Nevertheless, before peoples can communicate their cultures to others with whom they come into contact, they must first evolve these cultures. The question thus remains, What determines this evolution? In order to gain a proper perspective in this matter, we must for a moment consider the progress of human civilization as a whole. Archæological research shows that the modern era of steel and iron tools was preceded by an age of bronze and copper implements, which in turn was preceded by a stone age subdivided into a more recent period of polished, and an earlier of merely chipped, stone tools. Now the chronological relations of these epochs are extremely suggestive. The very lowest estimate by any competent observer of the age of Palæolithic man in Europe sets it at 50,000 years;[9] since this is avowedly the utmost minimum value that can be assigned on geological grounds, we may reasonably assume twice that figure for the age of human culture generally. Using the rough estimate permissible in discussions of this sort, we may regard the end of the Palæolithic era as dating back about 15,000 years ago. In short, for more than eight-tenths of its existence, the human species remained at a cultural level at best comparable with that of the Australian. We may assume that it was during this immense space of time that dispersal over the face of the globe took place and that isolation fixed the broader diversities of language and culture, over and above what may have been the persisting cultural sub-stratum common to the earliest undivided human group. The following Neolithic period of different parts of the globe terminated at different times and had not been passed at all by most of the American aborigines and the Oceanians at the time of their discovery. However, from the broader point of view here assumed, it was not relieved by the age of metallurgy until an exceedingly recent past. The earliest estimate I have seen does not put the event back farther than 6000 B. C. even in Mesopotamia. During nine-tenths of his existence, then, man was ignorant of the art of smelting copper from the ore. Finally, the iron technique does not date back 4,000 years; it took humanity ninety-six hundredths of its existence to develop this art. We may liken the progress of mankind to that of a man a hundred years old, who dawdles through kindergarten for eighty-five years of his life, takes ten years to go through the primary grades, then rushes with lightning rapidity through grammar school, high school and college. Culture, it seems, is a matter of exceedingly slow growth until a certain ‘threshold’ is passed, when it darts forward, gathering momentum at an unexpected rate. For this peculiarity of culture as a whole, many miniature parallels exist in special subdivisions of culture history. Natural science lay dormant until Kepler, Galileo and Newton stirred it into unexampled activity, and the same holds for applied science until about a century ago.