Morphic field

The theory of morphic fields conjectured that all living beings hade a field associated with them which defined their entire biology. Morphic fields provided a force that guided the development of an organism as it grew, making it take on a form similar to other species. DNA was not the source of structure itself, but rather a "receiver" that translated instructions in the field into physical form.

Morphic fields were reinforced by every successive generation. That is why there were so many humanoid races in the Universe. The Gallifreyans were the first sentient life to evolve in the universe, and so created a morphic field for humanoids. That made it more probable that the next race to evolve would be humanoid, and that made it much more likely that the following race would be humanoid, and so on. The majority of the non-humanoid races developed in environments where two legs, two arms and a head would have been a positive handicap. (NA: Lucifer Rising)

Jack Harkness explained to Rex Matheson that a disturbance in the morphic field was likely responsible for "Miracle Day". (TW: Rendition) It was revealed that inserting Jack's immortal blood into the Blessing at both ends disrupted the morphic field which resulted in Miracle Day. (TW: The Blood Line)

Behind the scenes

 * The theory of morphic fields and resonance originated as a serious Real World scientific proposal which was pioneered by Rupert Sheldrake PhD. However, most in the scientific community do not accept the idea and regard it as a fringe science or pseudoscience.