Experimental zoölogy

Preface

The great interest that has been shown during the last fifteen years in the study of Experimental Zoology has led to the rapid development of this branch of biology. An attempt is made in the following pages to bring together the results of this work. A series of about thirty-five lectures formed the basis for my treatment of the subject, and this will account, in part, for the way in which the matter has been handled ; many details have been omitted that an exhaustive treatment would demand ; and the plan has been to select the most typical and most instructive cases for presentation, when such a choice was possible. Nevertheless, I believe that the reader will find a fairly full account of the subjects considered.

Physiology has from the beginning made use of the method of experiment, and with notable success. Morphology has, up to the present time, followed mainly the historical and descriptive methods, although striking exceptions could be cited. While the historical study of zoology must always remain a legitimate field for activity, as human history has been a time-honored study, yet there can be little doubt that the more promising and searching method of zoological study in the future will be found in experiment.

The central problem of morphology — the causes of the changes in form, or at least the determination of the conditions under which changes in form occur — will furnish the main theme of the present treatise. Two fields of study that properly fall under this head are, however, not considered, viz. experimental embryology and the experimental study of regeneration. Both of these subjects have in recent years received comprehensive treatment in book form, so that it did not seem desirable to go over the ground again. More-

VI

Preface over, their consideration would have demanded too much space to be included with the present matter in a single volume. The fascinating study of the psychical side of living phenomena also belongs to the province of experimental biology, especially comparative psychology; but this subject has quite recently been fully dealt with by Loeb and by Jennings, who have themselves been largely instrumental in developing the subject, so that further treatment would be more than superfluous.

The excellent summaries and reviews of some of the topics discussed here, that have been published in recent years, have greatly facilitated my work. I need only mention Roux’s and Driesch’s analysis of the experimental method, Grafin v. Linden’s summary of the experiments on butterflies, Herbst’s excellent treatment of the subjects of “Formative Reiz,’’ Phillips’s very full review and literature on sex determination, and Cuenot’s, Lenhossek’s, and O. Schultze’s treatment of the same subject. I need scarcely add that, while using these and other reviews, I have made my own compilation almost exclusively from the original sources.

Covering as extensive a field as I have attempted to cover, it is probable that the different subjects have received unequal treatment, and I fear that some omissions may have been made. I trust, however, that no serious oversights or mistakes will be found.

It gives me great pleasure to express here my appreciation of the generous assistance in the correction of the manuscript and proof rendered by my wife, by Professor E. B. Wilson, by Professor C. B. Davenport, and by Professor C. E. Castle.

EXPERIMENTAL STUDY OF EVOLUTION

CHAPTER II The Influence of External Conditions in causing Changes in the

Structure of Animals . . • • • • • .12

Adaptive Responses

Non-adaptive Responses 1 5

Influence of Temperature on the Coloration of Butterflies . . 15

Effect of Temperature on Caterpillars ..... 24

CHAPTER III The Influence of External Conditions in causing Changes in the

Structure of Animals (^Continued) .29

Experiments on the Influence of the Food Plant ..... 29

The Influence of Light, Electricity, Centrifugal P'orce, Chemical Substances, and Oxygen on the Caterpillars and Pupae of Moths and Butterflies ........... 37

The Influence of Humidity on the Characters of Moths and Butterflies 38 Experiments with Flatfish ......... 40

