Individuality in Organisms

SCIENCE SERIES, established by the Trustees of the University, owes its origin to a belief that there should be a medium of publication occupying a position between the technical journals with their short articles and the elaborate treatises which attempt to cover several or all aspects of a wide field. The volumes of the series will differ from the discussions generally appearing in technical journals in that they will present the complete results of an experiment or series of investigations which previously have appeared only in scattered articles, if published at all. On the other hand, they will differ from detailed treatises by confining themselves to specific problems of current interest, and in presenting the subject in as summary a manner and with as little technical detail as is consistent with sound method. They will be written not only for the specialist but for the educated layman.

The Characteristics of the Organic Individual; Unity and Order in the Life of the Individual; Reproduction and Individuation; Metabolism and Protoplasm; Terminology. Theoretical Review and Critique; A Dynamic Conception of the Organic Individual. Susceptibility Gradients in Animals and Plants; Further Physiological Evidence for the Existence of Metabolic Gradients; Embryological Evidence for the Existence of Axial Metabolic Gradients; Developmental Gradients in Agamic and Experimental Reproduction; Conclusion.

IV. Physiological Dominance in the Process of Individuation 88 The Experimental Material; The Independence of the Apical Region; Dominance and Subordination in Experimental Reproduction; The Reconstitution of an Individual from an Isolated Piece; Some Modifying and Limiting Factors in Animal Reconstitution; Conclusion. V. The Range of Dominance, Physiological Isolation, AND Experimental Reproduction 127 Experimental Control of Spatial Relations of Parts and of the Range of Dominance; Experimental Obliteration and Determination of Axial Gradients and Dominance; The Extension of Dominance during Development; Experimental Physiological Isolation and Reproduction in Plants; The Localization of Experimental Reproduction in Relation to Different Axes; Conclusion.

The Nature of Dominance; The Nature of Inhibition; Origin of MetaboHc Gradients and of Dominance; Morphological Differentiation in Relation to Metabolic Rate; The Fundamental Reaction System; Agamic Reproduction in Relation to Physiological Isolation; Gametic Reproduction; Heredity, Evolution, and Other Problems from the Dynamic Standpoint. The present book is an attempt to state, and to present some of the evidence in favor of, a conception of the nature of organic individuality which has gradually developed in the mind of the writer during the course of some fifteen years' investigation of the simpler processes of reproduction and development in the lower animals. In these forms organic individuality appears in relatively simple terms, and it is here if anywhere that we must look for the key to the problem of individuality in the higher animals and man.

With the great variety of facts at hand and the limited space available, it has often been difficult to decide what particular points of the evidence to include in the consideration and what to omit. To those familiar with biological facts it will doubtless be evident that many data from various lines of investigation have been either barely mentioned or entirely omitted. The attempt has been made to show in some degree the wide range of applicability of this conception of individuality to various biological fields, and it is perhaps permissible to express the hope that, not only the physiologist and botanist, but also the neurologist, the psychologist, and the sociologist may find something of interest in it. Chaps, i and ii are necessarily somewhat abstract and condensed and may seem to some readers to demand too extensive a background of biological knowledge. A re-reading of these chapters after reading chaps, iii-vi will assist in decreasing this difficulty.

In the book Senescence and Rejuvenescence , recently published, the writer was chiefly concerned with the periodic changes of the age cycle in the organic indi- \ddual as one aspect of the life-cycle. The present book deals primarily with the problem of the nature of the unity and order in the organism, the constancy of character and course of development, the maintenance of individuality in a changing environment, and the processes of physiological isolation, disintegration, and integration or individuation in reproduction. The two books, concerned as they are v/ith these intimately associated aspects of the life cycle, are in many respects complementary and together constitute a presentation of the more important results and conclusions from the writer's investigations and a consideration of certain biological problems from the point of view attained.

