Finally, we get the best of Darwin — "Origin of Species." Here, he writes about reproduction and the differences between species in producing offspring. Variables such as food, predators, disease and climate change makes numbers go up or down. Ultimately, every species' fate is interconnected with that of every other species.
While Darwin might have been a bit off with his estimate that there would 19 million elephants in 750 years, the rest of it is spot on.