Experiments with Crustaceans ........ 40

Changes in Mammals and Birds . . . . . . . >41

CHAPTER VIII Experiments with other Mammals and with Birds .... 99

Experiments with Guinea Pigs . 99

Experiments with Rabbits ......... 106

Experiments with Rats . . . . . . . . .110

Experiments with Cats . . . . . . . . . • H3

Data for Other Mammals and Man . . . . . . .116

Experiments with Pigeons . . . 135

CHAPTER IX Experiments with Silkworms ........ 141

Experiments with Beetles . . . . . . . . •'^5^

Experiments with the Currant Moth . . . . . . -153

Experiments with Tephrosia . . . . . . . • *53

Chapter X

Blended Inheritance . . . . . . . . • • *5^

Mosaic Inheritance . . . . *57

Hybridization between Linnrean Species *57

Dimorphism *60

Reversal of Symmetry . . . . *^5

Conclusions regarding Mendelian Inheritance *66

CHAPTER XI Behavior of the Germ-cells in Cross-fertilization . . . • *73

Experiments with Amphibia . . . • • • • • *73

Experiments with Echinoderms . . . • • • • • *75

Factors involved in the Entrance of the Spermatozoon . . .178

Artificial Helps to P’ertilization and to Cross- fertilization . . • *81

Polyspermy

IX

Chapter XII

Experiments with Mice and Rats Experiments with the Pomace Fly Behavior of Germ-cells in Inbreeding .

CHAPTER XIII Influence of Selection

Fluctuating or Individual Differences .......

Selection of Fluctuating Differences .......

Elimination within the Species ........

Variation and Parthenogenesis .... . . .

The Results of Selection and Hybridization of Wild Elementary Species Selection under Domestication of Mutations, Saltations, Sports, and Discontinuous Varieties in General ......

CHAPTER XIV The Analogy with Artificial Selection

The Influence of the Environment ......

De Vries’s Results with CEnothera ......

Variation and Mutation in Helix .......

Evolution by Means of Definite Variation

The Selection Theory and the Theory of the Survival of Mutations

CHAPTER XV Introductory

Normal Growth .........

Senescence ..........

Length of Life in Different Species .....

Absorption of Parts by Larvm

CHAPTER XVI Influence of Food

Stimulants affecting Growth

Effects of Salt on Growth

Effects of Heat on Growth ....

PAGE 1 86

199 202 204

X

Effects of Light on Growth .

Growth toward the Light; Phototropism Influence of Gravity on Growth .

Effects of Electricity on Growth .

Pressure and Contact ....

The Formation of Galls

CHAPTER XVIII The Union of Different Regions

The Influence of the United Parts on Each Other : Formative Factors The Union of Parts of Different Species ......

Special Problems of Development

EXPERIMENTAL STUDIES OF THE INFLUENCE THE ENVIRONMENT ON THE LIFE-CYCLE

CHAPTER XIX Changes in the Life-cycle and Changes in the Environment

The Life Histories of Some Animals .......

Influence of Food on the Life-cycle of Lepidoptera . . . .

Influence of the Environment on the Time of Ripening of the Sexual Organs ....

CPIAPTER XX

Alternation of Sexual and Parthenogenetic For.ms

Alternation of Generations in the Aphids and Phylloxerans .

CHAPTER XXVI Condition of the Germ-cells ........ 393

Vigor of the Parents .......... 393

Effects of Inbreeding . . . . . . . . . . 394

Ratio of Nucleus to Cytoplasm ........ 396

The Extrusion of the Polar Bodies, and the Analogous Process in the

The Formation of Male and Female Producing Spermatozoa . . 401

CHAPTER XXVII The Internal Factors of Sex Determination (^Continued') . . 407

The Origin of Gynandromorphs 407

The Sex of Multiple Embryos . . . . . . . .410

The Sex of Human Twins and Double Monsters . . . . .411

Two Recent Theories of Sex Determination based on the Assumption

of Male and Female Eggs . . . . . . . .413

The Reduction Process in Parthenogenetic Eggs . . . . .415

Influence of the Cytoplasm . . . . . . . . .417

PAGE

Conclusions :

(1) The Morphological Conception of Sex Determination . 420

(2) The Physiological Conception of Sex Determination . . 421

(3) Analysis of the Results ....... 422

EXPERIMENTAL STUDY OE SECONDARY SEXUAL

CHARACTERS

CHAPTER XXVIII Correlation between the Secondary Sexual Characters and the Essential

