ABSTRACT
This is the essence of the species problem: rst, I will tell you with absolute certainty that there are millions of species on Earth right now that are undiscovered, undescribed, and unknown. I will also tell you with equal condence that species are not xed entities made by an omniscient creator and put on the Earth, each in its correct place. So how did they get there, and what makes them different? Also, are they all really different? In some groups, one thing fades into the next, while other species seem to be starkly different from their nearest relatives. How much of that blurriness is our inability to identify things, and how much is an incomplete boundary between lineages? Most species ‘users’ want to get on with other questions and we all want a simple answer. Unfortunately, this chapter only provides some explanation about the problem and the reasons the answer cannot be simple.
