, and after a brief period is fit only for the scrap-heap. Competition, the " struggle for existence," is severe among different species, and a species that is able to replace frequently its worn-out members with vigorous new individuals would maintain its level of efficiency, so to speak, at the highest point. More- over, where numbers count so heavily, the species that might be able to replace each worn-out veteran with a hundred or a thousand new recruits, would have a corresponding advantage over one that could not do So. Whether or not some such conditions as these may have been the cause, they indicate, at any rate, an K 129 130 GENERAL BIOLOGY advantage to the species of the phenomenon of reproduction, — the replacement of individuals by others from the same stock. Each individual passes through a cycle of birth, youth, maturity, senility, and dissolution, but before the final stages are reached, it normally produces other individuals to take its place. Biogenesis. — This phenomenon clearly implies a continual stream of life, in which individual succeeds individual like waves on the ocean. The physical connection of the individual with its ancestry is thus obvious. An aphorism of a century ago expresses it, — " Omne vivum ex vivo," " all life from [pre- existing] life." But a contrary view was also held until very recent times, viz. that living organisms may arise from non-living matter. This conception, which has been called spontaneous generation or abiogenesis, arose from incomplete or misinterpreted observation. Thus it is a matter of everyday ob- servation that maggots develop in rotting meat. Whence do they come, if not from the meat itself? A generation less well trained in the methods of exact research found no difficulty in accepting just such an hypothesis, but an ingenious Italian, Redi, showed that if the meat be covered with gauze so as to keep out the blow-flies, no maggots ever develop, since these are produced, not from the meat, but only from the eggs which the fly lays on the meat. In brief, it has been conclusively shown, in every instance, that no living forms are to be found to-day, ONTOGENESIS 131 except such as have arisen from preexisting individ- uals of the same species.1 Individuals give rise to other individuals either by simply cutting in two to form two half-organisms, each of which reorganizes its tissue so as to complete itself to the specific type, or by budding off a small portion of itself, which grows and differentiates to a form similar to its parent, or, finally, by budding off a single cell, which grows and differentiates into an individual like its parent. Reproduction as a Growth Process. — It was pointed out in a previous chapter that, in most cases, cell division is a consequence of cell growth or " cumulative in- tegration " and, in fact, may be considered as a phase of the growth phenomenon, discontinuous instead of continuous. The result is to increase the mass of a tissue with- plaj^; out differentiation, and without simple budding.— the alteration of the size-relations of its components, the cells. In the case of free- living one-celled organisms, cell division results from the same cause, — with the difference that the newly formed cell units are free individuals instead of elements of a mass. This is, however, a distinction without any real difference. On the other hand, certain unicellular organisms, after fission, remain 1 Or in rare cases as mutants from closely related species. See next chapter. 132 GENERAL BIOLOGY attached to one another in chains or masses. Such a mass might justly be called a tissue, except that it is convenient to restrict the use of that term to a cell-mass which, in turn, is a part of a still more highly organized complex. In certain Algae this connection of cells with one another is transitory and indefinite. In some forms, however, the con- nection is permanent, and the mass con- sists of a definite number of cells. In such a case we speak of the cell group as a colony. The com- bination of individ- uals in a colony is not only found in the Protista, but in many FIG. 47. — Budding in an animal develop from an outgrowth of the body- °f tne higher groups wall analogous to a plant rootstalk. as well, particularly Natural size. — (Herdman.) in such forms as are fixed to one spot (sea squirts, crelenterates, sponges, and most plants). Fission in Metazoa. — Not only in the Protista, but in metazoa also, are found examples of repro- duction by fission. In Ctenodrilus, one of the lower Annelids, the worm cuts in two by transverse fission much as an elongated protozoan. The separate por- tions of the original body then become transformed into complete individuals by a shifting and read- ONTOGENESIS 133 justment of the original tissues, some of which become modified to wholly different uses to what they served before. FIG. 48. — Vegetative reproduction (terminal budding) in an Annelid worm, Myrianida: A, an asexual individual which has produced by budding from a zone (z) a chain of twenty-nine zooids, the oldest being labelled 1, the youngest 29; B, a ripe male zooid ; C, a ripe female zooid. — (Malaquin.) In Myrianida, another worm of the same class, the divisions follow each other so rapidly that one 134 GENERAL BIOLOGY individual is not budded off before another constric- tion becomes visible, and the result is a chain of individuals in various stages of differentiation. It is evident that if, instead of separating from the parent organism, the newly formed " sub-individ- uals " remain attached, we would have a resultant organism of a different character, compounded, so to speak, of individual units held together by a sort of common bond. It is supposed by some that the segmental structure more or less evident in nearly all the animal kingdom came about originally through some such suppressed fission. Just as we have seen that whereas the stress of growth and the necessity for maintaining a certain relation between nucleus and cytoplasm results usually in the cleav- age of the cell, but on the other hand results some- times in the suppression of this cleavage through a distribution of the nuclear substance to form multi- nucleate cells or syncytia, so also conditions of existence that in the beginning brought about a cleavage of the whole organism (schizogeny) also may have made it advantageous for the separate parts to remain together. We can trace different degrees in such a condition, from Ctenodrilus, in which the separation is immediate, or Myrianida, in which it is temporarily retarded, to the tape- worm, in which the segments are permanently attached together to form a " segmented body." In the tape-worm, however, the segments at the posterior end may drop off without losing such vital functions as they are endowed with, and the number ONTOGENESIS 135 of segments in the body apparently has no influence on or relation to the individuality of the organism. In the higher animals this primitive segmentation has become overshadowed by the unity of the whole, so that no segment could be sacrificed without destroying the individual. This type of bodily structure is called metameric segmentation, and is the fundamental plan in nearly all the animal phyla,1 FIG. 49. — Diagram of metameric segmentation (as of the earth- worm) : A, longitudinal section of the body showing the alimentary canal and coslom divided by septa ; B, cross-section ; coe, coalom ; al, alimentary canal ; m, mouth ; an, anus. — (From Sedgwick and Wilson.) from the generalized Annelids, in which the segments bear to the body somewhat the same relation that the individual cars do to a vestibuled train, to the higher vertebrates, in which the segmental feature is plainly evident only in the developmental stages. Fission in Lower Plants. — In the important group of the Bacteria, multiplication takes place exclusively by fission. In the spherical type (Coc- cus) the planes of division may be repeatedly 1 The exceptions are the higher Mollusca, Flatworms, Crelenterates, and Sponges. 