For permission to reproduce or to make drawings based upon figures which have appeared in other books acknowledgments are due to publishers as follows: to the University of Chicago Press, for Figs. i6, 17, 25-27, 47, 48 in part, reproduced from Senescence and Rejuvenescence; to Wilhelm Englemann, for Fig. 13, drawn after von Graff's Fig. 8, Tafel XVIII, in Monographic der Turbellarien, I, Rhabdocoelida ; to Henry Holt & Co., for Figs. 14 and 15, reproduced from Lillie, Development of the Chick; to Gustav Fischer, for Fig. 86, reproduced from Hildebrand, Die Gattung Cyclamen; to B. G. Teubner, for Fig. 92, drawn after Fig. 84 in Goebel, Einleitung in die experimentelle Morphologic der Pflanzen. For certain figures drawn from botanical preparations and for other figures not original, acknowledgments are made in the legends. The writer is also deeply indebted to Dr. C. J. Herrick and to Dr. W. J. G. Land, both of the University of Chicago, for reading the manuscript and for suggestions and criticisms.

The organic world appears in the form of more or less clearly defined and limited and more or less complex entities which for lack of a better name we call individuals. The individual is not necessarily a single whole organism; it may be a part of a cell, a single cell, or a many-celled organ or complex part of the organism; or, as in most plants and some of the lower animals, a number of organisms possessing certain organs or parts in common, and therefore remaining in organic continuity with each other, may together constitute an individual. In at least most organic individuals a more or less orderly series of changes in structure and behavior which comprise the life-history occur, and in the course of these changes the individuals may give rise to new individuals by some sort of reproductive process.

In order to define the problem of the organic individual, it is necessary to inquire whether any fundamental identity or similarity is discoverable in all individuals and whether the changes which they undergo are subject to any general laws which we can at present apprehend. The term ''individual," meaning in its etymological sense something undivided or which cannot be divided, is open to various objections. Division of individuals to form new individuals is a characteristic feature of

living forms, and many individuals are made up of other individuals and these in turn of others. Divisibility is as characteristic of the organic individual as indivisibility. Nevertheless individuality is a very real thing in the organic world. There may often be difficulty in determining the presence or absence of individuality or the limits in space or time of particular individuals, but such difficulties do not in the least shake our faith in the existence of the individual. Whatever the anatomist and the histologist tell us concerning the constitution of the human body, of hundreds of organs and millions of cells, it is perfectly evident that each human being is an individual, because his behavior proves it= And the same is true for the single cell with its nucleus, cytoplasm, centrosomes, plastids of various kinds, "mitochondria," chromosomes, chromomeres, etc. We call the cell an individual because of its behavior. What then are the fundamental characteristics of this behavior ? In what does the individuality consist ? In the first place, the organic individual is alive and therefore consists essentially of the complex of substances termed in general protoplasm; secondly, it is more or less definitely limited in size; thirdly, it possesses a more or less definite morphology, a visible form and structure, which is associated in some way with dynamic and primarily chemical activity; fourthly, a greater or less degree of order, co-ordination, correlation, or harmony, as it is variously called, is perceptible in the character of its form and structure and in the dynamic activities of its constituent parts. In short, the organic individual appears to be a unity of some sort, its individuality consists primarily in this unity, and the process

of individuation is the process of integration of a mere aggregation into such a unity, for this unity is not simply the unity of a chance aggregation, but one of a very particular kind and highly constant character for each kind of individual. In all except the simplest individuals it determines a remarkable degree of uniformity and constancy, both in the spatial relations of parts and the order of their appearance in time, and also in co-ordination or harmony of functional relation of these parts after their development.

If this conception is correct, the fundamental problem of the organic individual is the problem of the nature of this unity. The first step in consideration of this problem is to inquire whether this unity is real or apparent. Conceivably it may be only an apparent or passive unity resulting from a pre-established harmony of some kind between the constituent parts, a unity like that of a house constructed from girders, stone, and other materials, each part of which is measured and cut beforehand according to a definite plan. The real unity here is in the plan, not in its material realization. This conception of the individual leads necessarily to the assumption of a creative entity of some sort which controls and orders the physico-chemical organism as man controls and orders the materials of the house which he builds. This dualistic or ''vitalistic" conception of the organic individual, carried to its logical conclusion, denies the possibility of solution of the problem of organic individuality by scientific methods. Some of its special forms will be briefly considered in another chapter.

