REGENERATION IN ANIMALS, EXPERIMENTAL EMBRYOLOGY.) Hitherto we have been implicitly considering the organism as stable; but in reality no organism is at all stable—even the long est-continued adult stage is but part of a changing life-cycle. We are accustomed to think of ourselves as the same individual which we were as a child, and to extend this identity backwards to embryo and egg. This is legitimate, but involves a rather new conception of individuality. The individual thus viewed ceases to be a static being; it is rather, as Le Dantec says, a history. What breaks in this history suffice to justify us in saying that a new individual has arisen? In higher animals there are no very abrupt changes from egg through embryo to adult. In frogs the change at metamorphosis (q.v.) is so violent that two states re ceive different names—tadpole and frog. In higher insects, the organs of the grub are actually broken down during pupal life, and those of the adult arise from rudiments previously dormant. In sea-urchins the adult arises as a separate lateral rudiment in the late larva; and in Nemertina (q.v.), the bulk of the larva is actually thrown off at metamorphosis.
We generally call such cases transformations of a single indi viduality ; but there is no hard line to be drawn between them and other cases, e.g., liver-flukes (see TREMATODA), whose life cycle consisted of a whole succession of forms, which here we usually style separate individuals. Finally, there are difficulties concerned with dedifferentiation (q.v.). A sea-squirt such as Clavellina dedifferentiates to an opaque simply-constructed mass, then redifferentiates to a normal Clavellina. Is it the same individual, or a new one? In plants, difficulties are even greater, since often there is no definitely shaped body, but only a tangled felt of filaments, e.g., some Algae, most fungi. Mushrooms are only fructifications; the plant's vegetative business is performed by a subterranean felt-work of wholly irregular construction. Even in higher plants this sort of thing may hold. Many trees grow by suckers, so that one "individual" may be in organic connection with several others. The climax is reached in the case of the banyan tree, in which the branches let fall aerial roots which turn into new trunks, so that a many-trunked colonial tree is produced.
These examples will show the impossibility of defining an indi vidual in any absolute way, or of believing that individuality is ever complete or perfect. We must rather say that there are degrees of individuality, and that it is very poorly exhibited by plants and many animals.
An individual must be in some degree a unitary whole with interdependence of parts; it must have a history of appreciable duration, singleness and continuity; it must be rather sharply marked off from other individuals; its working must be directed at least partly to continuing either itself or other systems like itself. In so far as portions of living substance exhibit these characters, they are individuals. The more definite the unity, the more obvious its continuity, the greater the interdependence of its parts, the greater its independence as a whole, the more indi viduality must we ascribe to it.
During the course of evolution, two, in a sense contrary, ten dencies have been at work—aggregation and individuation. In dividuation is the tendency towards making a given unit more of an individual. Aggregation is the tendency to fit together a number of individuals to make an aggregate, or unit of higher order (which may then be progressively individualized). We can distinguish individuals of three orders or grades. The lowest are single cells, e.g., free-living protozoa, bacteria. These first-grade units may become aggregated to form colonies; the colonies may show division of labour between their cellular units and may even be so co-ordinated as to behave and move as wholes (e.g., V olvox). When this stage is reached, we may properly speak of a second-order individual. All multi-cellular animals are sec ond-order individuals : and the evolutionary history by which a simple coelenterate has been transformed into specialized crea tures like insects or mammals is the history of the individuation of such units. These units, however, may again undergo aggrega tion. Colonies without division of labour are seen in Polyzoa, corals, etc.: colonies with some division of labour are frequent among hydroid polyps ; and colonies which can be called third order individuals are found in siphonophores.
The bonds holding the parts of such units together are physi cal; but with the development of efficient sense-organs and brains, the parts can be held together by psychical bonds. This, without division of labour, is seen in gregarious animals ; with division of labour, in the communities of social insects and of man, which thus also are incipient third-grade individuals.
As a general rule, where a unit of higher grade acquires more individuation, the lower-grade units composing it become less in dependent and more subordinate, and so lose individuality. The only exception to this occurs in human communities, in which the presence of language and conceptual thought has transformed the method by which the second-grade individuals (men and women) enter into relation with each other and with the whole.
In man for the first time mental life can become organized into a continuing unity with interdependent parts. Superposed upon (but interwoven with) the physical individuality is the mental individuality, which we call personality. Obviously degrees of individuation of the personality are possible, both as regards the integration of the different emotional tendencies and the intensity to which any one capacity is developed.
It is not necessary that personality should be intensified by living in a highly-organized society—indeed, the reverse is often the case—but it is true that only in highly-organized societies do the highest levels of individuation become possible. High degree of individuation undoubtedly seems to be a progressive character istic of life. It is thus important to notice that the individuation attained by the state or any other human community is, and apparently must always remain, far lower than that possible to its component members; and that therefore, biologically speak ing, the community exists for the individual more than the indi vidual for the community. Since both, however, are intimately interdependent, there are, from the purely biological point of view, many cogent duties of the individual to the community.
What we have said about the aggregation of similar units to form units of higher grade, applies also to the aggregation of dissimilar units, as in the development of symbiosis (q.v.). The only differences are that a good deal of division of labour is already given from the start in the difference between the two symbiotic species, but that their difference and separateness makes progress towards a higher degree of intimate union and mutual integration very difficult.
The idea of absolute individuality must thus be given up, just as much the idea of definite and unchanging objects in the inorganic world. A stone, e.g., is continuously losing or gaining matter and energy to or from its surroundings, and it is not the same to-day as yesterday. It is relatively but not absolutely distinct from the rest of the world. So the individual is rela tively separate, unified and independent, but never absolutely so. Among animals, the highest degree of individuation is found among second-grade units, but the greatest biological success is achieved by the combination of these to form third-grade units of low individuation (social insects, man).
Man's mental powers make it possible for him to merge his individuality with that of other individuals, with that of the community, and with general ideas; or, as it is often justly put, to lose his own individuality in something other, greater, or higher than himself.
The mental individuality (personality) is integrated out of a number of relatively separate emotional tendencies and systems of ideas; besides the central well-integrated portion, which alone has the right to be called personality, there exists a fringe of loosely-connected or unconnected mental life, and also subter ranean systems of ideas gathered round emotional tendencies which have been repressed instead of integrated with the rest. (See PSYCHOLOGY.) Peculiar sudden enlargements of mental indi viduality may occur if these unintegrated or repressed systems are brought into connection with the main personality. Con versely, certain conflicts may terminate in a total splitting of the personality into two or more parts—a process analogous to the re constitution of the halves of a bisected planarian or other low ani mal into complete individuals. (See REGENERATION IN ANIMALS.) Paradoxically, the mental individuality attains its highest de velopment only by entering into relation with the greatest pos sible number of facts and ideas outside itself (though it must of course organize them within its own system into a unified whole). If physical individualities are never at all completely separate or independent physically, mental individualities can only develop by the mutual interpenetration of subject and object in experience. Many ideas bearing on this last point of view will be found in A. N. Whitehead, Science and the Modern World.