|Style over substance|
The morphic field is the central concept in Rupert Sheldrake's theory of Morphic Resonance, the idea that living organisms take on forms due to... ah, I dunno. But it explains things better than evolution! (if you ignore the fact that there is not, in fact, any evidence at all for any such thing as a morphic field).
Not to be confused with the concept of a morphogenetic field, an important organising concept in early 20th-century developmental biology. Though the first under the name of the second will be familiar to readers of Pratchett.