"In science, "fact" can only mean "confirmed to such a degree that it would be perverse to withhold provisional assent." I suppose that apples might … - Stephen Jay Gould

In short, the root-head turns the crab into a Darwinian cipher, a feeding machine working entirely in the parasite's service. The castrated crab can make no contribution to its own evolutionary history; its "Darwinian fitness" has become flat zero. ...But ever so carefully, for the parasite must maintain the crab in constant and perfect servitude - not draining the host enough to kill this golden goose, but not letting the crab do anything for its own Darwinian benefit.

Contingency is rich and fascinating; it embodies an exquisite tension between the power of individuals to modify history and the intelligible limits set by laws of nature. The details of individual and species's lives are not mere frills, without power to shape the large-scale course of events, but particulars that can alter entire futures, profoundly and forever.

Why, then, have we been bamboozled into accepting the usual tale without questioning? I suspect two primary reasons: we love a sensible and satisfying story, and we are disinclined to challenge apparent authority (like textbooks!). But do remember that most satisfying tales are false.