Morphic field

The theory of morphic fields conjectures that all living beings have a field associated with them which defines their entire biology. Morphic fields provide a force that guides the development of an organism as it grows, making it take on a form similar to other species. DNA is not the source of structure itself, but rather a "receiver" that translates instructions in the field into physical form.

Morphic fields are reinforced by every successive generation. That is why there are so many humanoid races in the Universe. The Gallifreyans were the first sentient life to evolve in the Universe, and thus 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 next 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 it's 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.