136 GENERAL BIOLOGY transverse, producing strings or chains of cells (Streptococci), or they may be alternatingly in two vertical planes, producing groups of four, or, finally, the planes of division may occur in the three dimen- sions of space, producing groups of eight (Sarcina) in a cubical aggregate. FIG. 50. — Types of Bacteria, illustrating various modes of fission : A, simple coccus form; B, diplococcus (fission in one plane only, the pair separating as formed; E, chain form (streptococcus), the fission all in one plane, but the cells remaining together; C, division in fours (tetrads) in one plane ; D, division in three planes to produce cubes or bale-like masses (sarcina) ; G, division planes at random, to produce masses (staphylococcus) ; F, flagellated cocci (nitrogen-fixing bacteria of the soil); H, rod-form (bacillus) ; /, bacillus with flagella at one end (putrefactive bacteria); J, with flagella all over the cell body (typhoid bacillus). Budding. — From reproduction by fission of the parent body, to reproduction by cutting off of a small portion of the body, is a short step. In the latter case the individual identity of the parent organism is unaffected, and the part budded off develops into the specific type by individual growth and differentiation. Such a process of reproduction is known as gemmation or budding and is well-nigh universal in the plant kingdom as well as in those ONTOGENESIS 137 animal types that have a plantlike habit, such as the sponges, ccelenterates, and tunicates. Familiar examples are the " runners " of strawberries, the " shoots " of willows and other trees, the tubers FIG. 51. — Habit of growth of a grass: A, aerial stem terminating in a branched inflorescence; B, underground stem or rhizome sending up a new shoot from one of the nodes; C, section through the node of a stem that has been placed horizontal, showing the sheathing leaf base, I, and the beginning of the upward curvature of the stem. — (From Curtis.) of potatoes, etc. The most primitive condition in plants was doubtless that of a plant-body (thallus), dying or drying up in the middle, and leaving two 138 GENERAL BIOLOGY or more vigorous extremities to pursue an individual existence. The development of runners and shoots is a special modification of the same process. In the animal realm, one of the simplest examples of budding is found in the common fresh-water Hydra. In this form, the interstitial cells at a FIG. 52. — Budding Hydra, as seen under the low power of a com- pound microscope: B, attached end; Bl, fi2, buds; M, mouth; T, tenta- cles.— (From Hunter's Elements of Biology : American Book Co.) certain point begin to increase in numbers, and form a knoblike protrusion on the side of the hydra-body. By a shifting and rearrangement of the cells composing this lump, a cavity forms, in direct communication with the inner cavity of the Hydra. This bud grows until it attains a considerable size, when the cells at the extremity begin to differentiate into ten- ONTOGENESIS 139 tacles, a mouth opens, and the bud becomes in all respects an individual Hydra, able to carry on all the functions of life. The process is completed by a severing of the connection between bud and parent. Permanent Budding. — In the majority of the marine relatives of the Hydra the buds formed in this way do not separate from the parent stem, but FIG. 53. — Portion of Syllis ramosa, an annelid worm in which the col- lateral buds branch repeatedly. — (M'Intosh.) remain attached, forming colonies of many indi- viduals. Such a condition of persistent budding is characteristic of plants, but occurs as well in many Protozoa. In some instances the buds run together in such a fashion that it is impossible to discriminate one individual from another. In Syllis we have a highly organized worm that divides and subdivides, like the branches of a tree, while individual heads develop here and there. 140 GENERAL BIOLOGY Spore-formation. — In many types of Protista a modification of reproduction by fission is found, which is especially advantageous in certain condi- tions of existence. After repeated divisions of in- dividuals in a free state, the organism may surround itself with a thick wall, forming a cyst within which the protoplasm fragments into a multitude of minute particles called spores, each of which has the poten- tiality of developing into an individual like that which formed the cyst. In this way a species may tide over a critical period, as of drouth, in such an encysted condition. When favorable conditions again intervene (perhaps after several years), the multitude of emerging spores insure the immediate existence of a large number of individuals, and thus reduce the chances of extermination for the race In the bacteria, under certain conditions, the proto- plasm of the tiny cell condenses into one or more spores, which, on account of their minute size, may be blown about in the dust, and thus afford the most effective means for the dispersal of the species. Propagation by spores is, indeed, a feature of devel- opment throughout the plant kingdom. Plants are adapted in large measure to what wt call a "vegetative" existence; that is, with the exception of the Protophyta, most of which are motile, they are fixed in the place where they sprout and cannot seek food elsewhere if it fails, or avoid extremes of climate by migrating. This disad- vantage is compensated by the production of spores, which are frequently developed in enormous number ONTOGENESIS 141 and, being scattered by various agencies far and wide, bring about an extensive distribution of the species. The method of propagation just outlined, whether by spores or by buds, by simple or multiple fission, is termed vegetative or asexual reproduction. We may define it as the cutting off from an organism of a single mass of protoplasm (of one or many cells) which independently differentiates into an organism resembling its parent. The greatest diversity in vegetative reproduction is to be observed in the various groups of animals and plants, but in every case it can be explained as a special form of the growth process. SEXUAL REPRODUCTION In nearly all forms of organic life, reproduction is complicated by an accompanying phenomenon, the meaning of which is not at all clear. In the bacteria and a few other Protista vegetative reproduction is the only sort known. In all others we find that, either accompanying the vegetative reproduction, or alternating with it, there occurs the production of certain reproductive cells (germ-cells) to which the name gamete is given. Two of these gametes fuse together, either completely or partially, and from the fused cell (called the zygote) a new individual devel- ops, or, more accurately, the zygote transforms by growth and differentiation into a new individual. When the two cells fuse completely into one, in the formation of the zygote, the process is termed ho- logamy. When the fusion involves only the nucleus. 