On the other hand, the unity of the organic individual may be an active unity resulting from interactions between its parts, but such a unity may again conceivably be the result of a pre-established harmony in construction like that of a steam engine, or it may be a unity which itself determines, constructs, and harmonizes as a flowing t^ream sculptures its channel and develops a characteristic morphological structure and dynamic activity in mutual relation to each other.' According to this last purely mechanistic conception the problem of individuality is accessible to scientific investigation and may be solved by scientific methods.

While it is impossible to exclude absolutely the dualistic alternative as long as a complete mechanistic solution of the problem has not been reached, the advance of scientific knowledge has resulted in demonstrating the mechanistic character of one feature after another of the organism and in narrowing the field within which vitalistic assumptions are still possible. We know that actual energetic relations do exist between the different parts of the individual. These relations, which are often called physiological correlation, are of various sorts: mechanical, such as pressure or tension between parts; transportative, consisting in the transportation or exchange of substances between different parts; transmissive or conductive, consisting of changes, impulses, or excitations transmitted or conducted from molecule to molecule or from particle to particle. Physiological correlation of these different kinds unquestionably pla^ys a very important part in the unity of the individual, and the only possible method of procedure to determine whether the unity consists essen-

* Child, "The Regulatory Processes in Organisms," Jour, oj Morphol., XXII, 191 1. tially in such correlation is the scientific method, that of experimental analysis and synthesis of data. Only in this way is there any possibility of determining whether or not the mechanistic conception is adequate. My own experiments, together with the experimental and observational data already at hand, point the way toward a conception of organic unity which is somewhat different from current views, but still entirely mechanistic, and which, as I believe, makes possible further advance toward a solution of the problem.

Life in general consists of the life-histories of individuals. Individuals arise from other individuals, undergo a more or less definite and orderly series of changes known as development, usually reproduce new individuals in a more or less orderly way, and either undergo complete physiological disintegration into new individuals in the process of reproduction or else finally lose their unity by the cessation of their activity in death. The definitiveness and constancy, the degree of order in the behavior of the individual as regards the morphological and physiological relations of its parts in space and the sequence of the changes during its life, must be considered as, to some extent at least, a criterion of the degree of unity or individuality which it possesses. In the simplest individuals order is scarcely apparent; it is sometimes difficult to determine whether a particular aggregation of protoplasmic substance is an individual in the biological sense or merely an aggregation. Again, in some cases a given order is local or temporary and is

soon succeeded by another. Each pseudopodium of an amoeba, for example, is to some extent an individuation of a part of the amoeba protoplasm, but it soon disappears or gives way to another individuation, and so on. Between such simple and evanescent individuals as this, at the one extreme, and the human body with its amazingly complex structural and functional order and its relative permanency at the other, there are of course many intermediate conditions. In general, organic evolution appears from this point of view to consist in an increasing complexity and stability of order; in other words, the degree of individuation, the unity of the individual, increases in the course of evolution. The series of changes which constitute development becomes more and more definite, constant, and complex in appearance, and the product of these changes, the fully formed individual, shows an increasing complexity and stability of structure and an increasing variety and degree of interrelation of parts. In fact, a progressive morphological and physiological complication seems to occur both in individual development and in evolution. Between the unicellular organism and the adult human being the difference appears to be almost infinite, but the human individual is at the beginning a single cell with much less complex visible structure than many unicellular forms.

