Many attempts have been made to explain the behaviour of man as dictated by instinct. He is, in fact, moved by the promptings of such obvious instincts as self-preservation, nutrition, and sex enough to render the enterprise hopeful and its early spoils enticing. So much can so easily be generalized under these three impulses that the temptation to declare that all human behaviour could be resumed under them was irresistible. These early triumphs of materialism soon, however, began to be troubled by doubt. Man, in spite of his obvious duty to the contrary, would continue so often not to preserve himself, not to nourish himself and to prove resistant to the blandishments of sex, that the attempt to squeeze his behaviour into these three categories began to involve an increasingly obvious and finally intolerable amount of pushing and pulling, as well as so much pretence that he was altogether “in,” {17} when, quite plainly, so large a part of him remained “out,” that the enterprise had to be given up, and it was once more discovered that man escaped and must always escape any complete generalization by science. A more obvious inference would have been that there was some other instinct which had not been taken into account, some impulse, perhaps, which would have no very evident object as regarded the individual, but would chiefly appear as modifying the other instincts and leading to new combinations in which the primitive instinctive impulse was unrecognizable as such. A mechanism such as this very evidently would produce a series of actions in which uniformity might be very difficult to recognize by direct observation, but in which it would be very obvious if the characters of this unknown “x” were available. 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. BIOLOGICAL SIGNIFICANCE OF GREGARIOUSNESS.