This is natural selection, Darwin's great insight. All living creatures have the same purpose: to make more of themselves, ensuring their biological future by the only means available. And all living creatures have a maximum reproductive rate: the greatest number of offspring they can generate in a lifetime. (For people, she told the class, the maximum reproductive rate is about twenty children per couple per generation. The potential maximum for dachshunds is around 330: eleven pups per litter, three litters a year, for roughly ten years.)