Organs of Reproduction 432

CHAPTER XXIX Secondary Sexual Characters {Co7itinued') 439

Theories of the Origin of Secondary Sexual Characters . . . 439

EXPERIMENTAL STUDY OF EVOLUTION

Chapter I

The study of Zoology by experimental methods is not a new departure, for the method of experiment has been often applied to special zoological problems. On the other hand, the recognition that only by experimental methods can we hope to place the study of Zoology on a footing with the sciences of chemistry and of physics is a comparatively new conception, and one that is by no means as yet admitted by all zoologists. I do not wish to appear to disparage those studies that deal with the descriptive and with the historical problems of biology. They also offer a wide field for activity, and the more familiar we become with the structure and modes of development of animals, so much the better can we apply the experimental method. In fact, many of the problems of biology only become laiown to us as the result of direct observation. The wider, therefore, our general information, the greater the opportunity for experimentation.

It is undoubtedly true that many zoologists who have spent their lives in acquiring a broad knowledge of the facts of their science fail to make use of their information by testing the very problems that their work suggests. This is owing, no doubt, to their exclusive interest in the observational and descriptive sides of biology, but also in part, I think, to the fact that the experimental method has not been sufficiently recognized by zoologists as the most important tool of research that scientists employ.

. 3

Perhaps also the fact that the historical side of biology attracts such popular interest accounts, in part, for the neglect of more searching scientific methods of study.

Whether the method of observation or the method of observation and experiment is followed, seems to be also a question of the kind of interest aroused by living objects. If the number of collectors, naturalists, zoologists, anatomists, entomologists, ornithologists, mammalogists, conchologists, etc., be compared with the number of physiologists, physiological chemists, bacteriologists, it will be seen that the former have an enormous advantage in numbers. It is true that a few zoologists are experimentalists, and that some physiologists do not experiment at all, but the proportion remains about the same. In other words, interest in collecting and recording the results of observation and in the artistic side of nature is much more widespread than interest in the study of problems, or, if the interest is not lacking, the will to take the initiative in the formulation and solution of problems seems to be less cultivated in the biological sciences than the power to observe and to describe. In so far as the followers of the one or of the other method of investigation have made their selection as a matter of temperament, the disproportion will probably always remain ; but in so far as the result is due to imitation, or to following the line of least resistance, or to a failure to appreciate differences in aim and method, the proportion may to some extent be altered ; and I think it will be generally admitted that at the present time there is greater need for experimental work than for descriptive and observational study.

It is sometimes said that experimental study is the analytical study of problems, and this in a. sense is true, but it is only a part of the truth. It is rather the method of attacking problems that is the chief characteristic of experimental work, for is not the historical method also a study of problems? We demand in the case of a problem in experimental science that the conditions under which an event takes place be discovered, and that, if possible, we reproduce artificially the result by controlling the conditions. In fact the control of natural phenomena is the goal of experimental work.

In the studies of physics and chemistry the method of experiment is so familiar, that we think of, their advancement as taking place by experiment alone. In biology the situation is different, and new discoveries are looked for as often in the field of observation as of experiment. This difference is due to the higher stage of development that has been reached by the physical sciences, while biology is still, in large part, in the lower stages of its evolution where facts are insufficiently known. Nevertheless the amount of time still given to descriptive work is out of proportion to the present condition of development of biology.

A few examples may serve to illustrate the differences between the descriptive and the experimental study of zoology. The egg of an animal, if set free and fertilized, begins at once to develop. Descriptive embryology gives us the different stages through which the egg passes, but no matter how complete the description, we still know little or nothing of the causes that are operating to bring about the development. What, for instance, does the spermatozoon bring into the egg to make it develop ? What physical and chemical changes take place during cleavage ? What makes the embryo turn in at one pole ? Why do certain cells develop cilia ? These and a hundred other questions suggest themselves. Observation has failed to answer them.