The process of visible structural complication which occurs in both development and evolution is commonly known as differentiation. Different regions of the cell, different cells or cell groups, become different from each other and from the original undifferentiated or so-called embryonic condition. These differences are in general brought about by the formation and accumulation in or about the cell of substances not present in the undifferentiated cell. Differentiation is of course merely a visible indication of differences of some sort in physiological activity in different parts, although physiological differences may exist without visible differentiation. The physiological differences appear to consist at least in large part of specialization in activity, that is, the various fundamental activities of life which are all present to some degree in the unspecialized cell or part become more or less definitely distributed and localized among different parts, a process often called division of labor. Such specialization of parts is a characteristic feature of life in all except the simplest individuals, and even there it is probably present to some extent.

Physiological specialization and the differentiation which may result from it occur in any orderly way, and in fact constitute the fundamental evidence for the orderly character of the individual. The orderly course of specialization and differentiation proceeds very much as if there were underlying it a plan or scheme characteristic for each kind of individual which is worked out in a regular constant order, as the construction of a building according to a plan follows a regular course. The orderly localization of parts and the orderly sequence in their appearance with reference to certain directions in the developing individual indicate the existence of some sort of ordering capacity underlying and preceding the stages where the order becomes structurally visible. It is evident that this underlying order, plan, or whatever it may be that determines the developmental and physiological order in the individual

is the foundation of individual unity and order, but in attempts to solve the problem of the individual this fact has not always been clearly recognized. This underlying capacity for unity and order finds its primary expression in what is commonly called the polarity and S3rmmetry of the individual. These terms polarity and symmetry refer to the fact that in the appearance and maintenance of the structural and functional order in the individual certain geometrical relations, characteristic for each individual, are distinguishable. These relations are commonly expressed in terms of axes or planes. To say that an individual possesses a polar axis or a plane of symmetry is merely a convenient way of stating the fact that it is possible to conceive as drawn through the individual a line or a plane with reference to which order is perceptible. Such a geometrical conception is an abstraction from the fact that order is actually perceptible to a greater or less degree in all directions. It is merely a selection of those ideal lines or planes to which the order is most directly, simply, or permanently related, and these then serve as a system of co-ordinates, so to speak, to which the order is conveniently referred.

The geometrical relations of order differ in different individuals. In some cases the order is referable to a system of lines passing through a common center and is designated as radial or radiate. In other cases the order is referable to one or a certain number of axes and is therefore an axiate order, and it is often convenient to refer to planes instead of axes of symmetry. In the living individuals as they exist in nature various combinations of these relations occur. In the starfish, for

example, a high degree of order is apparent in relation to an axis drawn through the center of the body vertical to the plane in which the arms extend. Centering about this axis is an order referable to radii centering in this axis, and again the order in each arm is referable to a plane passed through the radius of each arm and the central axis of the body. In a bilaterally symmetrical individual, such as man and many animals, the order can be referred to three axes — longitudinal, transverse, and dorso-ventral — at right angles to each other, or perhaps better to a longitudinal axis, and two planes, transverse and ventro-dorsal, passed at right angles to each other through this longitudinal axis.

These axes and planes drawn through the individual as a whole represent merely the general plan of orderly arrangement. Geometrical relations are also distinguishable in the order of various parts and organs, and these relations do not necessarily coincide in direction with the geometrical scheme of the whole individual but differ from it in all conceivable ways. Evidently the geometrical relations of order in the organic individual, particularly where the structure is complex, are by no -means as simple as the general scheme might seem to indicate.

The reason for making a distinction between polarity and symmetry lies in the fact that in most axiate individuals one axis, the polar axis, is distinguishable as the chief or major axis of the body. In the direction of this axis the specialization and differentiation are most marked and the order in this direction is usually more conspicuous or more stable and commonly appears earlier than that in other directions. This axis is also very often the chief direction of growth, so that the body becomes elongated in this direction and the polar axis becomes the longitudinal axis. In short, the so-called polarity of the individual represents the direction of the chief or major order, while the axes of symmetry represent the directions of minor orders.