142 GENERAL BIOLOGY it is called karyogamy. It is doubtful, however, if pure karyogamy ever takes place. The total fusion of gametes, in turn, may be of various degrees of specialization, either between similar cells (isogamy) or between cells of different sizes (anisogamy), leading to the differentiation of sexually specialized cells (oogamy). ,-%: FIG. 54. — Bui!" letts : 1 , normal " monad " ; 2, same in the semi-amoeboid condition preparatory to conjugation; 3, two individuals in the process of coalescence; 4. the resultant zygote; 5, a zygote whose protoplasmic contents have divided into spores; 6, young monads escaping from the sporocyst. — (After Kent.) Total Conjugation. Isogamy. — Even in the " lowest " of the Protista, the generalized condition in which two similar adults conjugate to produce a zygote is very rare. In such a case, the entire organism functions as a gamete. One of the most primitive examples is Bodo lens. In this species, two ONTOGENESIS 143 free-swimming individuals (see fig. 54) come to a temporary resting position with one flagellum touch- ing solid matter. They then sway toward each other, meet, and fuse into one. Their flagella dis- appear, and a thick covering or cyst is secreted about the fused mass. After a period of rest, the contained protoplasm fragments into a number of spores, or into individuals like the parents, which escape from- the cyst as typical "monads" of a smaller size. Other Bodos show a certain difference in size between individuals of the same culture. Conjugation occurs apparently at random between these dissimilar in- dividuals and between similar ones. It is perhaps more usual for the fully developed microorganism, instead of fusing directly with another similar individual, to fragment into a number of minute, active, individuals called micro- gametes, which conjugate with one another. The latter procedure must be of advantage to the species, since on account of the much larger number of new individuals produced at one time, the race is not so likely to be exterminated. One of the most primitive examples is Stephano- splwera. which consists of a colony of eight flagellate individuals (fig. 55) arranged in a plate within a gelatinous sphere of which they form the equator. In reproduction, each cell of the colony divides into sixteen or thirty-two smaller individuals (gametes), all of the same size, which break out of the gelati- nous sphere, swim away, and conjugate, two by two, to form a zygote. Each zygote divides into four 144 GENERAL BIOLOGY free individuals, each of which secretes a gelatinous sphere and by successive divisions gives rise to a new eight-celled colony. Anisogamy. — A further advantage would accrue to the species that developed microgametes, in FIG. 55. — Stephanosphcera, a colonial flagellate which reproduces by isogamy : A, a mature 8-celled colony in equatorial view ; B, a colony whose individuals by repeated divisions have formed daughter-colonies within the mother colony ; C, a colony, whose individuals by repeated divisions have formed flagellated individuals that function as gametes ; D, single gamete on a larger scale ; ^ «Jx off one of the eye- // i^Sk ;fi^ sta^s> another will \ 7 jft grow to replace it, ^ t^J^\> but if, in addition ^*"*W. • *%^-— ~- »— -*\ fS N . to severing the eye- stalk, he also de- stroys the deeper- lying ganglion, the FIG. 70. — Regeneration of antenna-like resultant growth organ in place of eye-stalk, in Palcemon. — • „„* an „..„ ct-illr (From Morgan, after Herbst.) at all, but an antenna-like structure. (See fig. 70.) It is inter- esting to find that the new organ is still of the crus- tacean type. Such a diversion of the normal path of differentiation is termed heteromorphosis. Theories of Morphogenesis. — The nature of the changes involved in the ontogeny of both plants and animals has been briefly outlined. The ques- tion arises: Why does the germ always give rise to precisely the type characteristic of the species and no other? Nothing that our microscope can tell us of the young embryo within the eggshell of a pigeon or a sparrow gives us the slightest clue as to why the culmination of the development of one ONTOGENESIS 191 is different from that of the other. More funda- mental is the question : Why does the shapeless germ take form at all? Nothing that we can learn of its nature or its structure gives us any reason for believing, a priori, that it will shape itself into an individual that resembles its parents. This is an old problem, and its solution was attempted long before our modern instruments for research gave us the insight into developmental processes we now possess. Preformation. — Suggested perhaps by the struc- tures found within the flower-bud, or the insect chrysalis, the idea was long current that the germ contains within itself the whole organism in minia- ture and that development consists, simply, in an unfolding and enlarging of this preformed indi- vidual. The chief elaborator of this speculation was Bonnet (1720-1793), but such a great naturalist as Cuvier also subscribed to the doctrine.1 Since the generation " A " was preformed in the genera- tion " B," then its descendants must also have been there preformed, and so on. The germ-plasm, therefore, was conceived of as a sort of nest of Chinese 1 There was a division of opinion as to whether thp preformed indi- vidual existed in the egg or in the sperm. Some held that the former is the case and that the function of the sperm is merely to fructify or "fer- tilize" the dormant egg. These speculators were denominated "ovists." On the other hand the " animalculists " contended that the egg is merely dead matter serving for nourishment, whereas the sperm is active and "alive." Moreover, with the primitive microscopes of the time, they had no difficulty in distinguishing head, arms, legs, and other structures in the sperm. 192 GENERAL BIOLOGY boxes, each one inclosing a series of increasingly smaller ones. Indeed the theory was called the " encasement " theory. Epigenesis. — Wolff, the father of modern em- bryology, investigated the developing chick in the shell, discovering, among other things, that the heart actually comes into existence after development begins, and was forced to conclude that there is no evidence whatever of the preexistence of the chick in the germ of the egg. In his Theoria Generationis (1759) he advanced the hypothesis of epigenesis, according to which the development of the germ involves the coming into existence of new structures with each generation. It was thus in direct con- flict with the preformation concept held by the majority of eighteenth-century naturalists and philosophers. As for the means of this epigenetic development, he conceived of a specific internal energy or force (vis essentialis) that permeates living matter. Development (in the hen's egg) is not brought about by the heat of incubation, but by the operation of this somewhat mystic internal force. Wolff's results and speculations were a long time in gaining acceptance, but the gradual im- provement in microscopes, and in technique, made it impossible t« accept the naive preforma- tion of the earlier school, and the biological world became persuaded to the epigenetic way of thinking, without, however, accepting the " vis essentialis" ONTOGENESIS 193 Weismannism. — In the course of time the pen- dulum again swung back toward the preformation standpoint. The increase in knowledge of the data of heredity, and a more exact understanding of the cellular phenomena of zygosis and ontogeny, forced speculative biologists to refer back the structures that develop in morphogenesis to some sort of pre- existing structure in the gametes. Under the hand of Weismann (1834- ) this became an elaborate architecture of determining particles of ultra-micro- scopic size, each of which is the causal agent in bring- ing about the development of some part of the organ- ism. Weismann laid much emphasis on the concept of the continuity of the germ-plasm, in contrast to the soma, a matter already