Now, it is a striking fact that amongst animals there are some whose conduct can be generalized very readily in the categories of self-preservation, nutrition, and sex, while there are others whose conduct cannot be thus summarized. The behaviour of the tiger and the cat is simple, and easily comprehensible, presenting no unassimilable anomalies, whereas that of the dog, with his conscience, his humour, his terror of loneliness, his capacity for devotion to a brutal master, or that of the bee, with her selfless devotion to the hive, furnishes phenomena which no sophistry can assimilate without the aid of a fourth instinct. But little examination will show that the animals whose conduct it is difficult to generalize under the three primitive instinctive categories are gregarious. If then it can be shown that gregariousness is of a biological significance approaching in importance that of the other instincts, we may expect to find in it the source of these anomalies of conduct, and if we can also show {18} that man is gregarious, we may look to it for the definition of the unknown “x” which might account for the complexity of human behaviour.
III. B IOLOGICAL S IGNIFICANCE OF G REGARIOUSNESS.
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