Meanwhile, head-hunting has for these wild-folk a function and significance that are not to be understood so long as we consider it as a thing apart. The same canon of interpretation holds good of any other outstanding feature of the social life. Customs are the organic parts of a body of custom. To use a technical expression, they are but so many elements composing a single “culture-complex.” Modern research is greatly concerned with the tracing out of resemblances due to the spread of one or another system of associated customs. The method is to try to work back to some ethnic centre of diffusion; where the characteristic elements of the system, whatever might have been their remoter derivation, have been thoroughly fused together, in the course of a long process of adaptation to a given environment. Thereupon it becomes possible to follow up the propagation of influence as it radiates from this centre in various directions outwards. Now it may well be that the tradition rarely, or never, is imparted in its entirety. Selection, or sheer accident, will cause not a little to be left behind. On the other hand, the chances are all against one custom setting forth by itself. Customs tend to emigrate in groups. Thus head-hunting, and a certain mode of tattooing, and the institution of the skull-shelf, and the requirement that a would-be husband must display a head as token of his prowess, are on the face of them associated customs, and such as are suited to have been travelling companions. Hence it is for the ethnologist to see whether he cannot refer the whole assortment to some intrusive culture of Indonesian or other origin.