discussed. Development thus becomes a mere sorting out of determinants, and the organism is a sort of mosaic. One obvious corollary of such an hypothesis is the fact that nothing that may befall the soma after development begins can have any influence in modifying the result of development in a succeeding generation, since each generation develops in strict accordance with the determinants in the germ-cells. Most ex- periments seem to prove that this is so. On the other hand, the facts of regeneration and regulation, just cited, are a strong argument against such an inflexible mosaic development. For this and other reasons, the elaborate and complicated architecture which Weismann postulated to exist in the germ- cells is not considered by modern biologists really to exist. Nevertheless, for many reasons, particularly 194 GENERAL BIOLOGY on account of the discoveries in Mendelian inher- itance,1 a great many biologists believe that the development of the structural characteristics of animal and plant individuals is dependent upon the presence of " something " in the germ-cell to which the name determiner is given. This determiner is a physical entity of some sort, however, and very different from the vis essentialis of Wolff. Vitalism and Mechanism. — The more exact be- comes our knowledge of the processes of differentia- tion and development, the more wonderful appears the delicate adjustment of forces that brings about the final result. The correspondence of time and place in development is at present particularly difficult to comprehend. Why, for instance, does an organ, let us say a finger, develop at precisely the time and place necessary to produce a symmetrical whole ? Why does an organ develop in apparent anticipation of a subsequent need? The wtalists believe that no known laws of matter can account for the adaptation of means to the end, which we are constantly confronted with in the study of mor- phogenesis, and that it is necessary to postulate a non-mechanical principle or " vital force," to which various names are given, which is a guiding and con- trolling agency in directing the course, not only of development, but of life processes in general. A sculptor in modeling a statue must have a pretty clear idea in his mind of just what he expects to 1 See next chapter. ONTOGENESIS 195 realize in the completed work: he works toward a definite end, and his preliminary efforts are con- ditioned by the sort of final result he wishes. To many observers the processes of development seem equally conditioned upon the nature of the final result, and it is hard to see how such events could come to pass without the help of some guiding agency, like the sculptor in the previous comparison. Such a point of view is called teleological. Human action is so constantly purposive that the untrained mind unconsciously reads into all the activities of Nature a similar purpose. A bygone generation, but by no means an unintellectual one, could see no way of accounting for the movements of sun, moon, and planets except by postulating the assist- ance of angels who pulled and pushed them along their appointed courses. With the increase of knowledge of celestial mechanics it became clear that the intervention of the imaginary angels is not necessary, and the explanation of the move- ments of heavenly bodies became an impersonal or mechanical one. In the same way, to " explain " the complex processes of development, it is not necessary to call upon the guiding help of some hypothetical vital force, even though our knowl- edge of developmental mechanics is still far too inadequate to explain the observed phenomena. One of the most prominent students of the rela- tions of plants to their surroundings l says : " Each year the list of * vitalistic activities ' of plants iH. C. Cowles. 196 GENERAL BIOLOGY becomes more and more restricted through the estab- lishment of a definite physical or chemical cause for what had been thought to have a vitalistic explana- tion, while never in the history of science has any phenomenon, once explained on a physical or chemi- cal basis, later been found to be vitalistic." 1 Summary. — In each species of plant or animal there is a continuous and unbroken succession of individuals that constantly replace one another. There is no authentic instance of any individual form of life coming into existence except from a preexisting individual. The reproductive process is essentially one of discontinuous growth. In its simplest expression it involves the cutting in two of a parent organism to produce two " daughter " individuals. Instead of a half of the parent organ- ism, the source of the new individual may be a por- tion of the parental tissue (bud) or a single cell (spore or gamete). In the case of the spore the new individual arises by direct growth and meta- morphosis, but in the case of the gamete it arises from a zygote, which is the result of the partial or complete fusion of two gametes. In any event the specific form of the new individual is attained by differentiation from a relatively generalized to a 1 The author is aware that the above paragraph gives a very incomplete presentation of the vitalistic standpoint, particularly of that of the so- called Neo-ritalists. There are many kinds and degrees of vitalism, but to go into the subject in detail is quite outside the compass of a work of this sort. The interested student is referred to the works of Driesch, Bergson, Reinke, Lovejoy, etc. ONTOGENESIS 197 relatively specialized condition. There is a striking parallel in the course of the differentiating process in the ontogenesis of all species. The similarity of the steps in any two different forms is in direct ratio to the closeness of their relationship. Sexual reproduction (by gametes) is apparently an inci- dental specialization and not a necessity for the accomplishment of the reproduction process, since experiment has shown that the egg contains within itself all the required potentialities for individual development. It is probable that the same would be also true of the sperm except that specialization has deprived the latter of the necessary food supply to serve as a source of energy. The accomplishment of the specific form is not only brought about by the differentiation of a specialized cell, normally pro- duced by another individual (ontogenesis), but also is manifest in the restoration of structures in the same individual, when these are abnormally altered or destroyed (restitution). CHAPTER VII VARIATION AND HEREDITY Variation. — When we see twins that resemble each other closely, or two unrelated individuals that have many features in common, our attention is at once attracted, and the fact that. the phenomenon excites our interest attests its comparative rarity; in other words, we are accustomed to the fact that individuals do not resemble one another, and the occasional exception is therefore conspicuous. If we apply exact measurements or other criteria to any sort of plant or animal, or to a structural part, and compare with similar measurements on other individuals, we find that the same thing holds true, — that variation is a universal phenomenon, and duplication almost non-existent. The analysis of this fact of universal variation resolves itself into a comparison of structures, — the components, so to speak, that go to make up the individual. Thus, if we wished to compare the individual seeds in a handful of beans, we should describe their size, shape, color, texture, etc. These components are technically called " characters." It is obvious that a complex individual may be resolved into a large number of such characters, displaying all sorts of