Once considered a "degenerate" form of life, parasites are being
seen as important indicators of how evolution has progressed
over 4 billion years. Zimmer credits them with being the driving
force for biological diversity. He substantiates this claim with
a sweeping, evocative survey of what is known today about
parasites. That, he regretfully concedes, is little enough. What
is known is that many early conceptions about parasites needed
to be thrown aside as more information about this highly
adaptable and widely variable range of organisms emerges. While
we may recoil at the term "parasite", Zimmer identifies but one
villain in this book. Ray Lankester, a devoted Edwardian-era
evolutionist, postulated that parasites were a "regressive" form
of organism. He thought they shed evolutionary advantages as
they simplified their bodies through their life cycles.
Lankester thus set the tone for generations - biologists avoided
studying parasites as offering no additional information
revealing evolution's processes.