We had a great discussion on G+’s martial artist forums regarding the question posed above. On the kung fu side of things, first there was random and haphazard fighting. Eventually, people figured out (and more importantly, passed on) the idea that certain ways of moving and fighting were better for their purposes; folks moved from the typical “boxing” jabs and hooks to crystallized kung fu patterns like Black Tiger Steals Heart (a level punch to the chest at the Bow-Arrow stance) or Hang a Golden Star at Corner (a punch to the side of the temple at the Bow-Arrow stance) because of certain advantages (a protected groin, more agile footwork, and being able to use waist rotation to generate and send spiral force from the abdomen into the opponent to deal injury).
Xingyi Quan is considered one of the three Chinese internal martial arts according to the classification wrought by Sun Lu Tang. Whereas Baguazhang and Taijiquan are often characterized by circular motions and open palms, Xingyi Quan is seen as a far more linear, closed-fisted, and aggressive art.