Hence the real problem in understanding China's loss of political and technological preeminence to Europe is to understand China's chronic unity and Europe's chronic disunity.

The answer is again suggested by maps. Europe has a highly indented coastline, with five large peninsulas that approach islands in their isolation, and all of which evolved independent languages, ethnic groups, and governments: Greece, Italy, Iberia, Denmark, and Norway/ Sweden. China's coastline is much smoother, and only the nearby Korean Peninsula attained separate impor­tance.

Europe has two islands (Britain and Ireland) sufficiently big to assert their political independence and to maintain their own languages and eth­nicities, and one of them (Britain) big and close enough to become a major independent European power.

But even China's two largest islands, Tai­wan and Hainan, have each less than half the area of Ireland; neither was a major independent power until Taiwan's emergence in recent decades; and Japan's geographic isolation kept it until recently much more isolated politically from the Asian mainland than Britain has been from mainland Europe.

Europe is carved up into' independent linguistic, ethnic, and politi­cal units by high mountains (the Alps, Pyrenees, Carpathians, and Norwe­gian border mountains), while China's mountains east of the Tibetan plateau are much less formidable barriers.

China's heartland is bound together from east to west by two long navigable river systems in rich alluvial valleys (the Yangtze and Yellow Rivers), and it is joined from north to south by relatively easy connections between these two river systems (eventually linked by canals).

As a result, China very early became domi­nated by two huge geographic core areas of high productivity, themselves only weakly separated from each other and eventually fused into a single core.

Europe's two biggest rivers, the Rhine and Danube, are smaller and connect much less of Europe.

Unlike China, Europe has many scattered small core areas, none big enough to dominate the others for long, and each the center of chronically independent states.

Once China was finally unified, in 221 B.C., no other independent state ever had a chance of arising and persisting for long in China.

Although periods of disunity returned several times after 221 B.C., they always ended in reunification.

But the unification of Europe has resisted the efforts of such determined conquerors as Charlemagne, Napoleon, and Hitler; even the Roman Empire at its peak never controlled more than half of Europe's area.

Thus, geographic connectedness and only modest internal barriers gave China an initial advantage.

North China, South China, the coast, and the interior contributed different crops, livestock, technologies, and cultural features to the eventually unified China.

For example, millet cultivation, bronze technology, and writing arose in North China, while rice cultiva­tion and cast-iron technology emerged in South China.

For much of this book I have emphasized the diffusion of technology that takes place in the absence of formidable barriers.

But China's connectedness eventually became a disadvantage, because a decision by one despot could and repeat­edly did halt innovation.

In contrast, Europe's geographic balkanization resulted in dozens or hundreds of independent, competing statelets and centers of innovation.

If one state did not pursue some particular innova­tion, another did, forcing neighboring states to do likewise or else be con­quered or left economically behind.

Europe's barriers were sufficient to prevent political unification, but insufficient to halt the spread of technol­ogy and ideas.

There has never been one despot who could turn off the tap for all of Europe, as of China.

These comparisons suggest that geographic connectedness has exerted both positive and negative effects on the evolution of technology.

As a result, in the very long run, technology may have developed most rapidly in regions with moderate connectedness, neither too high nor too low.

Technology's course over the last 1,000 years in China, Europe, and possi­bly the Indian subcontinent exemplifies those net effects of high, moderate, and low connectedness, respectively."