variations when compared VARIATION AND HEREDITY 199 with other individuals. These may be described and catalogued, but words alone will hardly suffice to discriminate the finer shades of distinction be- tween so many classes. To seek order in such a chaos, some sort of mathematical basis must be devised. If we study a group of a hundred men with regard to a single character, such as stature, we find, of course, that all the individuals fall within rather definite limits, from the shortest man to the tallest, and we might classify them by arranging them in a row in the order of height. The line connecting the tops of the heads of such a row of men should be irregular and jagged and would defy analysis. Sup- pose, however, that we group such a lot of men in classes corresponding to the various statures, and place a representative of each class with his heels on a base-line. Then, grouping all of a class together (see fig. 71), one in front of another, we would find that the line connecting their heads, when viewed from above, is of a very different sort compared with the former one. Briefly, the shortest rows, that is, the fewest individuals, would be found in the shortest and tallest classes, and the longest rows in the inter- mediate classes. Viewed from above, the outline marked by their heads would describe a fairly regular curve, reaching its highest point in the middle, and curving down to the base-line in both directions. If a thousand individuals instead of a hundred were thus arranged, the line would be more even, since individual differences would tend to merge in the 200 GENERAL BIOLOGY general average. If one were to fire a thousand rifle shots at a target and then sort out the results and classify them with regard to the accuracy of the hits, it would be found that the most accurate and the least accurate ones are fewest in numbers, and that the greatest number of hits is somewhere in between. If we plot out the result on paper, in the same way FIG. 71. — Bird's-eye view of forty men arranged in files by classes of stature. — (Davenport.) that we arranged the men above described, a similar curve will be secured. The same result will be obtained from any large array of data, the distribu- tion of which depends upon chance. An ingenious device invented by Galton illustrates this mechan- ically. A shallow oblong box (fig. 72) is constructed, one VARIATION AND HEREDITY 201 side of which is of glass. Toward one end a number of longitudinal compartments are formed of strips of tin ; at the other end, a sort of funnel is constructed in the same way. Between the two is a field of pins inserted alternately. The ap- paratus is provided with a handful of shot before the glass cover is put on. When the box is inverted, the shot all run back into the com- partment behind the funnel. "Then, when the box is tilted, the shot passes through the funnel, and issuing from its narrow end, scampers deviously down through the pins in a curious and interest- ing way, each of them darting a step to the right or left as the case may be, every time it strikes a pin. The pins are disposed in a quincunx fashion, so that every descending shot strikes against a pin in each successive row. The cascade issuing from the funnel broadens as it descends, and at length every shot finds itself caught in a compartment immediately after freeing itself from the last row of pins." When we examine the disposition of the shot in the compartments, we find that the greatest number is to be found in the middle compartment (if the apparatus be held vertically), and that the com- partments on either side contain a diminishing FIG. 72. — Gallon's mechanical device for illustrating the law of the frequency of error, and the dis- tribution of variatea in the normal curve. 202 GENERAL BIOLOGY number. In other words the line that connects the tops of the columns describes the same sort of a curve 1 that we secure when we plot out the heights of a large number of men. Nearly all the obvious variation of organisms is of the kind just described. Mathematical analysis gives no clue to the nature of the individual variate, and for this reason such variation is frequently called fortuitous, i.e. random or unpredictable. Nev- ertheless the mathematical values obtained from the analysis of a mass of such data are very accurate and certain . -NORMAL CURVE K: 01 NUMBER OF VEINS MEAN a b FIG. 73. — The Normal Curve. Veins in beech-leaves. — (From Daven- port, after Pearson.) Types of Variation Curves. — The theoretical or normal binomial curve is perfectly symmetrical (tig. 73). The classes may be marked off along the 1 The curve is that known in mathematics as the binomial curve (the expansion of the expression [p -f- q]n). VARIATION AND HEREDITY 203 base-line (a-6) ; in the diagram these run from 9 to 23. These figures happen to represent the vari- ation in the number of veins in beech leaves, but they might represent the limits of weights of seeds in grams, or the number of spines in a fish's fin, or any other measurable character. The number of variates in each class is represented by the cross-lines, each FIG. 74. — Two symmetrical curves illustrating the value of " a " as a measure of variability (see text). line standing for forty individuals. It will be noted that the greatest number of veins falls on 16, which is very nearly midway between 9 and 23. This middle point is called the mode or mean. Again, there is a point on each half -curve where the curvature changes from concave to convex (point of inflection). Let us now compare the curves in fig. 74. In both of these curves the mode is the same. The higher curve, compared with the lower, shows that the variates in the former array are concentrated, as it 204 GENERAL BIOLOGY were, about the median dimension, whereas in the flatter curve they are distributed more evenly along the whole base-line. In other words, the bulk of the individuals of the first lot are much alike, or, as we say, are not so variable as in the other lot. It is evident that the flatter the curve, the farther away from the modal axis (M) moves the point of inflection. The measure of the line connecting this point and the modal axis (designated by the Greek letter f fli^ Fitt^g^" It may be better called, perhaps, the survival of the best adapted, since the criterion of survival, or of " Natural Selec- tion," as Darwin called it, is the degree to which the organism is adapted to its environment. Since " like tends to produce like," Darwin held that the individ- uals that have survived on account of their favor- able variations will tend to reproduce individuals of the same type. But the inorganic environment is no more stable than organic nature. Excluding the titanic changes which, Geology teaches us, have been going on for long periods of time, the minor changes of climate and physical conditions are constantly nullifying the delicate adjustments that have come into existence through the natural selection just described. New criteria for the selec- tion of the survivors in the struggle will become operative, and in consequence a different type will be preserved. It is not even necessary for the environ- mental changes to become profound. As we have seen, in connection with fortuitous variation, free interbreeding tends to maintain a single mode, but if a group of variants should be segregated and prevented from mutual intercrossing, no other factor than chance need be called upon to bring about a divergence of two modes. Darwin's atten- 316 GENERAL BIOLOGY lion was attracted to the problem in the first place by observing that in the various islands of the Galapagos group, off the western coast of South