Another method employed in recent years has been to attempt to find certain physical or chemical changes that seem to be similar to those observed in the developing egg. Thus it has been suggested that the spermatozoon brings a ferment into the egg ; that the cleavage is due to differences of surface tension ; and that the gastrulation fe caused by osmotic pressure. Machines that behave in somewhat similar ways have even been constructed to illustrate some of these changes. Interesting as the ideas derived from these sources may be, their scientific value lies only in their suggestiveness, until it can be shown that the changes in the embryo are really the outcome of similar processes; and the only way in which certainty can be gained on this point is by experiment on the organisms themselves. If a ferment starts the development it is our duty to isolate it, and to introduce it hypodermically into the egg. If this could be done and the development thereby started exactly as in normal fertilization, the hypothesis becomes at least probable that a ferment is brought in by the spermatozoon and starts development. Similarly, for cleavage, for gastrulation, and for every stage in the development, experiment alone can give a satisfactory answer.

The essence of the experimental method consists in requiring that every suggestion (or hypothesis) be put to the test of experiment before it is admitted to a scientific status. From this point of view the value of an hypothesis is to be judged, not by its plausibility, but by whether it meets the test of experiment. Its use is therefore primarily for the investigator, and not for the layman ; yet as a matter of fact the wildest speculations are likely to be the ones that excite most popular attention and applause.

It is sometimes said that an hypothesis is useful in proportion to the number of facts it brings under one point of view. This is true for the student rather than for the investigator. Such an hypothesis may have no scientific status. It belongs rather to a system of mnemonics. To the teacher, also, hypotheses are useful in arousing the interest of his hearers, so that by exciting their imdeveloped imagination he can make his dry facts more entertaining. But let us be careful to distinguish between the forensic and the scientific value of hypotheses.

Hypotheses may be useful, and have been used in various ways. They have been used, as just stated, to hold together a body of isolated facts ; as such they are in reality only fictions. They have been used in the reconstruction of supposed historical events, especially in biology in the setting up of family trees. In this case they can only claim to be more or less plausible suggestions. Hypotheses have been used to direct interest toward certain fields of study. As such they have often proven stimulating and have been useful in acting as a guide for others. But the hypothesis of real importance is the working hypothesis of the investigator. It is the test by means of which he tries to interpret his problem, and therefore it is essential that his hypothesis is one having a practical bearing, i.e. an hypothesis that can be shown to be true or false. It differs in this essential respect from purely fictitious and from metaphysical hypotheses.

The working hypothesis carries along with it its dangers as well as its advantages; since, while it may lead to discoveries, it may, if it is wrong in principle, blind us to the real conditions. Therefore the investigator must not only be an inventor of working hypotheses, but cultivate also a skeptical state of mind toward all hypotheses — especially his own — and be ready to abandon them the moment the evidence points the other way. And herein hes one of the differences between the recorder of observations and the experimenter. The work of the observer, if exact, is complete in itself, and stands forever as a monument to his ability, or at least to his industry; while the conclusions of the experimenter, if they are to bear fruit, must become modified with each new discovery. His results are absorbed in the current of the next advance, but his consolation will be that he has had at least a share in the causal study of living things, and in helping the human race toward the control of organic phenomena.

To return to our examples. The growth of animals and plants offers a wide field for experimental study. Under certain conditions we see a young animal continuing to grow larger until a certain size is reached, when growth slowly ceases. Although the animal may live for many years longer, it has ceased to grow. What makes it grow? Why does it stop growing? We have hardly begun experimental work along these lines; yet we shall see later that there is a promising field for work in this direction. After a time old age comes on and the animal dies. We say it dies a natural death, and this seems inevitable.

but only because we have found that death always takes place under ordinary conditions. Suppose, however, we change the conditions ; might we not hope to prolong the duration of life ? Improbable as this may seem, there are already experiments afoot that indicate that, however difficult, the problems may not be insoluble.

Why, on an average, in most animals, are equal numbers of two forms born — male and female ? Is there an internal mechanism? If so, what regulates it? Do external or internal conditions determine that one egg becomes male, another female? Even if an internal mechanism exists, it might be affected by external conditions, and in any case the cause of the production of the two types must be determined.

Observation has established that the evolution of animals and plants has, in all probability, taken place. But what factors are involved in the process are unknown. Only in the last few years by means of an experimental study of the subject has decided advance been made.

