This thread relates specifically to the genre of science fiction and the art of creating believable non-terrestrial creatures/beings. Flipping through my old Genetics I book from my days at the University of Florida, I came across an interesting portion that made me think about how I create my alien beings and the brethren that share their worlds. It concerns certain rules (of which there are many) that rule morphological arrangement and accoutrement of the beings of our planet, Earth. The ones that gave me pause are the following: 1) No higher order* of vertebrate shall have more than a maximum of four (4) appendages (Legs or arms. The head is not considered, as it is the apex of the vertebral structure and not an offshoot of said structure ) *Fish are arguable because they are vertebrates, and many have more than two sets of ventral fins which can be argued to be analogous to limbs. Vertebrates with four appendages are called tetrapods. 2) No tetrapod shall have more than a maximum of five (5) digits* at the end of any given appendage. *There is fossil evidence that there were very early tetrapods with more than five digits, but for reasons only Mother Nature can answer, they did not survive. 3) All mammals shall have no more and no less then seven (7) neck vertebrae. Now, obviously these are only a tiny subset of the rules which apply on Earth. But my point is, rules do exist which sometimes seem unbreakable even when it would be more advantageous for the rule to be broken. For example, a giraffe, which is a mammal, has the same number of neck bones as you or I, even though it would seem to make sense that if it had more, it would be more limber, more flexible. But, alas, the rule is unbreakable in mammals, and the poor giraffe must sprawl in the most ungainly manner just to take a sip of water. Now, obviously not speaking about the rules we have here on Earth, but would genetic rules of some kind apply to an alien biosphere? If so, how would it affect your literary creation of alien beings?