Natural Selection
From so simple a beginning endless forms most beautiful and most wonderful have been and are being evolved. ~ Charles Darwin in The Origin of Species (1859)
Preformationism eventually bowed to cell biology, though to this day evolution is rejected by Christian sects which cannot reconcile their delusional belief system with actuality.
For the more sober-minded, the mechanics of evolution have been the grounds for discovery and argument, revolving around 3 key questions: 1) whether evolution is random or adaptive; 2) whether the impetus behind evolution is competitive or environmentally-adaptive; and 3) whether evolution is purely genetic, or influenced by lifestyle prior to conception. The answers to these questions are known beyond doubt yet still argued over. Like physics, economics, and politics, evolutionary biology is as much religion as science.
