WE have now completed our investigation of the process through which civilizations grow and, in the several instances which we have examined, the process seems to be one and the same. Growth is achieved when an individual or a minority or a whole society replies to a challenge by a response which not only answers that challenge but also exposes the respondent to a fresh challenge which demands a further response on his part. But although the process of growth may be uniform the experience of the various parties that undergo the challenge is not the same. The variety of experience in confronting a single series of common challenges is manifest when we compare the experiences of the several different communities into which any single society is articulated. Some succumb, while others strike,out a successful response through a creative movement of Withdrawal-and-Return, while others neither succumb nor succeed but manage to survive until the member which has succeeded shows them the new pathway, along which they follow tamely in the footsteps of the pioneers. Each successive challenge thus produces differentiation within the society, and the longer the series of challenges the more sharply pronounced will this differentiation become. Moreover, if the process of growth thus gives rise to differentiation within a single growing society where the challenges are the same for all, then, a fortiori, the same process must differentiate one growing society from another where the challenges themselves differ in character.
A conspicuous illustration presents itself in the domain of art, for it is generally recognized that every civilization creates an artistic style of its own; and if we are attempting to ascertain the limits of any particular civilization in space or time we find that the aesthetic test is the surest as well as the subtlest. For example, a survey of the artistic styles that have prevailed in Egypt brings out the fact that the art of the Pre-Dynastic Age is not yet characteristically Egyptiac, whereas the Coptic art has discarded the characteristically Egyptiac traits; and on this evidence we can establish the time-span of the Egyptiac Civilization. By the same test we can establish the dates at which the Hellenic Civilization emerged from beneath the crust of the Minoan Society, and at which it disintegrated to make way for the Orthodox Christian Society. Again, the style of the Minoan artefacts enables us to delimit the extension in space of the Minoan Civilization in the various stages of its history.