Naturally, there are many important factors in world history that I haven't had time to discuss in 40 minutes, and that I do discuss in my book. For example, I've said little or nothing about the distribution of domesticable plants (3 chapters); about the precise way in which complex political institutions and the development of writing and technology and organized religion depend on agriculture and herding; about the fascinating reasons for the differences within Eurasia between China, India, the Near East, and Europe; and about the effects of individuals, and of cultural differences unrelated to the environment, on history. But it's now time to summarize the overall meaning of this whirlwind tour through human history, with its unequally distributed guns and germs.

The broadest pattern of history � namely, the differences between human societies on different continents � seems to me to be attributable to differences among continental environments, and not to biological differences among peoples themselves. In particular, the availability of wild plant and animal species suitable for domestication, and the ease with which those species could spread without encountering unsuitable climates, contributed decisively to the varying rates of rise of agriculture and herding, which in turn contributed decisively to the rise of human population numbers, population densities, and food surpluses, which in turn contributed decisively to the development of epidemic infectious diseases, writing, technology, and political organization. In addition, the histories of Tasmania and Australia warn us that the differing areas and isolations of the continents, by determining the number of competing societies, may have been another important factor in human development.

As a biologist practicing laboratory experimental science, I'm aware that some scientists may be inclined to dismiss these historical interpretations as unprovable speculation, because they're not founded on replicated laboratory experiments. The same objection can be raised against any of the historical sciences, including astronomy, evolutionary biology, geology, and paleontology. The objection can of course be raised against the whole field of history, and most of the other social sciences. That's the reason why we're uncomfortable about considering history as a science. It's classified as a social science, which is considered not quite scientific.

But remember that the word "science" isn't derived from the Latin word for "replicated laboratory experiment," but instead from the Latin word "scientia" for "knowledge." In science, we seek knowledge by whatever methodologies are available and appropriate. There are many fields that no one hesitates to consider sciences even though replicated laboratory experiments in those fields would be immoral, illegal, or impossible. We can't manipulate some stars while maintaining other stars as controls; we can't start and stop ice ages, and we can't experiment with designing and evolving dinosaurs. Nevertheless, we can still gain considerable insight into these historical fields by other means. Then we should surely be able to understand human history, because introspection and preserved writings give us far more insight into the ways of past humans than we have into the ways of past dinosaurs. For that reason I'm optimistic that we can eventually arrive at convincing explanations for these broadest patterns of human history.