America, although numbers of genera of land animals are to be found on each of the islands, as well as on the mainland, yet each island has its own species, differing slightly, but definitely, from those of ad- jacent islands. There is little doubt but that at one time the islands were all connected with one another and with the mainland, and populated with one species of rabbit or of grasshoppers or other forms. When the islands came into existence through the depression of the land, the consequent segrega- tion and isolation of the different groups, and their enforced inbreeding, called into being the new and divergent types now to be found there. Such a segregation need not even be physical. A mutual infertility may arise between groups of a species in a common habitat which would just as effectively segregate them, so far as reproducing the species is concerned, as if a physical barrier were erected. Darwin's hypothesis of T^fttiiral fiplprtirm ie thin a theory~o! the origin" of species (evolution -being assumed] through the mutual relation of organisms ;to their environment, such that the unadapteoTare eliminated and_a changing environment produces changing types. Its most important feature is th'e emphasis which it lays upon the passivity of the organism itself. The selection is purely mechanical, and the conclusions as regards the first two premises SPECIES AND THEIR ORIGIN 317 (the enormous overproduction of indi/viduals, and consequent elimination) are incontrovertible. ! Lamarck's Theory. — We have seen, in a previous chapter, that one of the fundamental qualities of living matter is that of response, and that environ- mental stimuli frequently call forth advantageous reactions (such as the callusing of the hand through friction). That the reaction may also be disad- vantageous is, of course, equally true. Moreover, the fact that the organism is, as a rule, exquisitely adapted to the particular environment in which it is found is one of the most conspicuous facts of nature. Lamarck contended that the continued effect of such response made its impression on the inheritance of the organism, or, to use a technical phrase, that such " acquired characters " are in- herited. Use and disuse thus play a large part in Lamarck's^theory. From another stanripomtT t£ej will f Spfgjgs" was published in 1859, and the entire first edition was sold on the day of issue. This was evidence of the acute public interest in the subject at that time. The publica- tion was merely the spark in the powder magazine, for the idea of organic evolution had been in the air for a century and a half, and had been steadily gaining strength. The following ten or fifteen years were occupied with controversy and heated polemic, for the vital argument was not so much the hypothe- sis of Natural Selection as the theory of Organic Evolution versus the doctrine of Special Creation. As to the conclusion of the issue there could be no d_oubt, and when, finally, the theory of Evolution was definitely established, that of Natural Selection was accepted along with it, although of course the former did not necessarily involve the latter.1 The chief result of Darwin's great generaliza- tion was an extraordinary development of natural science and the extension of the field of biological inquiry in every direction. Darwin had spent his life in the patient accumulation of data on which he based his generalization, and he was not unaware 1 " History warns us, that it is the customary fate of new truths to begin as heresies and to end as superstitions ; and, as matters now stand, it is hardly rash to anticipate that, in another twenty years, the new generation, educated under the influences of the present day, will be in danger of accepting the main doctrines of the "Origin of Species" with as little reflection, and it may be with as little justification, as so many of our contemporaries, twenty years ago, rejected them." — T. H. HUXLEY, "The Coming of Age of the 'Origin of Species.' " 1880. SPECIES AND THEIR ORIGIN 319 of weak links in his own argument which he candidly avowed. These became more emphasized as time went on, and a host of investigators continually added to the facts that were most difficult to explain by the theory of Natural Selection. We have space for but a few of these objections. (1) The basis of elimination or preservation is the usefulness of the organ whose variations serve as the criterion for selection, but there are thousands of very stable characters which must be of wholly indifferent value to the organism. One of the largest groups of the ground-beetles is divided into two sub-groups containing hundreds of species by the invariable distinction of the possession of one microscopic hair above the eye or of two. (2) Again, while it may be recognized that the emphasis on a certain structure may be, so to speak, of selection value, yet the minute differences of fluctuating variations can hardly count one way or the other. Thus one author calls attention to the polar bear, whose white coat must be of great utility to him in stealing upon his prey unobserved. Without doubt, this species has evolved from a type of the more usual coloration, but " did the fortuitous appearance in his coat of a spot of white hairs as large as a dollar or a pancake give some ancient brown bear such an advantage in the struggle for existence as to make him or her the forerunner of a new and better-adapted sort of bear ? " Darwin recognized this difficulty, but thought that the struggle for existence was so keen that the slightest difference, however slight, might 320 GENERAL BIOLOGY decide between survival and extermination. (3) Al- lowing for the fact that certain small variations are advantageous to their possessors, and granting that of the hosts of individuals born into existence, but a minute fraction can hope to survive, yet in many cases chance must play a larger part in their extermination than the possession of any kind of morphological or physiological character whatever. (4f Most significant of all, perhaps, is the experimen- tal demonstration that artificial (and by inference, natural) selection has narrow limits. Beyond a certain point (see p. 219) the pull of the mysterious factor of regression prevents any further progress in that direction. Critique of the Lamarckian Theory. — The chief characteristic of man as distinguished from other animals is the fact that he " looks ahead " and shapes means to his own ends. It is difficult to avoid unintentionally attributing the same purposes to the abstraction we call "Nature " or the "species." Animals and plants, react to many stimuli. Often these reactions are advantageous and these we note ; frequently they are quite the contrary, and these we sometimes fail to remember. Man stores up food to provide for a future time of want. When a potato stores up starch in the tuber, what more natural than to think of it in the same way, — the plant is anticipating its own needs? But it has been dis- covered that the formation of tubers is directly due to the presence of an infecting fungus and does not occur in its absence. SPECIES AND THEIR ORIGIN 321 The same dangers beset the path of the unwary who argue from the Lamarckian standpoint. The need for a useful organ is evident, but we are by no means justified in assuming that the " need " brought about the existence of the organ. Even man cannot " by taking thought, add one cubit to his stature." The Lamarckian argument rests upon the transmission, in heredity, of the results of environ- mental influences upon the somas in other words, the "inheritance of acquired characters." On d priori grounds, Weismann sought to show that