The persons seated in one compartment of a railway train during a long journey may be entirely strangers to one another at the outset; yet, even in the absence of conversation, they in the course of some hours will begin to manifest some of the peculiarities of the psychological group. To some extent they will have come to a mutual understanding and adjustment; and, when a stranger adds himself to their company, his entrance is felt to some extent as an intrusion which at the least demands readjustments; he is regarded with curious and to some extent hostile glances. If an outsider threatens to encroach on the rights of one of the company, the others will readily combine in defence of their member; and any little incident affecting their one common interest (namely, punctual arrival of the train at its destination) quickly reveals, and in doing so strengthens, the bond of common feeling. On a sea voyage the group spirit of the passenger ship attains a greater development, by reason of the longer continuance of the group, its more complete detachment and definition, the sense of greater hazard affecting all alike, the sense of dependence on mutual courtesy and good-will and sympathy for the comfort and enjoyment of all. Very soon the experienced traveller, contrasting and comparing this present company with those of previous voyages, sums up its qualities and defects and lays his plans accordingly. And by the time that an intermediate port is reached, where perhaps the most ‘grumpy’ and least entertaining member of the company disembarks, even his departure is felt by the rest as a loss that leaves a gap in the structure of the group. Such fortuitous and ephemeral groups apart, all others may be classed in the two great divisions of natural and artificial groups. The natural groups again fall into two main classes which partly coincide,—namely, those rooted in kinship and those determined by geographical conditions. The family is the pure example of the former; the population of a small island, the type of the latter kind. The main difference is that the bonds of the kinship group are purely or predominantly mental and therefore can, and commonly do, remain effective in spite of all spatial separation and of all lack of common purpose or of material benefits accruing from membership in the group. The artificial groups may be divided into three great classes, the purposive, the customary or traditional, and the mixed; those of the last kind combining the purposive and the traditional characters in various proportions.