It is sometimes stated that nature has already carried out innumerable and wonderful experiments, and that we can never hope to excel her in this power. Is it not better, therefore, to examine patiently and reverently what she has done, and in this way learn how her processes have been carried out? Let us not be blinded by rhetorical questions of this kind. No doubt nature has carried out prodigious experiments; but we can never be certain that we know how she has obtained her results until we can repeat the process ourselves. What would the chemist or the physicist say if he were told that nature has already carried out experiments on a much greater scale than he can hope to accomplish, and that he should drop his experimental methods and study his physics in a thunderstorm and his chemistry in a volcanic eruption !

I have brought up this point because it illustrates one side of the experimental method that is sometimes overlooked. Almost all of the phenomena with which the biologist has to deal are so complex that he cannot determine what part each factor plays in the result unless he study the effects of each under different conditions that can be controlled. Little by little, in this way we can hope to gain a clearer insight into the conditions that, taken all together, produce the result.

So much for the experimental method. It is pertinent to ask what is an experiment? If I cut an earthworm in two to see what will happen, have I performed an experiment ? Perhaps not, for the actual performance of cutting the worm in two is not the essential point. The essence of an experiment is a trial or test, and the conditions are so arranged that an answer is expected. If the worm is cut in two in order to study the physiological behavior of the two ends, as has been done in fact, with some interesting results, or in order to see what regenerates at the two cut ends, we have a distinct purpose in view, although no formulated problem. If we proceed farther and remove a definite number of segments in order to see how many come back, and then try to determine what conditions are involved in the results, we are clearly carrying out an experiment with a more definite aim. This illustration will serve to show that the most essential feature of an experiment is the anticipation of the results of a test. The operation may be so simple, and the conditions .so little known, that to call the performance an experiment may easily expose one to the ridicule of those unfavorably inclined to the claims of experimental work. If the process is carried out with scarcely a thought as to its purpose, and in complete ignorance of all the conditions entering into the problem, it can scarcely be called an experiment at all. At most it is only a preliminary testing of possibilities. Much of the pioneer work in experimental zoology has necessarily been of this kind, and crude as such preliminary work must be, it should be looked upon only as the first step toward a further and more critical analysis.

The carrying out of an experiment implies the formulation of a working hypothesis, and this usually presupposes some knowledge of the possible conditions that control the phenomena. The experimental work becomes more explicit and accurate the lO Experimental Zoology

more we know beforehand of the possible conditions that may enter into the result. The ability of the experimenter is shown in his insight into the possible factors that may be present. His ability may be the result of a correct estimate of the possible conditions, but for the highest order of work there is demanded also great imaginative power. Good judgment and accurate observation may lead to fine work, but constructive imagination seems to be required for the highest order of original work. This does not imply that accuracy of observation is not as requisite in original work as in descriptive and observational work, and should always be expected; but the man who sees new and overlooked combinations may open fields of research that will set to work an army of able “investigators.”

The branches of biology that have made most extensive use of the experimental method are physiology, bacteriology, and physiological chemistry. The zoologist and the embryologist have also to deal with physiological problems, and already the beginning of important experimental work has been carried out in this field ; but the most distinctive problem of zoological work is the change in form that animals undergo, both in the course of their development from the egg {embryology) and in their development in time {evolution). It will be granted, I think, that these formative problems are more difficult than those relating to function with which the physiologist has concerned himself in the main; but this is all the greater reason why the experimental method should be used in their study, especially after so much purely descriptive work has been already done.

The term “ morphology ” has been used in recent times to denote the study of form, as contrasted with physiology, that deals with functional changes. Morphogenesis has also been employed to signify a study of the changes in form through which organisms pass. It is mainly the experimental study of these changes in form that I propose to examine in the following pages. Experimental morphology would perhaps nearly indicate the field to be examined ; but since the line between experimental physiology and experimental morphology is often hard to draw, and since I shall not hesitate at times to enter upon the physiological side of many problems, I have chosen the somewhat broader title of Experimental Zoology to include the subjects to be treated.