this was impossible, since the germ-plasm gives rise to the soma and is unaffected by its accidents. Moreover, the greatest variety of experiment has been attempted for many years, in an effort to secure the hereditary transmission of any sort of such acquired characters, with universally negative results.1 One of the most elaborate of these was carried out by a Ger- man botanist who transplanted some "2500 different kinds of mountain plants to the lowlands and studied them for several years in comparison with their lowland relatives. He found that the alpine environment had made no permanent change in their habit or structure. It is not even necessary to assume such a dis- tinction as that between germ-plasm and soma- plasm, a distinction that is sometimes difficult to maintain, in order to appreciate the improbability of the inheritance of environmental effects. Ani- mals and plants are complexes of matter whose 1 With one very doubtful exception. T 322 GENERAL BIOLOGY nature is ultimately dependent upon some sort oi molecular organization of which we are profoundly ignorant. But we are almost equally ignorant of the nature of the organization of the simplest chemical compounds. We know nothing, for in- stance, of the relation existing between the oxygen and the hydrogen that gives water its peculiar properties. These properties are inherent, and were they to alter fundamentally, our whole body of chemical theory would be upset. On the other hand, the manner in which these properties are revealed to us is determined primarily by external (environmental) conditions, i.e. those of tempera- ture and pressure. We think of H^O as a liquid because that is the form that our usual conditions of temperature and pressure cause it to assume. But the liquid state is, of course, no more charac- teristic of the compound than the gaseous or solid state. If we should keep a quantity of water at a temperature of 0° C. for a hundred years, we should have no reason for supposing that its liquid nature would be altered in the slightest if the temperature were finally raised ten degrees. Indeed, we may say that it is the specific characteristic of water to be a solid, a liquid, or a gas, at definite calculable levels of temperature. In the same way, the adjustment of internal relations to external ones is a specific characteristic :>f the organism. To put it another way, the solid condition of ice is not " caused " or " produced " by the lowering of the temperature, except in a SPECIES AND THEIR ORIGIN 323 figurative sense. It is the essential nature of H2O to be a solid at low temperature. In the same way the climatic conditions of the mountains do not cause the profound modifications in the habit of alpine plants in contrast to their congeners of the lowlands. The nature of the species is to respond, morphogenetically, in one way to one sort of environ- ment, and in another way to another sort of environ- ment without being intrinsically altered by either. For the succession of individuals is a continuous stream, and there is no absolute break between one generation and another. In other words the species, like the organism, has a unity unaffected by its sur- roundings. This consideration does not affect the idea that species do alter with respect to their in- trinsic nature, and that in the course of time new species come into existence from preexisting ones. It emphasizes, however, the significance of the internal factors involved and the relative unimpor- tance (in a direct and permanent way) of the action of the environment. In conclusion it may be said that biologists are by no means so positive in giving their allegiance to one theory or another of the origin of species as they might have been a generation ago. The concept of Evolution, that is, of the progressive changeful- ness of organic Nature and the descent of present- day species by modification of preexisting types, forms the basis of all modern biological work. As to the method of the evolutionary process there are several opinions, and it may very well be that 324 GENERAL BIOLOGY each is but a part of a true explanation, the complete key to which will be discovered only by subsequent researches. The painstaking and brilliant work of Darwin can never be set aside in spite of the fact that we may be compelled to doubt the universality of his theory of Natural Selection. Instead of hi? fluctuating variations, however, it may be that the basis for selection is something like Johanssen's " genes," or those vague units of organization that reveal themselves to us as Mendelian unit-char- acters. But we must have a far deeper insight into the physics and chemistry of the organism than is now available before we can begin to formulate an hypothesis as to the real nature of these units. INDEX The numbers refer to pages ; illustrations are indicated by boldface type. Abiogenesis, 130. Acetylene, 49. Adaptation, general, 259 ; aquatic, 260; aerial, 262; subterranean, 263 ; protective, 266 ; for seed dispersal, 278. Adaptive response, 253. Adrenalin, 88. Aerial adaptation, 262. Aerobic organisms, 63. Agamy, 167. Alimentary system, in animals, 104; in plants, 126. Alisma, reproduction, 180. Alternation of generations, ani- mals, 171 ; plants, 175. Amitosis, 93. Amoeba angulata, 32 ; A. proteus, 31 ; reproduction, 166. Anabolism, 56. Anaerobic organisms, 63. Anesthetics, 67. Analogy, 47. Anisogamy, 144. Annelid, structure, 119. Anolis, 254. Anosia plexippus, 271. Ant-guests, 265. Antiseptics, 67. Anti-toxins, 256. Aphis, life-history, 167 ; with ants, 281. Apogamy, 183. Appendix vermiformis. 312. Aquatic adaptation, 260. Assimilation, 30. Associations, of animals, 279; of plants and animals, 293. Bacteria, fission, 136; nitrogen- fixing, 76; putrefactive, 74. Beans, pure lines in, 221. Beroe, 261. Biffin, experiments with wheat, 235. Binomial nomenclature, 279. Biogenesis and abiogenesis, 130. Blastogenic variations, 213. Blastopore, 162. Blast ula, 160. Bodo lens, 142. Bonnet, 191. Bose, R. C., 248. Bougainvillea, 172. Budding, 136; in Hydra, 138, in Syllis, 139; permanent, 139; in annuals, 132. Carbohydrates, 12. Carbon cycle, 72. Care of the young, 272. Carnivorous plants, 59. Cell, 18, 20 ; various kinds, 22. Cellular structure of leaf, 19. Cellulose, 41. Cell wall, an adaptive structure, 24. Centrosome, 21, 94; nature of, 98. ChcBtopterus, 169. Chemical agents, effect of, on growth, 103. Chemical environment, 245. Chromatin, 23. ( 'hromosome, 94. Chrysanthemum segetum, variation, 207. Ciona intestinalis, 40. Circulatory system, in animals, 115. 116; in plants, 126. Cleavage, of animal egg, 157. Clover as a fertilizer, 76. Colloids, 15. Coloration, protective, 267 ; ag- gressive, 269. 