Trotter derives the mental phenomena that are described as occurring in groups from a herd instinct ('gregariousness'), which is innate in human beings just as in other species of animals. Biologically this gregariousness is an analogy to multicellularity and as it were a continuation of it. From the standpoint of the libido theory it is a further manifestation of the inclination, which proceeds from the libido, and which is felt by all living beings of the same kind, to combine in more and more comprehensive units.[51] The individual feels 'incomplete' if he is alone. The dread shown by small children would seem already to be an expression of this herd instinct. Opposition to the herd is as good as separation from it, and is therefore anxiously avoided. But the herd turns away from anything that is new or unusual. The herd instinct would appear to be something primary, something 'which cannot be split up'. Trotter gives as the list of instincts which he considers as primary those of self-preservation, of nutrition, of sex, and of the herd. The last often comes into opposition with the others. The feelings of guilt and of duty are the peculiar possessions of a gregarious animal. Trotter also derives from the herd instinct the repressive forces which psycho-analysis has shown to exist in the ego, and from the same source accordingly the resistances which the physician comes up against in psycho-analytic treatment. Speech owes its importance to its aptitude for mutual understanding in the herd, and upon it the identification of the individuals with one another largely rests. While Le Bon is principally concerned with typical transient group formations, and McDougall with stable associations, Trotter has chosen as the centre of his interest the most generalised form of assemblage in which man, that Ϛὡον πολιτικὁν, passes his life, and he gives us its psychological basis. But Trotter is under no necessity of tracing back the herd instinct, for he characterizes it as primary and not further reducible. Boris Sidis's attempt, to which he refers, at tracing the herd instinct back to suggestibility is fortunately superfluous as far as he is concerned; it is an explanation of a familiar and unsatisfactory type, and the converse proposition--that suggestibility is a derivative of the herd instinct--would seem to me to throw far more light on the subject.