The principal topics to be discussed fall under the following six headings : —

I. Experimental Study of Evolution.

II. Experimental Study of Growth.

III. Experimental Studies in Grafting.

IV. Experimental Studies of the Influence of the Environ-

ment on the Life-cycle.

V. Experimental Study of the Determination of Sex.

VI. Experimental Study of the Secondary Sexual Characters.

y

Chapter II

THE INFLUENCE OF EXTERNAL CONDITIONS IN CAUSING CHANGES IN THE STRUCTURE OF ANIMALS

Animals and plants are so constituted that one of their chief characteristics is that they respond to their natural environment in such a way as to insure their continued existence. These responses are in the main physiological, and therefore in large part transitory; but in some cases the response is structural, involving a temporary or even a permanent change in form or structure that persists at least so long as the external conditions that called it forth remain. The question arises whether these changes, directly induced by the environment, may not give origin to the more fixed characters that have become the permanent inheritance of each species. May not these have been in the first instance adaptive responses to the environment? This leads to the further question of the origin of all the characters of the species, whether adaptive or non-adaptive. In this and in the following chapters the different sides of this question will be considered.

ADAPTIVE RESPONSES

External conditions sometimes cause adaptive structural changes in organisms. We are familiar with some effects of this sort in our own bodies. Pressure on the skin, if long continued, causes it to become thicker and more capable of resisting the injurious effects of pressure. Sunlight tans the skin and protects it from “burning.” It is said that cold causes the fur of some mammals to become thicker, and this change better protects them against the cold. Conversely, it is said that horses

and dogs lose their hair to some extent in very warm climates.

A number of Arctic animals become white in winter. This change seems to be in part due directly to the cold, for it has been found if these animals are transferred to warmer climates they show less marked changes on the approach of winter.

Flounders and some other fish and some amphibians become lighter in color on a light background and darker on a dark background. The most remarkable case of this sort is that of the pupas of certain butterflies. If the pupation takes place on a light background, the chrysalids are lighter ; and if on a dark background, they are darker. Experiments by Poulton ^ have shown that this effect is produced directly through the skin and not through the ocelli. Poulton thought in one case that even the color of the silk in which the caterpillar incloses itself is influenced by the color of the background, but this has been shown not to be the case.

It is popularly supposed that the African chameleon becomes green in green surroundings, and brown in a dark environment, but this is probably not true ; at least it has been shown in another lizard, Anolis, that can also change from green to brown and the reverse, that the animal is as a rule green if warm and brown if cold. The effects are produced by change in the pigment of the dermal pigment cells. In the cold the black pigment spreads out over the surface and conceals the stationary green pigment. In the warmth the black pigment migrating inward exposes the green. Light has a somewhat different effect. In both the African chameleon and in Anolis a strong light acts like a low temperature, causing the black pigment to migrate to the surface, and a faint light or darkness acts like a high temperature, causing the black pigment to wander inward, so that the animal becomes green. Other lizards give reverse effects in light. Parker and Starratt have shown for Anolis that when both light and heat act together, the results are as follows :

‘ For details of the experiment, see Poulton, “The Colours of Animals,” p. no, 1890; “Further Experiment upon the Colour Relation,” etc., Trans. Ent. Soc., p. 293, London, 1893; “An Inquiry into the Cause and E.xtent of a Special Colour Relation,” Proc. Roy. Soc., Vol. XII, 1887.

At a low temperature, io° C., Anolis changes from green to brown, irrespective of illumination. At a high temperature, 40° to 45° C., it turns from brown to green, irrespective of illumination. Thus heat is the controlling factor at these extremes. Between these extreme temperatures there is a range from 25° to 35° through which light is the controlling factor, although heat is not without its influence, as shown by the rate of the change. Parker and Starratt have discovered the astounding fact that the effects of the illumination may be produced when an area of the skin no larger than a square millimeter is exposed, the rest of the animal being in the dark.