A close parallel exists between the anatomical and psychological characteristics of living beings. In these anatomical characteristics certain invariable, or slightly variable, elements are met with, to change which the lapse is necessary of geological ages. Side by side with these fixed, indestructible features are to be found others extremely changeable, which the art of the breeder or horticulturist may easily modify, and at times to such an extent as to conceal the fundamental characteristics from an observer at all inattentive. The same phenomenon is observed in the case of moral characteristics. Alongside the unalterable psychological elements of a race, mobile and changeable elements are to be encountered. For this reason, in studying the beliefs and opinions of a people, the presence is always detected of a fixed groundwork on which are engrafted opinions as changing as the surface sand on a rock. The opinions and beliefs of crowds may be divided, then, into two very distinct classes. On the one hand we have great permanent beliefs, which endure for several centuries, and on which an entire civilisation may rest. Such, for instance, in the past were feudalism, Christianity, and Protestantism; and such, in our own time, are the nationalist principle and contemporary democratic and social ideas. In the second place, there are the transitory, changing opinions, the outcome, as a rule, of general conceptions, of which every age sees the birth and disappearance; examples in point are the theories which mould literature and the arts--those, for instance, which produced romanticism, naturalism, mysticism, &c. Opinions of this order are as superficial, as a rule, as fashion, and as changeable. They may be compared to the ripples which ceaselessly arise and vanish on the surface of a deep lake. The great generalised beliefs are very restricted in number. Their rise and fall form the culminating points of the history of every historic race. They constitute the real framework of civilisation. It is easy to imbue the mind of crowds with a passing opinion, but very difficult to implant therein a lasting belief. However, a belief of this latter description once established, it is equally difficult to uproot it. It is usually only to be changed at the cost of violent revolutions. Even revolutions can only avail when the belief has almost entirely lost its sway over men's minds. In that case revolutions serve to finally sweep away what had already been almost cast aside, though the force of habit prevented its complete abandonment. The beginning of a revolution is in reality the end of a belief. The precise moment at which a great belief is doomed is easily recognisable; it is the moment when its value begins to be called in question. Every general belief being little else than a fiction, it can only survive on the condition that it be not subjected to examination.