The two terminal regions of the polar axis exhibit in general distinct and characteristic differences in behavior and structure. In most plants, in sessile animals, and in radially symmetrical forms generally, these two regions are commonly called the apical and basal regions, while in bilaterally symmetrical motile animals they are usually known as anterior and posterior. The apical or anterior region is primarily the region of greatest dynamic or metabolic activity in the individual: in the plants it becomes the growling tip, the region of most active primary growth, while in the animals it becomes the most highly specialized and differentiated region of the body, and in those forms which possess a central nervous system, including all except the simplest animals, the chief part of the central nervous system, the cephalic ganglion or brain and the chief sense organs usually arise in this region; in other words it becomes the head and in motile forms usually precedes in locomotion.

The basal or posterior end, on the other hand, is primarily the least active region, although in many forms it may become secondarily a region of growth or specialized activity because of certain changes during the life of the individual wliich will be considered later. In sessile forms it is usually the region of attachment and may develop special organs of attachment, while in motile forms its activity is more or less under the control of the apical or anterior region. In fact it is impossible to escape the conclusion that certain general features common to most or all axiate individuals are similarly related to the polar apico-basal or antero-posterior axis, as it is variously called. As regards other axes, the differences in relation between them and the differences in behavior and structure in different individuals complicate the matter, but I shall show that there are good grounds for believing that an organic or physiological axis is fundamentally the same in all cases, whether it is an axis of polarity or symmetry of a whole organism or of a part. These geometrical relations serve primarily to express in a general way the fact that spatial order of certain kinds exists in the organic individual, but the orderly sequence of events in time is also referable to a greater or less degree to the geometrical scheme of the individual or part. In development the specialization and differentiation make their appearance and undergo their progressive changes in a more or less definite sequence with respect to the chief axes. In many cases the original geometrical plan of the individual undergoes modification or gives place to a different plan. Such changes are sometimes brought about by conditions within the individual and sometimes occur in response to changes in external conditions. In general, however, it may be said that in any given kind of individual the plan is always the same or undergoes the same changes and is always worked out in essentially the same way during development, provided external conditions are the same. Under altered external conditions departures from the plan may occur, and individuals result which differ more or less widely from the

usual form or structure. As a matter of fact, no two individuals are exactly alike, and there is abundant reason to believe that this is so because no two individuals are or have been subjected to exactly the same conditions. From whatever standpoint we view the facts we must always return to the conclusion that the unity and order so characteristic of the life of the organic individual are in some way or other an expression of a fundamental ordering and determining capacity of some sort which makes the individual what it is.

Reproduction, the formation of new individuals from parts of those already existing, occurs in all living forms, and the question of the origin of the new individual and of the process by which the part becomes a new whole individual is perhaps the most interesting aspect of the whole problem of individuality. In the agamic or asexual forms of reproduction, which give rise to new complete organisms in the plants and lower animals, we see the existing individual dividing into two or more, sometimes in a very regular and definite manner, sometimes apparently falling apart, as it were, into fragments or single cells; or it gives rise, sometimes in a particular region, sometimes in regions manifestly determined by chance factors, to one or more small outgrowths, buds, which increase in size and become new individuals, and these in some cases remain organically connected with the parent, in others become completely separated and independent. In many cases the character of these reproductive processes varies with the

physiological condition of the individual and often also with external conditions. In some organisms, for example, many kinds of reproductive processes occur, their character varying with internal and environmental conditions. - Very generally it is possible to distinguish more or less clearly an orderly character in these reproductive processes; some of them are in fact orderly to a high degree. But they differ so widely in different organisms that attempts to discover common fundamental factors underlying the various forms of the processes have not been very successful.

In addition to those reproductive processes which give rise to whole new organisms there are also those which result in reduplication or more or less complex parts. The repetition of radially arranged parts, such, for example, as tentacles in a sea-anemone, arms in a starfish, a whorl of leaves or the parts of a flower in a plant, and on the other hand the succession of parts along an axis, leaves or branches along the stem of a plant, or the segments in the body of the earthworm, are all reproductive processes and involve processes of individuation. All such reproductive processes must be included in any attempt at a theory of reproduction.