325 326 INDEX Combustion, 49; and respiration, 64. Commensalism, 280. Conducting organs, 39. Conjugation, cytoplasmic, 151 ; in animals, 155 ; nuclear, 152 ; partial, 151 ; in Paramecium, 165 ; in Protozoa, 164. Connective tissues, 41. Conservation of energy, 50. Cork, function of, 124. Corn-root louse, 282. Correlation, 209. Creation, special, 299, 306. Ctenodrilus, 132. Ctenophor, 261. Curve, variation, 202 ; skew, 204. Cycads, ciliated sperms, 313. Cycle of the elements, 68. Cytoplasm, 20. Darwin quoted, 307-308. Darwinism, 314 ; critique of, 318. Death, 4. Defects, Inheritance of, 239. Denitrification of the soil, 75. Determiner, 194. De Vries, Mutation theory, 208. Differentiation, in animals, 104; in plants, 121. Digestion, 29 ; specialization in, 42 ; in higher animals, 62. Dioncea, 59. Disease, inheritance of, 236. Dissimilation, 56. Division of labor, 36. Dodder, 292. Draba verna, species of, 305. Drosera, 59. Drosophila, 234. Ectoderm, 162. Egg and sperm, 150. Electric ray, 78. Electric response, 247. Electricity in organisms, 79. Elements, cycle of, in Nature, 68. Elephant, history of the, 310. Endoderm, 162. Endoskeleton, 110, 111. Energesis, 66. Energid, 25. Energy, conservation of, 50. Enterokiuase, 85. Environment, 242, 245. Enzymes, 82. Epigencsis, 192. Eudorina, 147. Eugenics, 240. Evolution, of plants, 184; of species, 307. Excretory organs, 117, 118; in Protozoa, 119. Exoskeleton, 112. Fats, production of, in the organism, 55. Ferments, 84. Ferns, reproduction, 176. Fertilizers, 71. Filial regression, 219. Firefly, 81-82. Fission, in Metazoa, 132; in lower plants, 135. Flagellata, 33. Flame cells, 119. Flowers, 178. Flying fish, 263. Foods, 60 ; fate of, in animals, 61 ; influence of, on response, 258. Fortuitous variation, 202. Fowl's combs, inheritance, 231. Galton, Francis 241 ; ancestral inheritance, 217 ; device illus- trating variation, 201. Galvanotropism, 251. Gamete, 141. Ganglia, 107, 108. Gastrulation, 161. "Genes," 324. Germ layers, 162. Germplasm, 150. Glands, 44 ; ductless, 86. Glycogen, 12. Grafting, 295. Growth, 90. Haemophilia, 239. Hawthorn, species, 305. INDEX 327 Heat of organisms, 79. Heredity, 214; racial, 216; Gal- ton's law of, 217 ; selection in, 220. Hermit-crab, 266. Heteromorphosis, 190. Hologamy, 141. Holozoic and holophytic nutrition, 58. Homology, 46. Hookworm, L'Mi. Hormone, 89. Horse, evolution of, 309. Huxley, T. H. (quoted), 318. Hydra, 43; budding, 138; re- generation, 186. Hydroid, colonial, 172. Hydrophytes, 275. Hypertrophy, compensatory, 253. ' Immunity, 255. Index of variability, 203. Ingestion, 29. Inheritance, 215; ancestral, 217; Mendelian, 223 ; sex-limited, 233 ; of disease, 236 ; of defects, 239. Insects, parasitism in, 287. Ions, 10; ion-proteins, 11. Irritability, 30, 244. Isogamy, 142. Karyogamy, 142, 152. Katabolism, 57. Katydid, 268. Kidney, 121. Lamarck's theory of evolution, 259, 317 ; critique, 320. Lantern fish. 81. Leaf, cellular structure, 19. Leucocytes of frog, 27. Lichens. 291. Life and death, 4. Light, effect of, on growth, 100 : on response, 245 : actinic rays, 101 ; in animals, 80. Linnaeus, 298. Linophryne lucifer, 81. Lipas, 83. Liverwort, reproduction, 175. Living and non-living, 2. Locomotor organs, 32. Loeb, J., experiments, 170; with Ciona, 40. Mean, 203. Malarial parasite, 284. Mastodon, 311. Mastiganwsba, 33. Maturation, 153. Mechanical tissue in plants, 124. Mechanism and vitalism, 194. Megagamete, 148; spore, ger- mination, 179. Mendelism, 223 ff. Meristic variation, 205. Mesophytes, 277. Metabolism, 57. Metamerism, 135. Metaplasm, 21. Microgamete, 148. Microspore, germination, 179. Milk, as an emulsion, 13. Mimicry, 269. Mitosis, 92, 95, 97 ; abnormal, 88, 97. Mode, 203. Monarch butterfly, 270. Morphogenesis, 185; theories of, 190. Morphogenetic response. 256. Mosses, reproduction in, 175. Mougeotia, 146. Movement, 77 ; of plants, 123. Muscle cells, 38. Muscles in insects, 114. Muscular system, 113. Mutations, 207. Myrianida, fission, 133. Nasturtiums, orientation, 249. Natural Selection, 315. Nematoda, 286. Xco-vitalism, 196. Nephridium, 120. Nervous system of a caterpillar 108. 328 INDEX Nettles, inheritance in, 228. Nitrogen-fixing bacteria, 76. Nitrogen loss through plant growth, 71. NodUuca, 79, 80. Normal curve, 202. Notochord, 112. Nucleo-plasma relation, 99. Nucleus, 20. Nutrient solutions, 70. (Enothera, mutations, 208. Ontogenesis, 129. Organic response, 242. Organic synthesis, 52. Organisms, destruction of, 72 ; putrefactive, 73. Organs, sensory, 107. Origin of species, 306. Oxidation, 48 ; chemistry of, 65. Oxygen, role of, in metabolism, 62. Ozone, 49. Palcemon, heteromorphosis, 190. Papilio turnus, 302. Paramecium, 34; electric response, 251 ; conjugation in, 165 ; pure " lines in, 223. Parasitic and saprophytic nutrition, 58. Parasitism, in Protozoa, 283 ; in worms, 285 ; in insects, 287 ; in plants, 292. Parthenogenesis, 167 ; in wasps, 168; artificial, 169, 170; in in plants, 183. Peas, Mendel's experiments, 225. Pelagic adaptation, 261. Pelagonrmcrtes, 261. Pepsin, 83. Phagocytes, 29. Phosphorescent animals, 79—82. Photophores, 264. Photosynthesis, 53. Phylloxera, 295. Pipa, 272. Plants, association among, 290; parasitism in, 292 ; response in, 250 ; sexual reproduction, 174 ; evolution of, 184 ; fission in, 135. Plastogamy. 151. Poisons, 66. Pollen-tube, 179. Polyembryony, 189, Polymorphism in species, 301 ; in ants, 303. Preformation, 191. Proteins, 10 ; production of, in the organism, 55. Protista, 26. Protoplasm, chemistry, 8 ; physics, of, 14; organization of, 17. Protozoa, 26 ; parasitism in, 284 ; movements of, 77 ; conjugation in, 164. Pseudopodia, 28. Pure lines, 221. Putrefactive organisms, 74. Reaction, 243. Reduction, 153. Regression, filial, 219. Regulation, 185; in Hydra, 186; Stentor, 187 ; sea-urchin blas- tulae, 188. Reproduction as a growth process, 131; in plants, 174; sexual, 141. Respiration, 64. Response, organic, 242 ; nature of, 246 ; electric, 243 ; unsymmetri- cal, 248 ; adaptive, 253 ; mor- phogenetic, 256. Reversion, 232. Rhinoceros beetles, variation, 205 Rust in wheat, 292. Sacculina, 289." Salts, inorganic, importance of, 10. Sea-urchin larvae, regeneration, 188. Secretin, 89. Secretion, 41 ; internal and ex- . ternal, 86. Seeds, stored foods in, 128 ; dis- persal, 278. Seed plants, reproduction in, 177, 180. Segmentation, metameric, 135. Selection, Natural, 316. INDEX 329 Sensory organs, 107. Sex-limited inheritance, 233. Sexual differentiation, 147; re- production, 141 ; in plants, 174. Sigma, in variation, 203. Skeletal structures, of animals, 110; in plants, 123. Skew curves, 204. "Sleep" in plants, 122. Soma-plasm, 150. Sparrow, variation, 211. Special creation, 299. Specialization, in conducting organs, 39 ; in digestion, 42 ; in loco- motor organs, 32 ; and generali- zation, 45. Species, Linnsean, 305 ; criteria of, 300 ff . ; elementary, 305 ; origin of, 306, 313; meaning of term, 296. Specific energy, law of, 247. Spermatocyte, 154. Spiral valve, 106. Spirogyra, 146. Spore formation, 140. Sporophyte, 175. Starch, synthesis of, in the leaf, 53. Stature, variation in, 200. Stentor, regeneration in, 187. Stephanosphcera, 144. Stimulus, 243. Stomias boa, 264. Struggle for existence, 129. Stylonychia, 35. Substantive variation, 205. Suckers, 137. Sundew, 59. Surinam toad, 273. Survival of the fittest, 315. Suspended animation, 7. Syncytium, 91. Synthesis in the organism, 52. Synthetic products, artificial, 3. Syllis ramosa, 139. Tardigrades, 6. Teleology, 195. Temperature, effect of, on response, 244; on growth, 101. Thyroiodin, 88. Tiger butterfly, 302. Tonus, 246. Torpedo, 78. Trichosphcerium, 151. Trochosphcera, 261. Tunicate, 40. Typhlosole, 106. Unit characters, 223. ' Use and disuse in evolution, 317. Variation, 198; causes of, 212; correlated, 209 ; discontinuous, 205 ff. ; effect of conditions upon, 211; Gallon's device, 201; in human stature, 200; in beech leaves, 202 ; types of curves, 202 ; substantive, 205. Vegetative reproduction, 133. Venus' flytrap, 59, 122. Vermiform appendix, 312. Vertebrate and invertebrate, 109. Vestigial structures, 312. Vitalism and mechanism, 194. Volvox globator, 149. Vorlicella, 37. Waller's criterion of life, 7. Water, in protoplasm, 9. Weismannism, 193. Wheat, inheritance 'in, 235. Wolff, K. F., 192. Worms, parasitic, 288. Xerophytes, 276. Xylolrupes, variation in, 205. Yeast, growth of, 131. Yolk, 159. Zaitha, 272. Zoochlorella, 294. Zoospore, 174. Zygosis, 145 ; in animals, 155. Zymtgen, 85. Printed in the United States of America. NOV1 OF CALIFORNIA LIBRARY -^Igfpax. j THE LIBRARY Ill I II II III! 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