How Radiometric Dating Works

Briefly, a radioactive isotope is a kind of atom which decays into a different kind of atom: for example. one called uranium-238 turns into one called lead-206. Because we know how long this takes to happen, we can think of the isotope as a radioactive clock. Radioactive clocks are rather like the water clocks and candle clocks that people used in the days before pendulum clocks were invented. A tank of water with a hole in the bottom will drain at a measurable rate. If the tank was filled at...

 A Great Summary of Taxonmy

Branches or types are characterized by the plan of their structure,Classes, by the manner in which that plan is executed, as far as ways and means are concerned,Orders, by the degrees of complication of that structure,Families, by their form, as far as determined by structure,Genera, by the details of the execution in special parts, andSpecies, by the relations of individuals to one another and to the world in which they live, as well as by the proportions of their parts, their ornamentation,...