African cavalry mounted on rhinos or hippos would have made mincemeat of European cavalry mounted on horses. But it couldnt happen.

Not until the beginning of the 20th century did Europe's urban populations finally become self-sustaining: before then, constant immigration of healthy peasants from the countryside was necessary to make up for the constant deaths of city dwellers from crowd diseases.

I decided that now is the time to start doing the things that really interest me and I find important. It was in the 10 years of the MacArthur grant that I began working on my first book... and I began putting more work into environmental history.

Today Charles Darwin is best known for establishing the fact of evolution and for recognizing the major role of natural selection in driving it.

All recognized famous inventors had capable predecessors and successors and made their improvements at a time when society was capable of using their product.

Some people have much more pull than other people. But when I say that the public has ultimate responsibility, I'm not saying it in a moral sense. I'm just saying it in the sense of what is it that's really going to bring change.

African cavalry mounted on rhinos or hippos would have made mincemeat of European cavalry mounted on horses. But it couldnt happen.