"Certainly paleontologists have found samples of an extremely small fraction, only, of the earth's extinct species, and even for groups that are most readily preserved and found as fossils they can never expect to find more than a fraction"

This quote by George G. Simpson is referring to the reality that paleontologists have just had the ability to uncover an extremely little fraction of the earth's extinct species. Even for groups that are most easily maintained and found as fossils, paleontologists can never ever anticipate to discover more than a portion of the species that as soon as existed. This is because of the reality that the fossil record is incomplete and lots of species have been lost to time. This quote works as a pointer of the vastness of the earth's history and the constraints of our current understanding of it. It also functions as a suggestion of the value of preserving the types that still exist today, as they may one day end up being extinct and lost to time.