A psychological crowd once constituted, it acquires certain provisional but determinable general characteristics. To these general characteristics there are adjoined particular characteristics which vary according to the elements of which the crowd is composed, and may modify its mental constitution. Psychological crowds, then, are susceptible of classification; and when we come to occupy ourselves with this matter, we shall see that a heterogeneous crowd--that is, a crowd composed of dissimilar elements--presents certain characteristics in common with homogeneous crowds--that is, with crowds composed of elements more or less akin (sects, castes, and classes)--and side by side with these common characteristics particularities which permit of the two kinds of crowds being differentiated. But before occupying ourselves with the different categories of crowds, we must first of all examine the characteristics common to them all. We shall set to work like the naturalist, who begins by describing the general characteristics common to all the members of a family before concerning himself with the particular characteristics which allow the differentiation of the genera and species that the family includes. It is not easy to describe the mind of crowds with exactness, because its organisation varies not only according to race and composition, but also according to the nature and intensity of the exciting causes to which crowds are subjected. The same difficulty, however, presents itself in the psychological study of an individual. It is only in novels that individuals are found to traverse their whole life with an unvarying character. It is only the uniformity of the environment that creates the apparent uniformity of characters. I have shown elsewhere that all mental constitutions contain possibilities of character which may be manifested in consequence of a sudden change of environment. This explains how it was that among the most savage members of the French Convention were to be found inoffensive citizens who, under ordinary circumstances, would have been peaceable notaries or virtuous magistrates. The storm past, they resumed their normal character of quiet, law-abiding citizens. Napoleon found amongst them his most docile servants.