The animal kingdom presents two relatively sudden and very striking advances in complexity and in the size of the unit upon which natural selection acts unmodified. These advances consist in the aggregation of units which were previously independent and exposed to the full normal action of natural selection, and the two instances are, of course, the passage from the unicellular to the multicellular, and from the solitary to the social.
It is obvious that in the multicellular organism individual cells lose some of the capacities of the unicellular—reproductive capacity is regulated and limited, nutrition is no longer possible in the old simple way and response to stimuli comes only in certain channels. In return for these sacrifices we may say, metaphorically, that the action of natural selection is withdrawn from within the commune. Unfitness of a given cell or group of cells can be eliminated only through its effect upon the whole organism. The latter is less sensitive to the vagaries of a single cell than is the organism of which the single cell is the whole. It would seem, therefore, that there is now allowed a greater range of variability for the individual cells, and perhaps, therefore, an increased richness of the material to be selected from. Variations, moreover, which were not immediately favourable would now have a chance of surviving.
Looked at in this way, multicellularity presents itself as an escape from the rigour of natural selection, which for the unicellular organism had narrowed {19} competition to so desperate a struggle that any variation outside the straitest limits was fatal, for even though it might be favourable in one respect, it would, in so small a kingdom, involve a loss in another. The only way, therefore, for further advantageous elaboration to occur was by the enlargement of the competing unit. Various species of multicellular organisms might in time be supposed in turn to reach the limit of their powers. Competition would be at its maximum, smaller and smaller variations would be capable of producing serious results. In the species where these conditions prevail an enlargement of the unit is imminent if progress is to occur. It is no longer possible by increases of physical complexity and the apparently inevitable sequence is the appearance of gregariousness. The necessity and inevitableness of the change are shown by its scattered development in very widely separated regions (for example, in insects and in mammals) just as, we may suspect, multicellularity appeared.
Gregariousness seems frequently to be regarded as a somewhat superficial character, scarcely deserving, as it were, the name of an instinct, advantageous it is true, but not of fundamental importance or likely to be deeply ingrained in the inheritance of the species. This attitude may be due to the fact that among mammals at any rate the appearance of gregariousness has not been accompanied by any very gross physical changes which are obviously associated with it.[3]
[3] Among gregarious insects there are of course physical changes arising out of and closely dependent on the social organization.
To whatever it may be due, this method of regarding the social habit is, in the opinion of the present writer, not justified by the facts, and prevents the attainment of conclusions of considerable fruitfulness.
A study of bees and ants shows at once how {20} fundamental the importance of gregariousness may become. The individual in such communities is completely incapable, often physically, of existing apart from the community, and this fact at once gives rise to the suspicion that even in communities less closely knit than those of the ant and the bee, the individual may in fact be more dependent on communal life than appears at first sight.
Another very striking piece of general evidence of the significance of gregariousness as no mere late acquirement is the remarkable coincidence of its occurrence with that of exceptional grades of intelligence or the possibility of very complex reactions to environment. It can scarcely be regarded as an unmeaning accident that the dog, the horse, the ape, the elephant, and man are all social animals. The instances of the bee and the ant are perhaps the most amazing. Here the advantages of gregariousness seem actually to outweigh the most prodigious differences of structure, and we find a condition which is often thought of as a mere habit, capable of enabling the insect nervous system to compete in the complexity of its power of adaptation with that of the higher vertebrates.
If it be granted that gregariousness is a phenomenon of profound biological significance and one likely therefore to be responsible for an important group of instinctive impulses, the next step in our argument is the discussion of the question as to whether man is to be regarded as gregarious in the full sense of the word, whether, that is to say, the social habit may be expected to furnish him with a mass of instinctive impulse as mysteriously potent as the impulses of self-preservation, nutrition, and sex. Can we look to the social instinct for an explanation of some of the “a priori syntheses of the most perfect sort needing no proof but their own evidence,” which are not explained by the three {21} primitive categories of instinct, and remain stumbling-blocks in the way of generalizing the conduct of man?
The conception of man as a gregarious animal is, of course, extremely familiar; one frequently meets with it in the writings of psychologists and sociologists, and it has obtained a respectable currency with the lay public. It has, indeed, become so hackneyed that it is the first duty of a writer who maintains the thesis that its significance is not even yet fully understood, to show that the popular conception of it has been far from exhaustive. As used hitherto the idea seems to have had a certain vagueness which greatly impaired its practical value. It furnished an interesting analogy for some of the behaviour of man, or was enunciated as a half serious illustration by a writer who felt himself to be in an exceptionally sardonic vein, but it was not at all widely looked upon as a definite fact of biology which must have consequences as precise and a significance as ascertainable as the secretion of the gastric juice or the refracting apparatus of the eye. One of the most familiar attitudes was that which regarded the social instinct as a late development. The family was looked upon as the primitive unit; from it developed the tribe, and by the spread of family feeling to the tribe the social instinct arose. It is interesting that the psychological attack upon this position has been anticipated by sociologists and anthropologists, and that it is already being recognized that an undifferentiated horde rather than the family must be regarded as the primitive basis of human society.